sugarcane trash management

Sugarcane Trash Management:

Consequences for Soil Carbon and Nitrogen

Final Report to the CRC for Sustainable Sugar Production

of the project Nutrient Cycling in Relation to Trash Management

(CRC Activity 2.2.1)

By Fiona Robertson

BSES & CRC SugarPO Box 86, (50 Meiers Rd),Indooroopilly, Qld 4068.Phone: +61 07 3331 3333Email: [email protected]

A CRC Sugar Technical Publication July 2003

CRC for Sustainable Sugar Production, Townsville. 39pp.

ISBN - 1876679271

CRCSUGAR

C R C S U G A R T e c h n i c a l P u b l i c a t i o n

Sugarcane Trash Management Consequences for Soil Carbon and Nitrogen iii

Table of Contents

Acknowledgements iv

SUMMARY 1

INTRODUCTION 3

METHODS 4

Field Experiments 4Climatic Conditions at Experimental Sites 6Analytical Methods 7Calculations and Statistical Analyses 8

RESULTS 10Trash Return from GCTB 10Trash Decomposition 11

Free Trash 11Incorporated Trash 15

Soil Organic C and Total N 17Soil Inorganic N 17Soil Microbial Biomass C 18Soil C Mineralisation 18Soil Net N Mineralisation 19Soil C:N Mineralisation Ratio 20Soil Bulk Density 21Soil Water 22Soil pH 22Soil Electrical Conductivity 22

DISCUSSION 23Trash returned after harvest 23C and N in Trash returned after harvest 23Trash decomposition 23Mineralisation and Immobilisation of N from Trash 24Soil C and N Cycling 25Long-term Soil C and N Status under GCTB 27Implications for Fertiliser Management under GCTB 30Water Retention 30Soil pH 30Stratification 31Other Effects of GCTB 31

Other Nutrients 31Organic Matter Accumulation 32

CONCLUSIONS 33

References 34

Publications Based on this Work 38

iv C R C S U G A R T e c h n i c a l P u b l i c a t i o niv C R C S U G A R T e c h n i c a l P u b l i c a t i o n

My thanks are due to the people whocontributed to and facilitated this work:

Graham Kingston, Alan Hurney, and LesChapman allowed this study to besuperimposed on their field experiments andprovided supporting crop and soil data. TheMcSwan family provided the fields for theHarwood experiments. Kaylene Harris, RuthMitchell, Kylee Sankowsky and Patricia Nelson

were at different times invaluable ResearchTechnicians on the project. Peter Larsen, JohnJackson, Jody Biggs, and Holly Ainslie helpedwith field work. Merv Probert made helpfulcomments on an earlier draft of this report. Inparticular, I thank Peter Thorburn for hisvaluable input and for a positive experience ofcollaborative research.

Acknowledgements

The Australian sugar industry is progressivelymoving away from the practice of burning thecrop before harvest to the system of green canetrash blanketing (GCTB). Since the residuesthat would have been lost in the fire arereturned to the soil, nutrients and organicmatter may be accumulating under trashblanketing. There is a need to know if this isthe case, to better manage fertiliser inputs andmaintain soil fertility.

The objective of this project was to determinewhether conversion from a burning to a GCTBtrash management system is likely to affect theability to maintain the soil organic matter andnitrogen (N) status of sugarcane soils. To thiseffect, five field experiments were conducted indifferent climatic zones (Tully, Mackay, andHarwood) to compare the carbon (C) and Ncycling in sugarcane soils where the crops weremanaged under either a (i) burnt or (ii) GCTBsystem.

In the experiments, 7-12 t/ha of trash drymatter (DM) was returned to the field underGCTB (approximately 10 t/ha for every 100t/ha fresh cane yield). This trash contained 3-5t C and 28-54 kg N /ha, and had a high C:Nratio (80-120). During one year, 82-98% of thetrash was decomposed. The decomposition ratewas influenced by rainfall and temperature andother factors not identified, and could bedescribed approximately by a relationship withthermal time. Net N loss from trash wasvariable among experiments until about 40% oftrash DM had disappeared, and thereafteraveraged 110 g N/ha/day (3 kg/month). Itscontribution to the soil mineral N pool is likelyto have been insignificant. In the olderexperiments (Mackay and Tully, 3-6 years old),soil organic C and total N were greater undertrash blanketing than under burning, to 10 or25 cm depth (most of this effect being in the top5 cm). The younger experiments (Harwood, 1-2years old) showed no significant trashmanagement effects. As a consequence of theimproved C availability under GCTB, soilmicrobial activity (CO2 production) and soilmicrobial biomass were increased under GCTB.Most of the trash C was respired by the

microbial biomass and lost from the system asCO2. The stimulation of microbial activity inthese relatively short-term GCTB systems (allwere in the first crop cycle) was notaccompanied by increased net mineralisation ofsoil N because of greatly increased microbialimmobilisation of N. It was calculated that,with standard fertiliser applications, the entiretrash blanket could be decomposed withoutcompromising the supply of N to the crop.Calculations of possible long-term effects ofconverting from a burnt to GCTB productionsystem suggested that, at the sites studied, soilC and N could increase by 3-23%, dependingon soil and climatic factors, and that it couldtake 10-35 years for the soils to approach thisnew equilibrium. Plant-available inorganic soilN would be expected to increase undermedium-long term GCTB, due tomineralisation of N from trash-derived organicmatter, to an amount approaching the annual Nreturn in trash. Small reductions in N fertiliserapplication will probably be possible in themedium-long term after adoption of GCTB,and the appropriate time scale for reducing Napplication will vary from site to site. Practicalrecommendations for fertilising under GCTBare made. Conversion from a burning to aGCTB management system is likely to improvethe soil organic matter and N status ofsugarcane soils.

Sugarcane Trash Management Consequences for Soil Carbon and Nitrogen 1

Summary

2 C R C S U G A R T e c h n i c a l P u b l i c a t i o n


burning to a GCTB trash management systemis likely to affect the soil organic matter and Nstatus of sugarcane soils. This objective wasachieved by conducting field experiments indifferent climatic zones (Tully, Mackay, andHarwood) to compare the carbon (C) and Ncycling in sugarcane soils where the cropswere managed under either a (i) burnt or (ii)GCTB system.

The work was done in close collaboration withPeter Thorburn of CSIRO SustainableEcosystems, who has contributed to thedevelopment of many of the ideas expressedhere. We have already presented the mainfindings of this work through various industryextension activities, and the publications listedat the end of the report.

In the Australian sugar industry, there is atrend away from conventional cultivation andburning crop residues before harvest to thesystem of green cane trash blanketing (GCTB),where residues are retained on the soil surfaceand cultivation is greatly reduced. Currently,around 70% of the Australian sugar crop isgrown under trash blanketing (Kingston andNorris, 2000), and this figure is still rising.

When sugarcane trash is burnt, most of theorganic matter and nutrients in the trash arelost. Mitchell et al. (2000) measured losses of70-95% of dry matter, nitrogen, potassium,phosphorus, magnesium, calcium, and sulfur.Conversely, with retention of trash, nutrientsand organic matter may be accumulating.There is a need to know if this is indeed thecase, to be able to manage fertiliser inputsappropriately to maintain soil fertility andminimise environmental contamination.Chapman et al. (1992) suggested that trashretention may increase total soil N and thatcane crops may benefit from this in the longterm. Wood (1991) reported increases in totalsoil N and C under a GCTB system in theHerbert region. Sutton et al. (1996) laterreported increased N mineralisation in thesame experiment. Retention of crop residueshas been shown to increase soil organic matterand nutrient content in several croppingsystems (Larson et al., 1972; Barber, 1979), butthe effect of trash conservation on the fertilityof canelands is not well known.

Nitrogen (N) is the nutrient most likely to limitproduction in cane farming systems and isapplied as a fertiliser in considerably greaterquantities than any other nutrient. This,together with the high solubility of N, makesthe risk of nutrient contamination in the off-farm environment greatest with N. Thedynamics of N in natural and agriculturalsystems are closely linked to carbon (C)dynamics, and the availability of N from cropresidues is controlled to a large extent by theavailability of C (Paul and Juma, 1981).

The objective of the work presented here wasto determine whether conversion from a

Introduction


Field Experiments

This research was conducted on 3 existing fieldexperiments in different climatic zones spreadover the main sugarcane-growing areas ofeastern Australia: Harwood (southern region),Mackay (central region) and Tully (northernregion) (Fig. 1, Table 1).

