Soil & Tillage Research 120 (2012) 99–111
Changes in soil carbon sequestration, fractionation and soil fertilityin response to sugarcane residue retention are site-specific
P.J. Thorburn a,*, E.A. Meier a,b, K. Collins a, F.A. Robertson c,1
a CSIRO Ecosystem Sciences and Sustainable Agriculture Flagship, GPO Box 2583, Brisbane, Queensland 4001, Australiab Meier IT Pty Ltd, PO Box 7153, Mt Crosby, Queensland 4306, Australiac BSES Ltd, 50 Meiers Road, Indooroopilly, Queensland 4068, Australia
A R T I C L E I N F O
Article history:
Received 7 February 2011
Received in revised form 31 October 2011
Accepted 25 November 2011
Available online 22 December 2011
Keywords:
Carbon fractions
Green cane management
Nitrogen mineralisation
Particulate organic matter
Soil organic matter
Crop residue
A B S T R A C T
Sugarcane crop residues contain substantial quantities of C and plant nutrients, but there have been
relatively few studies of how sugarcane residues enrich the soil and contribute to C sequestration, and
most studies have been undertaken at only one or a few sites. The purpose of this study was to address
these knowledge gaps by determining the magnitude and time scale of changes in soil concentrations of
total C, C fractions and plant nutrients following retention of sugarcane residues. C fractions were
determined by two different methods. We sampled soils from five experiments, in contrasting
environments, where sugarcane residues had been either retained or removed for between 1 and 17
years. Changes in the concentration of both soil C and plant nutrients were highly site-specific and not in
proportion to the period that residues were retained: for example, soil C (0–250 mm) decreased by
0.9 g kg�1 and 0.5 g kg�1 at sites where residues had been retained for 1 and 17 years, respectively, but
increased by 2.0 g kg�1 at a site with residues retained for 6 years. Soil C composition, defined by the
KMnO4 oxidation and particulate organic C-ultraviolet photo-oxidation fractionation (POC-UV)
schemes, appeared to be a more sensitive indicator of changes in residue management, indicating
that increases in readily-oxidisable C and particulate organic C, respectively, after 1 year of retaining
instead of burning residues. The two methods provided different information that was complementary
in understanding changes in soil C. The KMnO4 method identified downward movement of C fractions in
the profile to 250 mm, while the labile fractions measured by the POC-UV scheme appeared to be more
sensitive to early changes in residue management (after 1 year). While recent studies have found that
several concentrations of KMnO4 reduced all C fractions by a similar magnitude and thus concentrated
on the fraction oxidised by the 333 mM concentration of KMnO4, we found that use of both this and the
33 mM concentration enabled a greater understanding of changes in C pools due to residue
management.
Crown Copyright � 2011 Published by Elsevier B.V. All rights reserved.
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Soil & Tillage Research
jou r nal h o mep age: w ww.els evier . co m/lo c ate /s t i l l
1. Introduction
Sugarcane is an important tropical crop, and interest in the cropis growing because of its role in biofuel production (Hartemink,2008; Martinelli and Filoso, 2008). In many locations wheresugarcane is produced, the practice of burning residues at eachharvest is being replaced by retaining the residues on the soilsurface. Mature sugarcane crops include large quantities ofresidues (called trash) at harvest (13–20 t DM ha�1), which containuseful quantities of plant nutrients (0.46–0.54% N; 0.47–0.66% K;0.09–0.17% Mg; 0.18–0.41% Ca; 0.06–0.17% S; 0.05–0.09% P) and
* Corresponding author. Tel.: +61 7 3833 571; fax: +61 7 3833 5505.
E-mail address: [email protected] (P.J. Thorburn).1 Current address: Department of Primary Industries, Private Bag 105, Hamilton,
Victoria 3300, Australia.
0167-1987/$ – see front matter . Crown Copyright � 2011 Published by Elsevier B.V. A
doi:10.1016/j.still.2011.11.009
large quantities of C (�42%) (de Oliveira et al., 2002; Robertson andThorburn, 2007a). Retaining instead of burning residues providesseveral potential benefits (Hartemink, 2008; Rayment, 2003),including reducing atmospheric pollution, sequestering C, improv-ing various soil properties, and reducing fertiliser requirementsthrough recycling nutrients in the residues. While the atmosphericpollution and greenhouse gas benefits of not burning residues areclear and immediate, the extent of enrichment of sugarcane soilswith C and nutrients when residues are retained, and the time scaleover which this occurs, has been difficult to establish.