The experiments had been set up by otherresearchers to investigate the effects of trashmanagement on sugarcane productivity andratooning, and included several trashmanagement, cultivar, and cultivationtreatments. In this study (conducted betweenOctober 1996 and August 1998), 2 trashmanagement treatments were compared: (1)Burnt: trash burnt before or after harvest and(2) Trash blanketed: sugarcane harvestedwithout burning and trash retained on the soilsurface. In the Mackay and Tully experiments,the burnt treatments were cultivated and thetrash blanketed treatments were not (according

to usual grower practice). In Harwood, neitherof the treatments was cultivated. In Harwoodand Mackay, the trash management treatmentswere compared in separate experiments at 2times of harvest (early and late in the season).The Tully experiment was harvested late in theseason. All experiments were designed as

Methods

Table 1. Summary of the field experiments

Harwood Mackay Tully

Location northern New South Wales, central Queensland, northern Queensland,McSwan family farm BSES Experiment Station BSES Experiment Station

Grid reference 29.50 S, 153.20 E 21.10 S, 149.07 E 17.56 S, 145.56 E

Annual rainfall (mm) 1021 1668 4067

Trash management • Trash blanket • Trash blanket • Trash blankettreatments • Burnt (post-harvest) • Burnt (post-harvest) • Burnt (pre- and post-harvest)

Crop harvest • Late (December 1996) • Late (November 1996)treatments • Early (August 1997) • Early (July 1997) • Late (October 1996)

Replicates 4 4 3

Planting date July 1994 July 1992 July 1990

Crop class during • 1st ratoon (late) • 4th ratoon (late) • 6th ratoonthis study • 2nd ratoon (early) • 5th ratoon (early)

Years of trash • 1 (late) • 4 (late) • 6 (late)return (GCTB) • 2 (early) • 5 (early)

Irrigation no supplementary no

Soil texture Clay loam (26, 34, 40) Sandy loam (54, 26, 20) Clay (27, 34, 39)(%sand, %silt, %clay) Reps 1 & 2

Clay (18, 28, 54) Reps 3 & 4

Previous cropping vegetables, sugarcane (burnt) sugarcane (burnt) sugarcane (burnt)

Figure 1. Location of the experimental sites

N


randomised complete blocks. The Harwoodand Mackay experiments had 4 replicates. TheTully experiment had 3 replicates, but on mostoccasions, only 2 replicates were sampled in theburnt treatments, due to the inadvertentcultivation of one of the plots before post-harvest burning. Table 1 shows further site andexperimental information.

The sugarcane variety in each case was Q124,planted at a row spacing of 1.5 m. The cropswere fertilised within 6 weeks of harvest (160-200 kg N (as urea), 14-20 kg P, 90-125 kg K, and16-25 kg S). The fertiliser was applieddifferently at each site: on the surface on eitherside of the row in Harwood; in the centre of therow using a stool-splitter in Mackay; and oneither side of the row in Tully (below thesurface in the burnt treatments, and on thesurface of the trash treatments). The Mackayexperiments each received 70 mm of irrigation;the Harwood and Tully experiments were notirrigated. All crops were mechanically

harvested. In the GCTB treatments, the trashwas left as a blanket on the soil surface. In theburnt treatments at Harwood and Mackay, thetrash was burnt on the ground after harvest. Inthe burnt treatments at Tully, the cane wasburnt before harvest and residual trash burnton the ground after harvest.

At each site, the mass of trash deposited on thetrash blanketed plots after harvest wasmeasured by weighing the trash in 10randomly placed quadrats per replicate (1.50 x0.75 m). The average was used as the standardmass of trash for that site. At Harwood andMackay, the contents of the quadrats wasdivided into cut trash and whole tops, and astandard mass was calculated for each. Tomeasure trash decomposition, a standard massof trash was placed within multiple quadrats (7per replicate at Harwood, 8 per replicate atTully, 7 per replicate in Mackay (Early) and 9per replicate in Mackay (Late)), after firstraking off underlying trash. The quadrats were

Table 2. Sampling schedule for trash and soil

Sampling Harwood Mackay TullyDate Days after Date Days after Date Days after

harvest harvest harvest

Late-harvest

1st Trash 11 Dec. 1996 10 25 Nov. 1996 12 31 Oct. 1996 11st Soil 13 Dec. 12 29 Nov. 16 31 Oct. 12nd 11 Feb. 1997 72 15 Jan. 1997 63 27 Dec. 583rd 31 Mar. 120 17 Mar. 124 6 Mar. 1997 1274th 15 May 165 1 May 169 17 Apr. 1695th 23 June 204 4 June 203 28 May 2106th 6 Aug. 248 14 July 243 23 July 2667th 25 Sept. 298 16 Aug. 276 4 Sept. 3098th 27 Nov. 361 19 Sept. 310 8 Oct. 3439th 21 Oct. 342 30 Oct. 36510th 4 Nov. 356

Early-harvest

1st Trash 23 Aug 1997 9 16 July 1997 121st Soil 24 Aug 10 18 July 142nd 26 Sept 43 18 Aug 453rd 26 Nov 104 17 Sept 754th 12 Jan 1998 151 12 Nov 1315th 13 March 211 27 Jan 1998 2076th 30 Apr 259 30 March 2697th 29 June 319 18 May 3188th 3 Aug 354 6 July 367

Table 3. Rainfall (mm) during the experimental period compared with the long-term average.

Experimental Period Long-Term Average

1996 1997 1998 (Aug.-Dec.) (Jan.-Dec.) (Jan.-Aug.)(Aug.-Dec.) (Jan.-Dec.) (Jan.-Aug.)

Harwood 292 951 503 327 1021 736

Mackay 404 1753 1083 397 1668 1296

Tully 692 3267 773 4067 3428

held in place by tent pegs, and covered withbird netting (2 cm mesh).

On 7 to 9 occasions over the following year, onequadrat was destructively sampled perreplicate (Table 2). First, all trash that had notbecome incorporated with the soil (‘free trash’)was removed from the quadrat and retained.The soil was then sampled within the quadratby taking 12 cores (3.0-3.4 cm internaldiameter) in two regular transects from the topof one row, across the inter-row, to the top ofthe next row (Fig. 2). The cores were cut intodepth layers (0-2, 2-5, 5-10, and 10-25 cm) andsamples pooled from each depth. Trash thathad become incorporated with the soil(‘incorporated trash’) was then sampled byremoving soil and trash from a strip (17.5 cmwide x 150 cm long x about 2 cm deep)between the holes left by the soil sampler (Fig.2). After sampling, trash from adjacent guardrows was placed over the disturbed area. Soilin the burnt plots was sampled in exactly thesame way, from an area of approximately 1.50 x0.75 m.

In the Mackay (Early) experiment on 20 April1998, soil was sampled for bulk density

estimation by taking undisturbed cores (70 mmdiameter x 50 mm deep) in the 0-5, 5-10 and10-20 cm depth layers, in the row, shoulder, andinter-row positions.

Climatic Conditions at ExperimentalSites

Monthly rainfall and temperatures during theexperimental period are shown in Figs 3 and 4.Long-term mean annual rainfall was 4060 mmat Tully, 1670 mm at Mackay and 1020 mm atHarwood. Rainfall was generally within 25% ofthe long-term average (Table 3), except inFebruary 1997 at Mackay, when it was twicethe average. Temperatures decreased in theorder Tully > Mackay > Harwood, with thegreatest difference being in winter minimumtemperature (Fig. 4, Table 4). The number of


Figure 2. Plan of the position of trash quadrats, andsoil and incorporated trash samples.

Figure 3. Air temperature during the experimentalperiod.Data are maximum and minimum monthly means.


rainy days decreased in the order Tully >Harwood > Mackay. Cumulative temperaturewas greater in early- than in late-harvestedcrops, but cumulative rainfall was greater inlate-harvested crops (Table 4).

Climate data for the Mackay and Tullyexperiments were recorded on site (bothBureau of Meteorology recording stations).Climate data for the Harwood experimentswere measured on site (Graham Kingston,unpublished data), but contained some longgaps due to failure of the equipment. Thesegaps were filled by data from the nearestBureau of Meteorology site (Bushgrove forrainfall, Grafton for temperature), modifiedaccording to the equation obtained by linearregression of site data and Bureau of

Meteorology data for summer and winter.

Analytical Methods

Soils were sieved (7 mm) and any large piecesof organic material other than trash wereremoved. Small pieces of trash retained on thesieve were reincorporated with the soil. Soilswere stored at 4°C for 1-10 days beforeanalysis. The water contents of trash and soilwere determined by drying for 24 hours at 70and 105°C, respectively.

Soil inorganic N (ammonium + nitrate) wasdetermined by extracting 15 g portions of field-moist soil in 45 ml of 2 M potassium chloride,filtering the extract through Whatman 42 filterpaper, and measuring ammonium and nitrateby automated colorimetric analysis (Raymentand Higginson, 1992, method 7C2).

Soil microbial biomass C (a measure of the totalsize of the microbial population, includingbacteria, fungi, protozoa etc.) was determinedby the chloroform fumigation-extractionmethod of Vance et al. (1987), which uses 0.5 Mpotassium sulfate as an extractant. Organic C inthe extracts was measured by dichromateoxidation (Heanes 1984) andspectrophotometry. The determinations weredone on 15 g portions of field-moist soil, madeup to a standard gravimetric water content,approximately 80% of water holding capacity(30% for Mackay soils, 35% for Harwood soils,and 40% for Tully soils). This method yieldedsome negative values if C from the

Table 4. Cumulative days, temperature, rainfall, rainy days, and (rainfall – evaporation) at the experimental sites.

Harwood Mackay Tully

Late harvest∑ Days 353 344 364∑ TemperatureA 6776 7582 8565∑ RainfallB 969 1502D 3694∑ Rainy daysC 112 96 163∑ (Rainfall – evaporation)E -618 -371 1910

Early harvest∑ Days 347 355∑ TemperatureA 7094 8129∑ RainfallB 814 1261D

∑ Rainy daysC 101 89∑ (Rainfall – evaporation)E -789 -708

A Cumulative mean daily temperature (ºC). C Cumulative number of days where rainfall > zero. E Cumulative daily (mm)B Cumulative daily rainfall (mm). D Includes irrigation (70 mm).