Similar to other cropping systems (Baker et al., 2007; Yang et al.,2008; Dalal et al., 2011), the practice of retaining rather thanburning sugarcane crop residues may increase total soil organic C,for example by 2 g kg�1 (0–200 mm) after 6 years (Robertson andThorburn, 2007b), by 5 g kg�1 (0–200 mm) after 8 years (Galdoset al., 2009), or by 9.2 g kg�1 after 55 years (Canellas et al., 2010).However, the amount of C sequestered is affected by climate, soil
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P.J. Thorburn et al. / Soil & Tillage Research 120 (2012) 99–111100
texture, N fertiliser management, and the period of time thatresidues have been retained on the soil surface (Thorburn et al.,2002; Robertson and Thorburn, 2007b; Galdos et al., 2009). Forexample, soil organic C concentrations were not significantlydifferent between sites where sugarcane residues had been burntor retained at Pernambuco State, Brazil after 1 year, or Tully or Ayr,Australia after 7–10 years (0–250 mm; Blair et al., 1998), or at a sitein South Africa after 60 years (0–300 mm; Graham et al., 2002b),possibly because of cultivation and resultant differences in bulkdensity between treatments. In addition, small changes in total soilorganic matter or C may be difficult to detect because of generallyhigh background levels and natural soil variability (Blair et al.,1995). For this reason many attempts have been made to use sub-pools, or fractions, of soil organic matter or C as more sensitiveindicators of the impact of crop residue retention on soil C (Galdoset al., 2009; Graham and Haynes, 2006; Skjemstad et al., 1999).
Total soil organic C can be fractionated by various methods,including physical (e.g. Gaunt et al., 2001; Skjemstad et al., 1993,1999) and chemical methods (e.g. Gachengo et al., 2004), or bysusceptibility to oxidation by chemical (e.g. Blair et al., 1995;Lefroy et al., 1993; Loginow et al., 1987) and ultraviolet radiation(e.g. Skjemstad et al., 1993, 1999) treatments. While different Cfractionation methods may be used, they generally have the aim ofdividing soil C into pools with different turnover rates in order toidentify the effect of different practices. The results can also beuseful for parameterising soil C models (e.g. Skjemstad et al., 2004;Huth et al., 2010). The different pools thus identified range fromthe readily oxidised or active ‘labile’ pools, to those requiringstronger treatments to characterise the relatively resistant andpassive ‘inert’ pools. Other pools with intermediate turnover ratesand other properties may be identified between the active andresistant ends of the spectrum. The labile pools can contribute tosoil fertility through nutrient cycling when net mineralisationoccurs, while more stable C pools may be associated with longerterm C sequestration (Sobocka et al., 2007). In sugarcane, labile Chas increased after 1 year of retaining residues (e.g. Blair et al.,1998), while changes in total soil C have been measurable onlyafter a longer period (e.g. Meier et al., 2006; Robertson andThorburn, 2007b; Blair, 2000).
The enrichment of soil by nutrients from sugarcane residues isalso variable. K is most rapidly available as it occurs in plant tissuesin ionic form or in weak complexes which are readily leached inhigh proportions from plant residues (de Oliveira et al., 2002;Marschner, 1995). Ca, Mg, S and P are more slowly released fromsugarcane residues, by 40–80% after a year of decomposition (deOliveira et al., 2002). Most plant N, P and S are assimilated intoplant proteins, nucleic acids and other compounds or theirprecursors (Marschner, 1995), so mineralisation of these nutrientsfrom sugarcane residues occurs when the microbial productsformed during the initial decomposition are subsequentlydecomposed. Despite the quantity of nutrients returned to thesoil when sugarcane residues are retained instead of burnt, therehas been limited investigation of the contribution of these residuesto the concentrations of these nutrients in the soil. Total N,mineralisable N, organic P, K and Ca increased in the surface 50 mmof soil after 60 years of retaining residues at a site in South Africa(Graham et al., 2002a); Mg increased after 6 years at Tully,Australia, but Ca and K were not significantly different from burntcane soils (Noble et al., 2003); and retaining sugarcane residuesfrom four crops had no effect on soil concentrations of K, Mg or Cain the Herbert Valley, Australia (Wood, 1986).