Figure 4. Monthly rainfall during the experimentalperiod.


unfumigated soil (Cunfum) was subtracted fromC from the fumigated soil (Cfum), as is the usualprocedure. However, Cfum was well correlatedwith Cfum – Cunfum (Table 5). Therefore Cfum ispresented as an index of microbial biomass C(e.g. Fig. 17). Where microbial biomass isshown as kg/ha (e.g. Fig. 15), this has beencalculated from positive values of (Cfum –Cunfum ) only, using a Kc factor of 0.38 (Vance etal.,1987).

The ability of the soils to mineralise N and Cwas measured in laboratory incubations. Soil Nand C mineralisations were measured as thechange in inorganic N (ammonium + nitrate)and the release of carbon dioxide, respectively,during a 7-day aerobic incubation at 25°C.Again, 15 g portions of field-moist soil wereused, made up to the standard water contentsabove. Carbon dioxide was trapped in 1 Msodium hydroxide and determined by titrationagainst dilute hydrochloric acid (Zibilske 1994).In soils from several samplings (Mackay lateharvest days 124 and 169, and Tully days 1, 210and 266), some mineralisation of inorganic Noccurred during storage. In those soils, thestarting inorganic N content for the incubationwas taken as that determined soon aftersampling instead of that determined on the daythe incubations were set up. This increased theN mineralisation value, but had no effect ontreatment differences or time trends.

The C:N mineralisation ratio of the incubatedsoils was calculated as the ratio of Cmineralisation: N mineralisation. Where net Nmineralisation was negative (i.e. netimmobilisation of N occurred), and where netmineralisation was <0.2 µg N/g/day, the valueof the N mineralisation was set to 0.2 µg

N/g/day. This was done to provide a semi-quantitative measure of gross mineralisationand net immobilisation, but had the effect ofreducing the treatment differences.

Incorporated trash was separated from the soilby soaking and agitating the sample in 7 L ofwater for 2 minutes, then removing floatingorganic matter by hand and suspended organicmatter on a 2 mm sieve. This procedure wasperformed 4 times per sample. The trash wasthen dried at 70°C. Any obvious non-trashmaterial was removed manually from the driedsample.

Total C and N analyses were done on driedsubsamples of soil ground to <250 µm andtrash ground to <500 µm. Total N in soil wasdetermined by Kjeldahl digestion followed byautomated colorimetric analysis (Rayment andHigginson 1992, method 7A2). Organic C in soilwas determined by the Heanes dichromateoxidation method, which measures totalorganic C but not carbonate (Heanes 1984;Rayment and Higginson 1992, method 6B1).Total N and C in trash were determined using aLeco induction furnace.

Soil pH and electrical conductivity weremeasured in 1:5 soil-water extracts (Raymentand Higginson 1992, method 3A1). The pH ofsome soils was also measured in 1:5 soil-calcium chloride extracts (Rayment andHigginson 1992, method 4B2).

Calculations and Statistical Analyses

The effects of trash management, soil depth,and sampling time were tested separately foreach experiment by analysis of variance for arandomised complete block design using theSystat program (SPSS Inc.). Allowance wasmade for the unequal number of replicates inthe Tully experiment. Relationships betweenselected variables were investigated usingPearson correlation and linear regression.Unless stated otherwise, effects were taken assignificant where Bonferroni probabilities wereP≥ 0.05.

Cumulative trash returns since theestablishment of the original field experimentswere estimated using the average relationshipbetween trash DM and fresh cane yield

Table 5. Correlations between Cfum and (Cfum –Cunfum) in the soil microbial biomass assay.Negative values of (Cfum – Cunfum) omitted. Pooled datafrom each experiment.

Experiment Correlation P ncoefficient

Harwood (Late) 0.89 0.001 167

Mackay (Late) 0.94 0.001 278

Tully 0.88 0.001 138

Harwood (Early) 0.92 0.001 245

Mackay (Early) 0.91 0.001 221

measured in the current experiments:

Trash DM returned = (Cane fresh weight) x 0.1

Cumulative returns of C and N in trash wereestimated by assuming concentrations of C =44% and N = 0.54% (means from the 5experiments).

Accumulated thermal time (∑temperature) wascalculated from mean daily air temperature,without subtraction of a base temperature.



Trash Return from GCTB

After harvest, about 7-12 t of trash dry matter(DM) was returned per ha (means forindividual experiments, Fig. 5 (c), (d)).Approximately 10 t of trash DM was returned

for every 100 t fresh cane yield (r2=0.54). Forindividual experiments, however, this figureranged between 8 and 12 t DM/ha.

Trash DM estimates were variable within plots(CV 23-26% at Harwood, 33-60% at Mackay,

Results

Figure 5. Dry matter (DM) decomposition in free trash during one year.Bars indicate LSD (P= 0.05) within experiments.


and 45% at Tully). Trash water content was 19-30% (fresh weight basis) after harvest (senescedtrash 8-47%, green tops 21-72%). During theexperiments, trash water content was 5-86% atthe time of sampling.

The trash returned after harvest contained 3-5 tC/ha and 28-54 kg N/ha (Figs 6 and 7). Thereturn of N in trash was not significantlyrelated to cane yield. The concentration of Cwas very consistent (44.1-45.0%), but theconcentration of N was more variable (0.38-0.67%) among experiments (Fig. 8). The trashC:N ratio was 70-117 (Fig. 8).

Trash Decomposition

Free Trash

During one year, 82-98% of the free trash(surface trash, not incorporated with the soil)was decomposed (Fig. 5 (a), (b)). The rate ofDM decomposition between samplingoccasions varied between zero and 0.78%/day(54 kg/ha/day, Fig. 5 (e), (f)). In the late-harvested crops, decomposition rate wassignificantly (P<0.05) related to mean airtemperature (r2=0.36), and mean daily rainfall(r2=0.36) in the previous decomposition period

Figure 6. Carbon in free trash during one year.Proportion of original mass remaining (a, b), and mass remaining (c, d). Bars indicate LSD (P= 0.05) within experiments.

(combined data from 3 experiments). Theserelationships were generally stronger for theindividual sites (Fig. 9). The proportion of rainydays in the decomposition period had asignificant effect on decomposition rate inMackay (Late) only (Fig. 9). The influence ofrainfall on decomposition (as seen by the slopeof the regression line, Fig. 9) increased in theorder T < M < H, in line with decreasingrainfall at the site. The influence of temperatureon decomposition (slope) was not markedlydifferent among sites. In early-harvested crops,decomposition rate was not correlated with anyclimatic variables.

Cumulative trash DM decomposition in the 5experiments was more closely related toaccumulated thermal time, ∑(temperature),than to accumulated time (Fig. 10). Trash DMloss could be approximated from the equation

LDM = 0.012 * ∑(temperature)

where LDM is percentage loss of DM from trashsince harvest (day 0). Decomposition could notbe described by a single relationship withaccumulated rainfall (´∑(rainfall), data notshown).

During the year, the concentration of C in freetrash declined and that of N increased, the C:N


Figure 7. Nitrogen in free trash during one year.Proportion of original mass remaining (a, b), and mass remaining (c, d). Bars indicate LSD (P= 0.05) within experiments.


Figure 8. C and N concentration, and C:N ratio of free and incorporated trash during one year.Bars indicate LSD (P= 0.05) within experiments.


ratio falling to around 40 in late-harvestedcrops and 23-29 in early-harvested crops (Fig.8).

At the end of the year, 84-98% of the original Cand 67-95% of the original N were lost fromfree trash. In other words, 50-800 kg C/ha and1-20 kg N/ha remained in free trash at the endof the year (Figs 6 and 7). The rate of C lossfrom free trash was almost identical to the rateof DM loss (r2=0.99, P<0.001).

Unlike DM decomposition, net loss of N fromfree trash could not be predicted adequatelyacross all sites by one relationship with eitheraccumulated time or ∑(temperature). AtHarwood and Mackay, net N loss from freetrash was linearly related to∑(temperature)throughout the year. At Tully, however, therewas no net N loss from free trash until afterday 126 (∑(temperature) 3400), when around

40% of the DM had disappeared, and the C:Nratio had fallen to around 55. Thereafter, the Nloss conformed to the relationship with∑(temperature) shown at Harwood andMackay (Fig. 11 (b)):

LN = 0.0085 * (∑(temperature)

where LN is percentage loss of N from trashsince harvest.

Likewise, the relationship between DM lossand N loss from free trash was similar in theHarwood and Mackay experiments, and theTully experiment did not follow this trend untilabout 40% DM loss had occurred. Thereafter, Nand DM were lost from free trash at aconsistent rate in all experiments, and N lossfrom free trash could be described by theequation:

LN = 1.36 [LDM] - 49

Figure 9. Effect of climatic variables on decomposition of free trash DM.(a) Air temperature, (b) rainfall, and (c) proportion of rainy days, in late-harvested crops. Data are means of the periodsbetween sampling dates.


where LN and LDM are percentage loss of N andDM, respectively, from free trash since harvest(Fig. 11 (c)). The rate of N loss from trashduring this period ranged between 100 and 130g N/ha/day, and averaged 110 g N/ha/day (±standard error of 6 g N/ha/day) across allexperiments.

Incorporated Trash

Incorporated trash (trash incorporated into thetop 2 cm of soil) contained 1.0-2.6 t DM/ha,370-860 kg C/ha, and 12-27 kg N/ha, with no

apparent time trends (Table 6). The Cconcentration was also variable (26-41%) withno trends in time. The N concentration (0.9-1.4%) tended to rise during the year (Fig. 8).

Table 6. Dry matter (DM), C and N in free and incorporated trash during one year in late- and early-harvestedcrops.

Days after Incorporated trash Free + incorporated trashharvest

DM C N DM C N(t/ha) (kg/ha) (kg/ha) (t/ha) (kg/ha) (kg/ha)

Harwood – Late harvest

360 1.49 472 21.1 2.31 816 29.7

Mackay – Late harvest

276 1.90 731 21.6 4.77 1957 50.2310 2.13 842 24.1 4.87 2015 51.0342 2.23 746 23.6 4.94 1860 52.2356 1.95 752 23.8 3.86 1543 43.6

Tully – Late harvest

309 2.03 692 19.9 2.94 1025 28.0343 1.74 545 17.9 2.50 801 25.1365 2.57 861 26.8 2.71 911 28.1

Harwood – Early harvest

8 2.32 826 24.7 10.55 4530 79.143 1.48 540 16.7 9.47 4102 73.6104 0.99 372 12.7 5.81 2437 53.3152 2.27 651 26.2 6.48 2374 71.6211 1.97 505 22.2 5.59 1906 59.9260 1.33 368 16.6 4.47 1557 56.1319 1.51 469 20.9 4.13 1498 53.5354 1.80 637 24.2 3.29 1030 44.3

Mackay – Early harvest

12 1.64 627 15.6 9.66 4272 64.145 1.08 421 11.5 8.09 3594 51.575131 1.12 456 12.9 6.37 2749 48.2207 2.16 740 21.6 4.88 1851 42.7269 1.57 507 17.5 3.49 1240 36.4318 1.50 490 18.0 2.66 921 30.2367 1.39 467 19.9 2.70 918 35.4

Table 7. Average soil organic C and total Nconcentrations at the experimental sites (0-10 cmdepth).

Site Organic C (%) Total N (%)

Harwood 2.4 0.20

Mackay 1.4 0.08

Tully 1.4 0.10


Figure 10. Relationships between free trash DM loss and accumulated (a) time and (b) thermal time(∑(Temperature)).Data from all experiments. Broken line on figure (b) is 95% confidence interval for mean value of Y at any given value of X.

Figure 11. Relationships between N loss from free trash and (a) days after harvest, (b) thermal time(∑(temperature)) and (c) DM loss from free trash.Tully data excluded from figure (b). All data shown in figures (a) and (c), but regression in (c) refers to DM Loss ≥40%. Brokenlines are 95% confidence interval for mean value of Y at any given value of X.


The C:N ratio of incorporated trash remainedwithin a fairly narrow range, declining fromaround 30-40 at the start of the year to around20-30 at the end. By the end of the year, theincorporated trash contained as much or moreC and N than the free trash. At this time, theC:N ratio of incorporated trash was generallyslightly lower than that of free trash (Table 6,Fig. 8).

Soil Organic C and Total N

Soil organic C and total N were greater undertrash blanketing than under burning, in the 0-2and 2-5 cm depth layers of the older (Mackayand Tully) experiments (Figs 12 and 13). Thistrash effect was generally significant whenaveraged to 10 cm at Mackay and to 25 cm atTully (data not shown). At Harwood, soilorganic C and total N were generally notsignificantly affected by trash management. Inthe late-harvested experiment at Harwood,however, there was a trend for greater soil Cand N in burnt plots than in GCTB plots. Thisis not considered to be a true treatment effect,

but rather an effect of the growth of algae andmoss on the soil surface, and the presence ofbroadleaf weed plants and seeds, whichoccurred in the burnt plots but not in the GCTBplots.

Soil organic C and total N were greater atHarwood than at Mackay and Tully (Table 7).Stratification of soil C and N by depth wasmore pronounced under GCTB at Tully andMackay (Tully data shown in Fig. 14 (c, d)).

The trash effect on soil organic C and total Nincreased with cumulative C and N returnsfrom trash (Fig. 15 (c, d). The slope of theregression lines suggested that soil organic Cincreased by 126 kg/ha for every t C returned,and soil N increased by 0.79 kg/ha for every kgN returned.

Soil Inorganic N

Soil inorganic N concentration was initiallyhigh following fertiliser application, anddeclined to a low level within about 2-3 months

Figure 12. Soil total organic C in the 0-5 cm depth.Asterisk indicates trash management treatments aresignificantly different (P<0.05).

Figure 13. Soil total N in the 0-5 cm depth.Asterisk indicates trash management treatments aresignificantly different (P<0.05).


Figure 14. Soil C and N measurements to 25 cm depth.Data from Tully experiment. Asterisk indicates trash management treatments are significantly different (P<0.05).

Figure 15. Relationships between cumulative trash C and N inputs and trash effect on soil C and N (0-10 cm).

(Fig. 16). Variation amongreplicates was extremely large afterfertilisation due to the presence ofundissolved fertiliser granules.There were no consistentdifferences between trashmanagement treatments ininorganic N concentration (somestatistically significant differenceswere observed, but they can not beconsidered true treatment effects).

Soil Microbial Biomass C

The soil microbial biomass C indexwas increased by trash blanketingat Mackay and Tully in the 0-2 and2-5 cm layers (Fig. 17 shows meansfor 0-5 cm). The effect wasgenerally still significant whenaveraged over the top 10 cm of soil(data not shown). The microbialbiomass index was not significantlyaffected by trash management atHarwood.

The Tully soils had more microbialC than the Harwood and Mackaysoils. There was considerabletemporal variation in microbialbiomass (except Harwood (Early).Microbial C in burnt and GCTBtreatments was generally wellcorrelated (overall r=0.81, P<0.05).

Like total organic C, the microbialbiomass C index decreased moremarkedly with depth in GCTB thanin burnt soils (Tully data shown inFig. 14). The magnitude of the trashresponse was directly proportionalto cumulative C returns in trash,with microbial biomass Cincreasing by about 5 kg C/ha forevery t trash-C returned. (Fig. 15).

Soil C Mineralisation

At most sampling dates, Cmineralisation (Fig. 18) was

increased by trash blanketing in the 0-2 cmdepth, and the effect was generally significantwhen averaged over the top 5 cm. As with totalorganic C, stratification of C mineralisationactivity was greater under trash (Tully datashown in Fig. 14 (a)). The size of the trash effectincreased with increasing cumulative Creturned in trash (Fig. 15 (a)).

Soil Net N Mineralisation

Soil net N mineralisation generally did notdiffer significantly between trash managementtreatments in any depth layer or at anysampling time, though it tended to be lower inGCTB soils than burnt soils from 0-2 cm atMackay and Tully (Table 8). Net Nmineralisation did not change significantlywith depth, but the GCTB and burnt treatments


Figure 16. Soil inorganic N in the 0-25 cm depth.Note different scales on the Y-axis. Asterisk indicates trash management treatments are significantly different (P<0.05). Barsare LSD (P=0.05).

were more similar in the 10-25 cm depth thanin the shallower depths. Variability amongreplicates was large, and there were no trendswith time and no differences between sites.

Soil C:N Mineralisation Ratio

The C:N mineralisation ratio of the surface 10cm of soil was greater in GCTB than in burnttreatments, particularly in the 0-2 cm depth andin the Mackay and Tully experiments. Meansfor the 0-5 cm depth are shown in Fig. 19. TheC:N mineralisation ratio showed no consistent


Table 8. Soil net N mineralisation during incubation,0-2 cm depth.

Site Net N Mineralisation SignificanceA

(µg/g/day)

Burnt GCTB

Late harvest

Harwood 0.38 0.32 n.s.

Mackay 0.32 -0.12 **

Tully 0.20 0.03 n.s.

Early harvest

Harwood 0.31 0.38 n.s.

Mackay 0.37 0.12 n.s.

A Significance of trash management effect as determined byindependent t-test: **, P<0.01, n.s., P>0.05. Data from all samplingdates included (except data from the two sampling dates afterfertiliser application)

Figure 17. Soil microbial biomass C index in 0-5 cmdepth.Asterisk indicates trash management treatments aresignificantly different (P<0.05). Bars are LSD (P=0.05) forcomparing sampling times.

difference below 10 cm. There were no regulartime trends.

Soil Bulk Density

In the Mackay (Early) experiment, soil bulkdensity was very slightly greater under GCTBthan burnt management (overall mean 1.46 and1.40 g/cm3, respectively, P<0.05). Most of theeffect was due to differences in the inter-rowposition and below 5 cm depth (Table 9).


Figure 18. Soil C mineralisation potential in 0-5 cm depth.Asterisk indicates trash management treatments aresignificantly different (P<0.05). Bars are LSD (P=0.05) forcomparing sampling times.

Figure 19. Soil C:N mineralisation ratio during 7-dayincubation.Data are means 0-5 cm depth and all sampling times.Experiments: HE, Harwood Early; HL, Harwood Late; ME, MackayEarly; ML, Mackay Late; TL, Tully Late. Asterisk indicates trashmanagement treatments are significantly different (P<0.05).

Table 9. Soil bulk density in Mackay (Early), 20 April1998.LSD (P=0.05) for management x depth x position interaction = 0.05g/cm3.