Existing research into the effect of sugarcane residues on soilnutrients and C has often focused on plant uptake rather thanchanges in the soil, and has not identified consistent trends in soilproperties with the period that residues were applied. This studyaims to address these knowledge gaps by determining the
magnitude and time scale of changes in C sequestration and soilfertility when sugarcane residues are retained instead of burnt.This was achieved by measuring soil fertility parameters and Cfractions (using two techniques) in experiments comparing sitesacross sugarcane producing regions in Australia where residueshad been retained or burnt.
2. Methodology
2.1. Sites
Soil samples were taken from five pre-existing experimentslocated across a wide range of environments in the Australiansugarcane growing regions, from the North Queensland wet tropicsthrough to sub-tropical northern New South Wales (Table 1). Therewere two treatments: sugarcane residues were either (1) retainedon the soil surface at harvest and the plots managed under zerotillage, or (2) removed at each harvest (usually by burning) and theplots then tilled (except at the Woodford Island site where neithertreatment was tilled). The treatments commenced when sugar-cane residues were first applied to the soil at the harvest of theplant crop, and were imposed for times ranging from 1 (atWoodford Island) to 17 (Abergowrie) years. The experiments alsodiffered in soil type and texture, soil C and nutrient concentrations,site history, average rainfall and irrigation management (Table 1,Appendix A). The Ayr, Mackay, and Tully sites were replicatedexperiments located on Bureau of Sugar Experiment StationLimited research stations. At the time of sampling both the Mackayand Tully sites were in their first crop cycle, while the Ayr site wasin its second. The other two sites were located on operationalfarms. The Woodford Island site was a replicated experimentwhere residues been retained for only one season (the first ratooncrop) at the time of sampling. The Abergowrie site was in its thirdcrop cycle when sampled, and to our knowledge was the oldest sitein Australia comparing the two methods of sugarcane residuemanagement. The experimental design at Abergowrie was notrandomised like the other four sites, but consisted of adjacent‘strips’ (>300 m long and 10 rows, �16 m, wide) of sugarcane inwhich residues were either retained or burnt. In 1997, residuesceased to be burnt on the previously burnt strip. For thecontinuation of this experiment, residues were raked by handfrom four quadrats (�25 m2), at 10, 100, 200, and 300 m along thecane rows, soon after harvest to provide a continuation of the burnttreatment. Removal of residues from the quadrats by raking wouldhave removed more residues from the plots than burning (�25% ofresidues remain after burning: Mitchell et al., 2000), but burningtrash remained the most important effect for the treatment atAbergowrie due to the long period of that management. Soilsamples were taken from the treatment where residues wereretained at the same distances along the strip to give a four-pairedcomparison for statistical analysis.
2.2. Soil sampling
Soil samples were taken with 50 mm diameter steel ‘pushtubes’ from the two residue treatments, immediately after harvest.For each plot a total of six cores were taken, three from the row andthree from the inter-row, to a depth of 1500 mm. A further 12 coresto a depth of 250 mm were taken to provide additional soil foranalyses done on the shallower depths. Samples from each corewere divided into depth increments (0–20 mm, 20–50 mm, 50–100 mm, 100–250 mm, and then in 250 mm increments to1500 mm), and bulked by depth increment for each replicate.They were stored in plastic bags under refrigeration until analysis,which started within seven days of sampling. Bulk density wasdetermined by dividing the gross sample weight adjusted for soil
Table 1Details of experimental sites. Sites are presented in order of increasing duration of residue retention.