Depth Soil Bulk Density (g/cm3)(cm)

BURNT GCTB

Row Shoulder Inter- Row Shoulder Inter-row row

0-5 1.20 1.35 1.36 1.29 1.31 1.42

5-10 1.38 1.44 1.43 1.42 1.49 1.58

10-20 1.39 1.46 1.50 1.44 1.49 1.57

0-20 1.34 1.43 1.45 1.40 1.44 1.54

Figure 20. Soil water content in the 0-5 cm depth.Asterisk indicates trash management treatments aresignificantly different (P<0.05).

Soil Water

The water content of the surface soil wasconsiderably greater under a trash blanket thanunder burnt management until canopy closure(Fig. 20). In Tully, this difference remained allyear. Most of the difference was in the top 5cm, but was usually still significant whenaveraged over the top 25 cm (data not shown).For calculation of volumetric water, bulkdensity for Mackay soils was assumed to be thesame as measured in the Early experiment,bulk density in the Tully soils was taken as1.12, 1.21, and 1.22 g/cm3 for 0-5, 5-10, and 10-25 cm, respectively (Mike Braunack,unpublished data). Bulk density at Harwoodwas assumed to be the same as Mackay.Volumetric water content gave identicaltreatment trends, and on the day of soilsampling, there was 0-30 mm more water in thetop 25 cm of trash blanketed soil than burntsoil.

Soil pH

At Tully, trash blanketing lowered soil pH(water) to 25 cm depth (significant for the first6 sampling dates). Trash management had nosignificant effect on pH at Harwood or Mackay(Fig. 21). Soil pH in the experiments fell withinthe range 4.5-5.5. The pH (calcium chloride) ofTully soils from the 2-5 and 5-10 cm depthsshowed the same trends as pH (water) (Fig. 22).

Soil Electrical Conductivity

There was no consistent effect of trashmanagement on soil electrical conductivityexcept in the 0-2 cm depth at Tully, whereelectrical conductivity in GCTB soil was greaterthan that in burnt soil for most of the year (Fig.21). Some individual sampling dates showedsignificant trash management effects (data notshown), where increased electrical conductivityunder either GCTB or burning coincided withhigh inorganic soil N content.


Figure 21. Soil pH (water) and electrical conductivity(EC) to 25 cm depth.Data are means of all sampling dates. Asterisk indicates trashmanagement treatments are significantly different (P<0.05).

Figure 22. Soil pH (calcium chloride) in the 2-10 cmdepth at Tully.Asterisk indicates trash management treatments aresignificantly different (P<0.05).


Trash returned after harvest

The quantity of trash DM returned underGCTB was fairly variable among experiments,but could be predicted approximately from thefresh cane yield. The average trash return inthis study of 10 t DM / 100 t fresh cane waslower than the 13 t DM / 100 t fresh canereported by Paul Nelson (unpublished data) incommercial canefields in the Burdekin region.His study spanned a wider range of crop yieldsthan the present study, so his estimate may bemore appropriate for generalising to othersituations.

The variability in trash DM was due to severalfactors: the gappy nature of the crops andirregular plant size (Mackay and Tully); thefrequent stopping and starting of the harvesterat experimental plots; irregular feeding of canethrough the harvester (Mackay-Late); anddifferences in harvester set-up and operation.Because a repeatable estimate of trash mass isdifficult to obtain under field conditions, thestarting mass of trash needs to be controlled inany attempt to measure its decomposition.Trash water content is so variable that freshmass is clearly not a useful indication of trashload.

C and N in Trash returned afterharvest

The trash from newly harvested cane containedsignificant quantities of N (48-55 kg/ha inHarwood and Mackay, 27 kg/ha in Tully). Thesmall N return in Tully reflected the small yield(79 t/ha in the year of this study, comparedwith an average of 111 t/ha in previous years,Alan Hurney, unpublished data). The N intrash represents around 60% of the total above-ground plant N (Chapman et al., 1994), so thatN loss from the system is considerably reducedby trash retention.

The return of N in trash, however, could not bepredicted adequately from the cane yield. Thereturn of C in trash could be estimated as 4400

kg C/ha per 100 t/ha fresh cane yield.

Most of the trash DM, C and N would havebeen lost under the burning system. Dependingon the severity of the fire, 77-97% of the DMand N may be lost by burning sugarcane trash(Mitchell et al., 2000). The losses were probablygreater at Tully, where the standing crop andresidual trash were burnt, than at Harwoodand Mackay, where the trash was burnt on theground only.

The quantity of trash returned under GCTB (upto 12 t DM/ha in this study, 25 t/ha in theBurdekin region (Fiona Robertson, unpublisheddata), and up 15 t DM/ha measured by Spainand Hodgen (1994)) and Jessica Klock(unpublished data), is large in comparison withthe harvesting residues from most agriculturalcrops, e.g. 2-8 t DM/ha from wheat and barley(Buyanovsky and Wagner, 1986; Bolinder et al.,1997). Corn (Zea mays) can also return largeamounts of residue (e.g. 14 t DM/ha,Buyanovsky and Wagner, 1986).

The C:N ratio of fresh trash was high - up to117 - therefore quality in terms ofdecomposition was low. Sugarcane trash C:Nratio can be greater than this, eg. 170 (Spainand Hodgen, 1994) The residues of other cropswith similarly high C:N ratios (eg. wheat (73)Amato et al., 1987; barley (94) Christensen,1985) result in net immobilisation of N duringdecomposition, so N immobilisation could beexpected to predominate during thedecomposition of sugarcane trash. The Cconcentration was almost identical in thedifferent experiments, but the N concentrationwas variable, which resulted in some variationin trash quality (trash returned in Tully havingthe lowest quality).

Trash decomposition

It took a year for essentially all (82-98%) of thetrash blankets to decompose. This is inagreement with the 81% decomposition of a 15t DM/ha trash blanket in the Herbert region ina little less than a year recorded by Spain and

Discussion

24 C R C S U G A R T e c h n i c a l P u b l i c a t i o n24 C R C S U G A R T e c h n i c a l P u b l i c a t i o n

Hodgen (1994). At several sites in the WetTropics, Jessica Klock (unpublished data)measured 95% DM loss from trash during oneyear.

Rainfall and temperature had a significantinfluence on the rate of free trashdecomposition in the late-harvested crops, withtheir effect increasing in the order Tully <Mackay < Harwood, in line with decreasingtemperature and rainfall (Fig. 9). Thisobservation concurs with the established notionthat water availability and temperature aremajor controlling factors for organic matterdecomposition (Stott et al., 1986). The extent towhich wheat residues decompose during oneyear was found to be positively related torainfall, over 12 locations in southern Australia(Amato et al., 1987). The more completedecomposition of trash at Tully than at theother sites was probably an effect of the greatertemperature and rainfall. The distribution ofrainfall during the decomposition period (theproportion of rainy days in a decompositionperiod) was apparently a significant control ondecomposition in the late-harvested crop atMackay only, probably because this was themost water-limited experiment. Such an effectof rainfall frequency has also been observed byVanlauwe et al. (1995) in dry tropicalconditions.

However, the relationships between free trashdecomposition rate and temperature andrainfall varied among harvest seasons and sites,indicating that other factors (soil, trash, ormanagement) also influenced decompositionrate, and were sometimes more important thanclimate. Climate and soil interact in theproportion of soil pore space habitable bymicroorganisms, a factor shown to be animportant control of microbial activity anddecomposition (Young and Ritz, 2000). Therelative influence of all factors is likely to havevaried throughout the year.

Notwithstanding the fact that it was influencedby factors other than climate, trashdecomposition could be estimatedapproximately from the relationship withthermal time found in this study (Fig. 10 (c)).The climatic range represented by Tully,Mackay and Harwood encompasses a largepart of the sugar industry, therefore it is likely

that this relationship would hold at other sites.Fully irrigated conditions, however, may bedifferent.

Mineralisation and Immobilisation ofN from Trash

The N measured in free trash included not onlythe remaining trash-N, but any soil or fertiliserN (including that immobilised bymicroorganisms) associated with the trash. Thechange in free trash N through time thereforecould have represented N movement from anyof these sources.

At the start of the year, a delay in apparent Nrelease from free trash occurred in allexperiments except Mackay (Early), lastingabout 6 weeks in Harwood and Mackay and 16weeks in Tully (Fig. 7). This is most likely tohave resulted from retention of fertiliser andsoil N within the trash through microbialimmobilisation (because there was DM lossduring this period, indicating activedecomposition). This immobilisation mayinitially have been greater in Harwood andTully because the fertiliser was applied on topof the trash (at Mackay, the fertiliser wasapplied below the soil surface). Such an effectof fertiliser placement would not have enduredbeyond day 100-150, by which time localisedhigh concentrations of soil inorganic N haddisappeared (data not shown).

After the initial period of decomposition whereN retention and loss were variable amongexperiments (until about 40% of trash DM haddisappeared), the trash C:N ratios almostconverged. Thereafter, N loss from free trashwas more easily predicted, from∑(temperature) or from DM loss (Fig. 11). Theaverage N loss from free trash during thisperiod (110 g N/ha/day) was remarkablysimilar among experiments.

N loss from free trash probably occurredthrough a combination of two processes: Firstly,movement of N from free trash to soil as aresult of the growth of hyphae of (soil-based)fungi through the trash layer, a process whichhas been demonstrated to be important duringdecomposition of wheat straw (Frey et al.,


2000). The declining C:N ratio of free trash withtime indicated decomposition of free trash eventhough most of it was not in direct contact withthe soil. The second process is transfer of Nfrom the free to the incorporated trash poolthrough the consolidating effect of rainfall, themixing effect of earthworms and other soilfauna, or partial decomposition bymicroorganisms (mostly fungi). Transition fromincorporated trash to soil would have occurredthrough further decomposition by fauna andmicroorganisms. For most of the year,incorporated trash was in a more advancedstage of decomposition than free trash, as seenby its lower C:N ratio and smaller fragmentsize. At the end of the year, the C:N ratios offree and incorporated trash were similar (23-42for free and 23-32 for incorporated trash), andlow enough that some net mineralisation of Ncould be expected from their decomposition(Parr and Papendick, 1978). The size of theincorporated trash pool remained fairly stableduring the year (Table 6), suggesting that eitherthe rate of addition and decomposition ofincorporated trash were similar, or that littledecomposition occurred.

The N loss from free trash of 110 g N/ha/day(<3 kg N/ha/month) is a maximum for Nmineralisation from free trash (assuming N loss= N mineralisation). A significant portion ofthis mineralised N would be immobilised bysoil organisms, so that trash would havecontributed very little N to the mineral Nalready in the soil (5-32 kg N/ha in the top 25cm, 3-14 kg N/ha in the top 5 cm, after theeffect of fertiliser had diminished). Thus, Nmineralisation from trash is likely to have beenof little significance for plant growth. Thisagrees with the finding of Ng Kee Kwong(1987) and Chapman et al. (1992) that less than10% of the N in isotopically labelled trash wastaken up by the plant.

Soil N dynamics in sugarcane systems willundoubtedly be affected by release of C fromplant roots through rhizodeposition (exudationof soluble organic compounds, sloughing of oldcells, and death of roots). Rhizodepositioncontributes large quantities of C to the soil (e.g.15-30% of photosynthetically fixed C, or 1000-3000 kg C/ha) in crops such as wheat andmaize, and grasses (Keith et al., 1986; Jensen,

1994; Qian et al., 1997; Kuzyakov et al., 2001),with important consequences for microbialactivity and N transformations (Qian et al.,1997; Kisselle et al., 2001). Large root masseshave been measured in sugarcane (Ball-Coelhoet al., 1992), but almost nothing is known aboutrhizodeposition, and until this knowledge gapis corrected any assessment of the importanceof trash as a C source in sugarcane systems willbe incomplete.

Soil C and N Cycling

The GCTB system caused total soil C and N toincrease in the oldest experiments (Mackay andTully) but not in the youngest experiments(Harwood). The response at Mackay and Tullywould have been due both to input of C and Nas trash and to the protective effect of reducedcultivation on soil organic matter. The lack ofresponse at Harwood may have been becausethe experiments were only 1 or 2 years old,because the burnt treatments were notcultivated, or because the soil at this site hadrelatively large contents of C and N.

Increased total C and N in the surface soil afterseveral years of GCTB suggests that thesesugarcane systems are responding in a similarway to residue retention as has been foundwith crops such as cereals and maize in manyparts of the world (e.g. Franzluebbers et al.,1995; Chan et al., 1992; Ortega et al., 2002).

This increase in total C and N in the Mackayand Tully experiments could not be explainedby the presence of incorporated trash in thesurface soil. Incorporated trash accounted for17-27% of the C and 6-14% of the Naccumulated under GCTB (0-5 cm). Similarly,growth of the microbial biomass could onlyaccount for 4-5% of the extra C in the GCTBsoils. In other words, at least 80% of the trash-derived C and N under GCTB was in anadvanced state of decomposition (<2 mm insize and not visually recognisable as trash).

The soil microbial biomass showed similartemporal variation in burnt and GCTB systems.Large seasonal variations such as the peakaround the middle of the year in the late-harvested experiments have been reported inother cropping systems and may have been


associated with microbial substrate availabilityor grazing by soil fauna (Buchanan and King,1992; Beare, 1997).

As a consequence of the improved Cavailability under GCTB, soil microbial activity(C mineralisation) and, to a lesser extent, soilmicrobial growth (microbial biomass) wereincreased. Most of the trash C was in factrespired by the microbial biomass and lost fromthe system as CO2 during the following year.The stimulation of microbial activity was not,however, accompanied by increased productionof soil mineral N (soil N mineralisation). Thisdid not mean that N was not mineralised fromdecomposing trash: although the Nmineralisation measurement was little differentin GCTB and burnt soils, the ratio of Cmineralisation: N mineralisation showed that Nimmobilisation by the soil microorganisms wasmuch greater in the GCTB soils (i.e. gross Nmineralisation was increased by GCTB, but netN mineralisation was not). This difference inmicrobial immobilisation between the GCTBand burnt systems would have been evengreater under field conditions, where surfacetrash (which was excluded from the laboratoryincubations) would have contributed additionalC to the soil, thereby increasing the ratio inGCTB soils.

The amount of N required for the microbialbiomass to decompose a trash blanket can beroughly calculated (Fig. 24). Say the trashcontains 9 t DM, 4000 kg C/ha and 55 kg N/ha(typical figures from this study). If themicrobial biomass has a C:N ratio of 8 (Allisonand Killham, 1988), and the microbial biomassrespires 65% and assimilates 35% of C usedduring decomposition (Alexander, 1977), thenthe microbial biomass will need 175 kg N.Around 55 kg of this microbial N demandcould be supplied from the trash, leaving 120kg N to be supplied from fertiliser and soilorganic matter. If 180 kg N is applied asfertiliser to a 100 t/ha crop with a Nrequirement of 180 kg N (Calcino, 1994, withallowance made for roots), and if the cropreceives 30% of its N from fertiliser and 70%from soil organic matter (Vallis et al., 1996),then a minimum of 126 kg N must bemineralised from the soil organic matter. Sototal N demand (crop + microorganisms)would be 355 kg N, and total N supply (trash +

fertiliser + soil) would be 361 kg N. Thus, theentire trash blanket could be decomposedwithout compromising the supply of N to thecrop. In reality, the total N demand in GCTBsystems is probably easily satisfied by the Nsupply (the estimated N from soil organicmatter in the above example was a minimum).This agrees with the finding that equally goodcrops can be grown under the burnt and GCTBsystems using the same N fertiliser applicationrates (Chapman, 1990). These calculations do,however, also suggest that very largereductions in N fertiliser application would notbe possible under GCTB.

The results here showing greaterimmobilisation of N under GCTB are consistentwith findings of increased N immobilisationwith residue retention in other crops(Christensen, 1986; Jensen et al., 1997). In somesystems (eg. cereal production in the DarlingDowns of Queensland), residue retention canstimulate immobilisation to the detriment ofthe crop, necessitating larger N fertiliserapplications where residues are conserved(Thompson, 1992). Sugarcane crops do notsuffer N stress because of trash conservation,perhaps because of the relatively large fertiliserinputs, or because the sugar soils have largerreserves of mineral or mineralisable soil N, orbecause the trash blanket is decomposedgradually over a whole year.

The proportion of free trash-C remaining on thesoil surface in the GCTB treatments after oneyear (2-18%) was about the same as theproportion that remains after trash is burnt (2-20%, Mitchell et al., 2000). However, some ofthe C lost from free trash became incorporatedtrash, microbial biomass, and soil organicmatter, thereby effecting the increase in soil C.Also, in contrast to the biologically labilenature of the C returned under GCTB, the Cremaining after a fire may have contained asignificant quantity of charcoal-like materialthat was biologically inert (Skjemstadt et al.,1999). Thus, the apparently similar surfaceretention of trash in the GCTB and burntsystems belied the important contribution ofthe GCTB system to soil organic matter status.A greater proportion of the trash-N wasretained on the soil surface (5-37%) at the endof the year, due to the relative N-enrichment oftrash during decomposition.


The magnitude of the trash effect on soil C andN depended on the cumulative amount of trashreturned to the soil. The regression ofcumulative trash returns against measuredincreases in soil total C and N suggested thatabout 13% of cumulative C inputs and 79% ofcumulative N inputs were retained in the soil(Fig. 15). Most of the remaining C would havebeen lost through microbial respiration, and asmall proportion may have been leached below25 cm. The remaining N could have been lostby denitrification, leaching or crop uptake.However, this apparent retention of trash Cand N was quite variable (10-20% for C and 40-100% for N), with no allowance made forfertiliser N inputs. Similar rates of retention ofresidue C (11-18% of inputs) have beenreported during the first 10 years of residueconservation in cereal cropping systems intemperate regions (Barber, 1979; Sørensen, 1987;Duiker and Lal, 1999). In the semiarid UnitedStates, Rasmussen et al. (1980) reported 18%retention of C and 75% retention of N fromwheat straw returns.

The GCTB effect on soil microbial biomass Cand C mineralisation also increased withincreasing cumulative trash returns. Thissuggests that part of the GCTB effect on theseproperties was due to old trash (>1 year old).

i.e. old trash was biologicallyactive, or else altered the soilenvironment in wayspromoting microbial activity.In most residue conservationsystems with minimumtillage, including GCTB,some of the increase in soil C(relative to burnt andconventionally cultivatedsites) will be due topreservation of soil organicmatter from reducedcultivation. At the Mackaysite, Blair (2000) measured areduction in soil organic Cafter the burnt and GCTBplots were cultivated for 4years. In a wheat croppingsystem Chan et al. (2002)found that cultivation had agreater effect in reducingtotal soil C than stubble

burning, though Dalal et al. (1991) found thatcultivation had little effect.