Item Experimental sites
Woodford Island Mackay Tully Ayr Abergowrie
Location 153.132 E; 29.487 S 149.119 E; 21.164 S 145.923 E; 17.934 S 147.405 E; 19.576 S 145.864 E; 18.476 S
Reference Kingston et al. (1998)
(Harwood site)
Chapman et al. (2001)
(Sugar Experiment
Station trial)
Robertson and
Thorburn, 2007b
McMahon and Ham
(1996) (Sugar Experiment
Station trial)
Wood, 1986 (Trial 1)
Average rainfall (mm year�1) 1020 1670 3480 1080 1700
Irrigation (mm year�1) None <100 None >1500 None
Year planted 1995 1992 1990 1988 1980
Year treatments started 1996 1993 1991 1989 1981
Date sampled 1/98 8/98 11/97 11/98 10/98
Years of trash addition 1 5 6 9 17
Crop cyclea at time of sampling 1 1 1 2 3
Estimated cumulative
residue (t dry matter)
8 40 60 112 120
Replicates 4 4 3 4 4
Plot size approx. (m2) 100 100 100 600 100
Average soil properties
Soil texture (0–300 mm) Silty clay Loam/clay loam Silty clay Sandy loam Loam
Soil classification
(Isbell 1998)
Not available Brown chromosol Brown dermosol Not available Red dermosol
Moisture at �33 kPa
(m m�3, 50–100 mm)
0.34 0.21 0.31 0.14 0.19
Moisture at �1.5 MPa
(m m�3, 50–100 mm)
0.17 0.08 0.19 0.05 0.08
Selected soil propertiesb for the control (burnt) sites, averaged over 0–250 mmc
C (g kg�1) 24.9 11.2 10.7 10.2 10.3
CL1 (g kg�1) 1.33 0.70 0.63 0.88 0.55
CL (g kg�1) 2.90 1.56 2.08 2.30 0.85
POC (g kg�1) 2.19 1.94 0.13 0.44 0.83
Humic C (g kg�1) 11.38 4.29 3.17 4.77 3.37
N (g kg�1) 2.0 0.7 1.0 0.7 0.7
pH 5.2 4.9 5.5 6.2 4.5
CEC (cmolc kg�1) 12.8 4.7 4.5 10.0 4.0
EC (mS cm�1) 92.6 41.7 24.5 36.3 55.1
Na (cmolc kg�1) 0.30 0.10 0.07 0.22 0.04
a A crop cycle is defined as the period from planting to destruction of the crop. Thus a crop cycle will consist of a plant crop followed by a number of ratoon crops.b Defined in Table 2.c Variations in properties in the depth increments within this range are given in Appendix A.
P.J. Thorburn et al. / Soil & Tillage Research 120 (2012) 99–111 101
water, by the volume of the auger tube. The bulk density for eachincrement in soil depth was averaged over all replicates withineach site, and the averaged values were used in all calculationsinvolving bulk density.
2.3. C analysis
Soil C fractions were determined using two different methods,the KMnO4 oxidation technique and the particulate organiccarbon-ultraviolet photo-oxidation technique, detailed below.Total soil C (CT) was determined by dry combustion using a massspectrometer. All C results were determined on oven dry soils andexpressed as concentrations (e.g. g C kg�1; Table 1).
2.3.1. KMnO4 oxidation method
The KMnO4 oxidation method was based on that described byLoginow et al. (1987), Lefroy et al. (1993), and Blair et al. (1995). Inbrief, the soil was reacted with an excess of KMnO4 and thereduced permanganate, which is proportional to the amount of Coxidised, was measured by spectrophotometry (Blair et al., 1998).Two KMnO4 concentrations (33 mM and 333 mM) were usedbecause lower concentrations are sufficient to oxidise the morelabile C (Blair, 2000). The amount of oxidisable soil C wasdetermined at the two concentrations of KMnO4 on five soil depths(0–20 mm, 20–50 mm, 50–100 mm, 100–250 mm, 500–750 mm),for each replicate in both treatments.
Using this approach, CT was divided into four different fractionsbased on susceptibility to oxidisation (Table 2). Two ‘labile’ C
fractions were established through KMnO4 oxidation: (1) theamount of C oxidisable by 333 mM KMnO4 (CL), and (2) the amountoxidisable by 33 mM KMnO4 (CL1). The CL fraction has previouslybeen referred to as ‘C3’ (Moody et al., 1997) or labile C (CL; Blairet al., 1995, 1998; Conteh et al., 1997, 1998, 1999), while CL1 haspreviously been referred to as ‘C1’ (Bell et al., 1999; Moody et al.,1997). The difference between CL and CL1 defines a third C fraction,CL2 = CL � CL1. Thus the CL fraction is essentially split into twoparts, with CL1 representing the most readily labile C and CL2 theremaining labile C. The amount of soil C that remained unoxidisedby KMnO4 represents a final, non-labile fraction: CNL = CT � CL
(Blair et al., 1995, 1998; Conteh et al., 1997, 1998). The division of Cinto these fractions enabled CT to be considered as the sum of threefractions (summarised in Table 2), CT = CL1 + CL2 + CNL, with adecreasing tendency to oxidation of the C contained in eachfraction.
An additional index was calculated to indicate the potential forC in the system to mineralise (Blair et al., 1995). This is referred toas the ‘lability of C’ (L), and expresses the amount of labile C (CL)relative to the amount of non-labile C (CNL) present in a given soil(Conteh et al., 1997), L = CL/CNL.
2.3.2. Particulate organic carbon-ultraviolet photo-oxidation method
Soil C fractions were also determined by the particulate organiccarbon-ultraviolet photo-oxidation (POC-UV) techniques (Skjem-stad et al., 1993, 1999; Table 2) on samples from four depths (0–20 mm, 20–50 mm, 50–100 mm, 100–250 mm). To obtain suffi-cient sample mass, soil samples were bulked across replicates.
Table 2Summary of C fractions definitions for the KMnO4 oxidation and particulate organic carbon-ultraviolet photo-oxidation techniques.
Fraction Description Derivation Replicates Maximum soil
sampling depth
(mm)
Number
of samples
KMnO4 oxidation technique
CT Total soil C content Dry combustion Individual reps
analysed
1500 9
CL1 Amount of C oxidised by 33 mM KMnO4 Oxidation with 33 mM KMnO4 750 5
CL Amount of C oxidised by 333 mM KMnO4 Oxidation with 333 mM KMnO4 750 5
CL2 The additional C oxidised when the KMnO4
concentration increased from 33 to 333 mM
=CL� CL1 750 5
CNL Non-labile C content; the unoxidised soil C component =CT� CL2� CL1
(i.e. CT� CL)
750 5
Particulate organic carbon-ultraviolet photo-oxidation technique
POC Particulate organic carbon; Amount of C
in particles greater than 53 mm
=CT� <53 mm Reps bulked and
resultant sub-sample
analysed
250 4
<53 mm Amount of C in particles less than 53 mm Oxidation of OM <53 mm 250 4
Inert C Amount of C present after UV photo-oxidation
of the <53 mm fraction; a sub-fraction
of <53 mm fraction
Oxidation of OM <53 mm 250 4
Humic C The UV oxidisable C component of the <53 mm
fraction; a sub-fraction of <53 mm fraction
=<53 mm � Inert C 250 4
P.J. Thorburn et al. / Soil & Tillage Research 120 (2012) 99–111102
The samples were first physically fractionated into two fractionsusing a 53 mm sieve. The ‘<53 mm’ fraction (Skjemstad et al., 1999,2001) represented the C content of the material that passed throughthe 53 mm sieve while in suspension. Organic material retained onthe sieve was particulate organic carbon (defined by Skjemstad et al.,2001), and was calculated as the difference between CT and the<53 mm fraction: POC = CT � <53 mm.
Material that was <53 mm was further fractionated using highenergy ultraviolet (UV) photo-oxidation, resulting in two more C
Table 3Linear correlations (r2 values with P-values shown in parentheses) between changes (reta
in Table 2 concentrations derived through the KMnO4 oxidation and particulate organic
other variables that may be associated with changes in the fractions. The change in C fract
indicate those attributes that correlate at P < 0.05 and P < 0.10, respectively.
Independent Dependent variable
CT CL1 CL
CL1 0.26
(0.21)
CL 0.25
(0.23)
0.90(0.00)
CL2 0.20
(0.35)
0.65(0.00)
0.92(0.0
CNL �0.25
(0.23)
�0.90(0.00)
�1.0(0.0
POC 0.45(0.03)
0.40(0.05)
0.27
(0.2
<53 mm �0.45(0.03)
�0.40(0.05)
�0.2
(0.2
Humic C �0.11
(0.62)
�0.38
(0.07)
�0.1
(0.4
Inert C �0.46(0.02)
0.01
(0.98)
�0.0
(0.6
Period of residue retention (years) 0.00
(0.93)
0.00
(0.91)
0.43
(0.2
Estimated cumulative residues retained (t) 0.01
(0.85)
0.23
(0.41)
0.75
(0.0
Average clay content (%) 0.07
(0.67)
�0.13
(0.55)
�0.3
(0.3
Change in pH �0.11
(0.59)
0.00
(0.97)
�0.4
(0.2
Change in CEC (cmolc kg�1) 0.46
(0.21)
0.00
(0.99)
0.14
(0.5
Change in EC (mS cm�1) 0.34
(0.30)
0.34
(0.30)
0.96(0.0
Estimated cumulative degree days (days) 0.00
(0.95)
0.01
(0.90)
0.44
(0.2
fractions. The C that remained after this further oxidationrepresented the C resistant to UV oxidation in the soil, and waslabelled ‘inert C’ (Baldock and Skjemstad, 1999). The differencebetween the UV-oxidisable component of the CT that was <53 mmand the inert C was considered ‘humic C’ (Skjemstad et al., 2004)and calculated as humic C = <53 mm � inert C.