Long-term Soil C and N Status underGCTB

The rates of accumulation and mineralisation ofC and N under GCTB measured in theseexperiments can only be considered indicativeof the first crop cycle (4-6 years) afterconversion from a burnt to a GCTB system. Atthe end of a crop cycle, sugarcane soils arenormally cultivated several times to 150-250mm depth, the effect of which may be to reducedifferences in soil C and N between burnt andGCTB treatments (e.g. Pankhurst et al., 2002).Since the sampling for this study wascompleted, the Mackay (Early) experiment hasbeen ploughed out and re-planted, and thetrash management effects on soil total C and Nare still evident (Table 10). Rates of C and Naccumulation in GCTB systems must beexpected to decrease with time and reach anequilibrium level, as is known to happen inother crop and pasture systems (Russell, 1980;Jenkinson, 1991). Accumulation rates decreasebecause mineralisation and loss of C and Nfrom the soil (through respiration, leaching,

Figure 24. Representation of N demand and supply under GCTB.See section Soil C and N Cycling for explanation.


denitrification, plant uptake) increase. Theimportance of trash as a N source lies in the oldtrash which has accumulated over the years.

In order to explore a range of possible long-term responses of soil C and N to trashblanketing, measured rates of decompositionand accumulation were combined withassumptions about the mineralisation of C andN from trash left from previous years, andequilibrium C and N balances were calculatedfor the top 25 cm of soil at each site(assumptions were supported by measuredretention of isotopically labelled residues inother cropping systems: Jenkinson, 1965;Jenkinson, 1971; Broadbent and Nakashima,1974; Jenkinson and Ayanaba, 1977). At thestart, the Mackay and Tully soils contained40,000 kg C /ha and 2,400 kg N /ha, and theHarwood soils contained 76,000 kg C /ha and6,500 kg N /ha. Annual trash returns of 10 tDM, 4500 kg C, and 55 kg N/ha were assumed.It was assumed that the systems were alreadyin steady state conditions under the burntsystem (i.e. that total soil C and N remainedconstant through time). Two decompositionscenarios were compared: (1) a ‘retentive’system (slow decomposition and/or loss ofdecomposition products), where for each crop,100% of trash-N and 20% of trash-C is retainedin the soil in the year following harvest, and90% of the remaining trash-N and 90% of theremaining trash-C is retained in subsequentyears and (2) a ‘non-retentive’ system

(relatively rapid decomposition and/or loss ofdecomposition products), where 100% of trash-N and 10% of trash-C from each crop isretained in the soil in the year followingharvest, and 80% of this trash-N and 80% ofthis trash-C is retained in subsequent years.The effects on soil C and N were calculated (noallowance made for fallow effects) until thesystems had reached equilibrium.

It was also assumed that the trash-inducedincrease in soil microbial biomass N was 3% ofthe trash-induced increase in total soil N atequilibrium. Trash-derived N potentiallyavailable for plant uptake was calculated astrash N returned minus trash-derived Nretained in the soil minus trash-derived Nimmobilised by the microbial biomass.

At equilibrium under the ‘retentive’ scheme,total soil C would have increased by 22% atMackay and Tully and by 12% at Harwood.Total soil N would have increased by 23% atMackay and Tully and by 8% at Harwood.Under the ‘non-retentive’ scheme, atequilibrium soil C would have increased by 5%at Mackay and Tully and by 3% at Harwood.Soil N would have increased by 11% at Mackayand Tully and by 4% at Harwood. The‘retentive’ system approached equilibrium after20-35 years, and the ‘non-retentive’ system after7-12 years (Fig. 23 (a) and (b)). Thus, usingthese scenarios the expectations for increasedsoil total C and N under long-term GCTB atthese sites varies from negligible to small.

During the first 2-5 years under GCTB, theamount of N potentially available for plantuptake (if not lost from the system by leachingor denitrification) was negative. i.e.mineralisation of trash-derived N did notcompensate for microbial immobilisation oftrash-derived N. Net mineralisation of trash-derived N gradually increased to 38 kg N/haunder the retentive scenario and to 47 kg N/haunder the non-retentive scenario (Fig. 23 (c)).

When equilibrium is attained, accumulation oftrash N in the soil ceases, because inputs fromnew trash are balanced by mineralisation andloss from old trash. This maximum rate of netN mineralisation also represents the maximumamount by which N fertiliser rates couldpossibly be reduced in GCTB systems (with

Table 10. Soil total C and N in Mackay (Early) beforeand after ploughout-replant (0-25 cm).

Before-ploughout data is from the final sampling in this study (6July 1998). After-plough-out is one year after cultivation and re-planting of all treatments (soil sampled 1 July 1999), data of RossMitchell (unpublished). Asterisk indicates trash managementtreatments are significantly different (P<0.05).

Measurement Treatment

Burnt GCTB

Before plough-out

Organic C (%) 1.09 1.14 (*)

Total N (%) 0.064 0.069 (*)

After plough-out

Organic C (%) 1.01 1.14 (*)

Total N (%) 0.067 0.071 (*)


respect to the recommended N fertiliser rate,based on the burnt system). In the abovescenarios, this point would be achieved after12-35 years, though smaller reductions infertiliser application could be made before thistime. However, it is possible that fertiliserapplications may not be able to be reduced tothis extent if, for example, N loss is greater inGCTB than in burnt systems. It has beensuggested that leaching (Vallis et al., 1996) anddenitrification (Chapman and Haysom, 1991;Weier et al., 1998) may be promoted underGCTB due to the increased C and watercontents.

The decomposition scenarios used above are, ofcourse, a simplification of the real systems. e.g.trash would not decompose as 2 separate pools(new and old trash) with individual retentionrates, but as a continuum of new to very oldtrash with a corresponding continuum ofretention rates. The above scenarios were alsocalculated assuming 3 trash pools (new, 1-year-old, and old trash) with the same retentionrates for new and old trash and an intermediateretention rate for 1-year-old trash. This resultedin slightly greater retention of C and N, butotherwise very similar results (not shown).

The concept of soil retentivity is useful as usedin the above what-if scenarios because itencompasses the effects a wide range of soiland environmental factors which do not needto be explicitly known. Identifying theretentivity of individual soils, however, is moredifficult, as retentivity cannot be measured as asingle quantity. Furthermore, it may changefrom year to year in response to climatic andmanagement factors. The rate of trashdecomposition and the rate of loss ofdecomposition products are what controlretentivity. Retention of trash C and N in thesoil would be promoted by slow trashdecomposition (eg. due to adverse temperatureor water conditions; or a limiting nutrientsupply) and, perhaps more importantly, byslow loss (eg. due to ‘protection’ of C and N insoils of high clay and silt content (Hassink,1996; Saggar et al., 1996); immobilisation in alarge soil microbial biomass; minimumcultivation; and minimal losses from leaching,denitrification, erosion, and runoff).

The measured increases in total soil N due toGCTB suggest that the Tully and Mackay soilswere fairly retentive (130-350 kg N after 3-6years). However, accumulation of total soil N issimilar under the retentive and non-retentivescenarios for about the first 4 years.

It should be noted that the scenarios describedabove are based on the assumption that totalsoil C and N was not changing under the burntsystem. The scenarios describe only the extra(trash derived) N in the GCTB system - thetotal soil N may increase, remain unchanged,or decrease through time depending on non-trash-related factors. In other words, trashretention will not necessarily increase total soilC and N, but it will reduce any decline in Cand N. Such a case was reported after 20 yearsof wheat cropping in Queensland, where totalsoil N declined under both stubble retentionplus minimum cultivation and stubble burningplus conventional cultivation, though thedecline was less in the former (Dalal, 1992).

The factors influencing retention andmineralisation of trash C and N are clearlyinterrelated, and very complex. Whilst theabove scenarios are useful in exploring possibleresponses to trash retention, the systeminteractions are likely to be better understood

Figure 23. Calculated cumulative increase in (a) totalsoil C, (b) total soil N, and (c) inorganic (plant-available) N under long-term GCTB.See text (Discussion, Long-term soil C and N status underGCTB) for more explanation.


with the aid of simulation models such asAPSIM (Keating et al., 1999; Thorburn et al.,2000; Thorburn et al., 2001). Modellinginvestigations supported by this and other fieldstudies are in progress (Peter Thorburn,Elizabeth Meier, and others, CSIRO SustainableEcosystems) and will not be discussed in thisreport.

The effects of crop residue management on soilC and N have been little studied in sugarcaneproduction systems. One exception is a fieldexperiment in South Africa which has beenrunning for 60 years (Graham et al., 2000;Graham et al., 2002), which compared theGCTB and trash burning systems compared inthe present study. After 59 years, the GCTBtreatments showed a 4% increase in organic C,30% increase in total N, and over 100% increasein net N mineralisation and microbial biomassC and N, to 30 cm soil depth, when comparedwith the burnt treatments (Graham et al., 2000;Graham et al., 2002). These increases in total Nand microbial biomass are larger than found inthis study, as could be expected from thedifference in age between the experiments. Theincreased N mineralisation under GCTBdiffered from this study, but agreed with thepredictions from the long-term scenarios, andindicated that the South African GCTB systemwas at or approaching equilibrium. That trashretention will result in a small increase in totalsoil C and N and a somewhat greater increasein mineralisable soil N in the long term, assuggested in this study and measured in theSouth African experiment, also accords withfindings in other cropping systems, e.g.Powlson et al. (1987) measured an increase of5% in organic C, 10% in total N, and over 40%in net N mineralisation and microbial biomassN after 18 years of barley straw incorporationin Denmark. In barley cropping systems inQueensland, Thompson (1992) reportedincreases of 5% in organic C, 11% in total N,and 34% in net N mineralisation after 8 years ofresidue return and zero tillage.

The effects of GCTB on soil N suggested by thisstudy are broadly similar to the conclusionsreached by Vallis et al. (1996) from simulationsof burnt and trash blanketed sugarcane systemsusing the CENTURY model.

Implications for FertiliserManagement under GCTB

One of the hopes held for the GCTB systemwas/is that the return of nutrients in trashwould increase soil nutrient availability to anextent that would allow fertiliser applicationsto be reduced (Chapman et al., 1992; Vallis etal., 1996). This study suggests that, for the samecrop yield, small reductions in N fertiliserapplication will be possible in the medium-longterm, and that the appropriate time scale forreducing N application will vary from site tosite. A suggested ‘best bet’ scheme for Nfertiliser application under GCTB is shown inTable 11.

These recommendations have been developedin conjunction with Peter Thorburn, CSIRO,and are supported by his work using theAPSIM-Sugarcane systems simulation model.

Water Retention

The greater soil water content under GCTBthan burnt management in these experimentsconfirms numerous such observationselsewhere (Wood, 1991; Ball-Coelho et al., 1993;Chapman et al., 2001). The reduction inevaporation from the soil surface due to thetrash blanket was important for the first half ofthe year, until the canopy of the crop closedover. In the Tully experiment, soil water wasgreater under GCTB throughout the year,probably because of the large number of gapsin the crop. The extra water under GCTBwould be of practical significance for cropgrowth in drier regions. In the Mackay

Table 11. Suggested ‘best bet’ scheme for N fertiliserapplication under GCTB.

Time since adoption Reduction from BSES of GCTB recommended N application

for ratoon crops

1-2 crop cycles None(5-10 years)

2-3 crop cycles 10-15%(10-15 years)

≥ 4 crop cycles 20-25%(≥ 20 years)


experiments, Chapman et al. (2001) measured amean increase of 14 t cane/ha in the GCTBplots (relative to the burnt plots), andsuggested that the extra water under GCTBwas equivalent to 2 ML of irrigation.

Soil pH

Long-term application of crop residues(including residues with high C:N ratio) cancause soil acidification, due to accumulation oforganic matter and acid production during itsdecomposition (Williams, 1980). The findingthat soil pH was not affected by trashmanagement in Mackay but significantlyreduced under GCTB in Tully was probablydue to a combination of soil and environmentalfactors. A reason sometimes given for soilacidification is the removal of basic cations inthe harvested crop (Bolan et al., 1991; Noble etal. 1997), but this can not explain the results inthe Tully experiment because crop yields weresimilar in both trash management treatments(Alan Hurney, unpublished data). If morenitrate was leached in the GCTB than in theburnt treatments (the greater water content ofthe GCTB soils makes this possible),acidification may have been greater also (Nobleet al.. 1997). The main reason for soilacidification under GCTB, however, is likely tobe the production of hydrogen ions duringdecomposition of the trash blanket. This is aprocess that would have occurred in Mackayalso, but the soil had a greater pH bufferingcapacity than the Tully soil. Application oflarge quantities of wheat residue has beenreported to result in less acidification in soilssubjected to frequent wet-dry cycles (such asthe Mackay soil) than in continuously moistsoils (such as the Tully soil) (Paul et al., 2001).Thus, the response of soil pH to GCTB mayvary with climate.

The acidification of the soil in the Tullyexperiment is an alert that soil pH should bemonitored under GCTB, and corrective actiontaken if necessary.

Stratification

Most of the effects of GCTB observed in this

study were concentrated in the surface soil: aconsequence of the undisturbed trash blanket.This is a phenomenon reported in othercropping systems where minimum tillage andresidue retention are used (Granatstein et al.,1987; Doran et al., 1998). The effects ofsugarcane trash can be expected to becomemeasurable deeper in the soil as GCTB iscontinued for longer periods, due to organicmatter accumulation and cultivation of the soilbetween crop cycles.

This depth gradient of organic matter, nutrientsand water under GCTB may haveconsequences for crop growth. For example, itmay promote more shallow rooting, as hasbeen observed in other crops in response towater availability (Proffitt et al., 1985). In thepresent experiments, growth of roots in andimmediately below the trash blanket wereclearly visible, whereas no such superficialrooting occurred under burnt management.

The surface nature of the GCTB effect meansthat the choice of sampling depth is importantwhen attempting to measure the effects ofGCTB on soil properties. Sampling the soils asone 0-25 cm depth layer, as has traditionallybeen done in the sugar industry, would havemasked much of the GCTB effect in theseexperiments. Some studies in the Herbert andMackay regions have found no significantincrease in organic C in the 0-25 cm depth aftermany years of GCTB (Andrew Wood, personalcommunication; Les Chapman, personalcommunication). This may have been becauseof the choice of sampling depth, or because thesoil was non-retentive.

Other Effects of GCTB

Other Nutrients

Not all nutrients in trash will follow thepatterns shown in this study for N. Forexample, potassium is very soluble and easilyleached from trash, so its availability to thecrop and persistence in the soil is likely to bevery different from that of N (Christensen,1985; Spain and Hodgen, 1994; Ross Mitchelland Peter Larsen, unpublished data). On theother hand, phosphorus and sulfur in trash arelikely to behave as N does, being similarlydependent on decomposition and C cycling for


release to the soil (Russell and Williams, 1982).

Organic Matter Accumulation

Accumulation of organic matter in the soilunder GCTB will have many consequencesbesides those dealt with in this study. Thefinding of increased microbial growth underGCTB is consistent with the increase inearthworms and other soil fauna measured byother researchers (Wood, 1991; Robertson et al.1994), and demonstrates improved biologicalconditions in the soil (Weigand et al. 1995). Thisin turn may eventually lead to a more resilientsoil system, less susceptible to outbreaks ofpests and diseases and other stresses (Swift,1994; Degens et al., 2001). Long-term sugarcanemonoculture under a burning system can resultin soil degradation by organic matter depletion(Holt and Mayer, 1998). This study suggeststhat adoption of the GCTB system may help toalleviate the problem.

Soil faunal (particularly earthworm) activityhas been shown to increase residuedecomposition and N mineralisation inresidues from other crops (Tian et al., 1995;Brown et al., 1998; Whalen et al., 1999), and itseems reasonable to speculate that the rate ofdecomposition and N release from trash mayincrease as faunal populations increase undermedium- or long-term trash blanketing.Similarly, repeated return of trash to the soilmay result in more rapid trash decompositiondue to acclimatisation of the soil microbialbiomass, a phenomenon which has beendemonstrated in cereal cropping (Killham et al.,1988).

The GCTB system can be difficult to manage insome situations e.g. with very large crops,problems may be encountered duringharvesting (Norris and Davies, 2001); in furrowirrigated systems, parts of the crop may bewatered inadequately because of slow watermovement down the drills; and in coolerclimates such as northern New South Wales,the productivity of early-harvested crop maybe reduced (Kingston and Norris, 2000;Graham Kingston, unpublished data). Untilthese obstacles are overcome, removal of trashfrom canefields will be the preferred option forsome growers.

There is currently a lot of interest in reducing

global CO2 emissions in order to reduce theseverity of the Greenhouse Effect, including bypromoting storage of C in the soil inagricultural production systems (Csequestration) (Lal, 1997). This study suggests asmall positive effect in this regard fromadopting GCTB.


Under the GCTB system, a large quantity oftrash of high C:N ratio was returned to thefield, most of which was decomposed duringone year. The rate of decomposition wasinfluenced by temperature, rainfall, and otherfactors not identified here. Most of the trash Cwas lost as CO2. Nitrogen release from thecurrent year’s trash was variable andinsignificant until about 40% of trash DM haddisappeared. During the rest of the year, N wasreleased at a slow rate, though its contributionto the soil mineral N pool was likely to remainunimportant. Soil organic C and total N in thesurface soil was increased by trash blanketingrather than burning, in the older experiments.As a consequence of the improved Cavailability under GCTB, soil microbial growthand activity were stimulated. Netmineralisation of N was not increased underGCTB due to greatly increased microbialimmobilisation of N. The entire trash blanketcould be decomposed without compromisingthe supply of N to the crop. Calculations of

possible long-term effects of converting from aburnt to GCTB production system suggestedthat, at the sites studied, soil C could increaseby 2-18% and soil N could increase by 4-23%,depending on soil and climatic factors, and thatit could take 10-35 years for the soils toapproach this new equilibrium. Inorganic soilN would be expected to increase undermedium-long term GCTB, due tomineralisation of N from trash-derived organicmatter, to an amount approaching the annual Nreturn in trash.

Small reductions in N fertiliser application willprobably be possible in the medium-long termafter adoption of GCTB, and the appropriatetime scale for reducing N application will varyfrom site to site. Conversion from a burning toa GCTB trash management system is likely toimprove the soil organic matter and N status ofsugarcane soils.

Conclusions


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