As with the KMnO4 oxidation technique, the final result offractionation by the POC-UV fractionation scheme was that CT wasdivided into three fractions (Table 2), as CT = POC + humic C + inert C.
ined residue treatment less removed residue treatment values) in C fraction (defined
carbon-ultraviolet photo-oxidation techniques, with changes in the fractions and
ions has been calculated for the 0–250 mm soil layer. Values in bold and bold + italics
CL2 CNL POC <53 mm Humic C
0)
00)
�0.92(0.00)
0)
0.10
(0.63)
�0.27
(0.20)
7
0)
�0.10
(0.63)
0.27
(0.20)
�1.00(0.00)
8
1)
0.03
(0.87)
0.18
(0.41)
�0.76(0.00)
0.76(0.00)
9
8)
�0.16
(0.45)
0.09
(0.68)
�0.25
(0.23)
0.25
(0.23)
�0.43(0.04)
3)
0.78(0.05)
�0.12
(0.57)
0.03
(0.77)
0.21
(0.44)
�0.52
(0.17)
6)
0.63
(0.11)
�0.05
(0.71)
0.00
(0.94)
0.32
(0.32)
�0.25
(0.39)
1
3)
�0.21
(0.44)
0.30
(0.34)
�0.17
(0.49)
0.00
(0.95)
0.56
(0.14)
4
3)
�0.95(0.00)
�0.01
(0.87)
�0.01
(0.90)
�0.50
(0.18)
0.13
(0.54)
3)
0.29
(0.35)
0.42
(0.24)
0.50
(0.18)
0.01
(0.90)
0.00
(0.95)
0)
0.74
(0.06)
0.08
(0.65)
0.04
(0.74)
0.42
(0.23)
�0.03
(0.80)
2)
0.79(0.04)
�0.11
(0.58)
0.03
(0.79)
0.23
(0.41)
�0.50
(0.18)
P.J. Thorburn et al. / Soil & Tillage Research 120 (2012) 99–111 103
2.4. Soil fertility
Total nitrogen (NT) was determined on soil samples from alldepths to 1500 mm by dry combustion with a mass spectrometer.
Potential N mineralisation and microbial biomass was mea-sured to a depth of 250 mm (0�20, 20�50, 50�100, and100�250 mm) at the two oldest sites, Abergowrie and Ayr.Aerobic and anaerobic mineralisation was determined by measur-ing the net amount of NO3–N and NH4–N produced during 7-dayincubations under moist and constant temperature conditions, innear field capacity and water logged circumstances, respectively(Bremner, 1965). Microbial biomass was measured by placing 15 gof soil in an evacuated desiccation chamber with chloroform for24 h, after which the chloroform was removed and the sampleunderwent mineral N extraction (based on Brookes et al., 1985,except that soils were incubated at field moisture (0.23–0.24m3 m�3) and extracted with 2 M KCl). NO3–N and NH4–N in the soilcores (to 1500 mm depth), in the two potential N mineralisationmethods, and in the microbial biomass, were determinedcolorimetrically (Henzell et al., 1968) after extraction from thesoil with 2 M KCl (Catchpoole and Weier, 1980).
A range of other soil fertility attributes were determined at allsites. Exchangeable Ca, Mg, Na and K were determined using thetechnique of Haysom (1982). Soil pH and electrical conductivity ina 1:5 soil/water suspension, calcium phosphate-extractable S, BSESacid-extractable P (0.005 M H2SO4), and cation exchange capacityby 0.01 M silver–thiourea were analysed using the proceduresdescribed by Rayment and Higginson (1992). All analyses wereconducted on the depth increments of 0–20, 20–50, 50–100, and100–250 mm, for both treatments and all replicates.
All measurements were made on oven dry soil, except formeasurements of mineral N which were determined on field moistsoils. Measurements in field moist soil were adjusted for soil watercontent, and all results were expressed as concentrations in ovendry soil.
2.5. Statistical analysis
Statistical analysis sought to address the question of whetherretaining or removing residues had an effect on soil chemicalproperties. As samples were taken both from sites where residueshad been retained for varying time periods of time, and for the fullrange of depths to 1500 mm, it was possible to also consider ifthere were trends associated with increased time under amanagement practice or with depth.
The soil chemical data for the Ayr, Mackay, Tully and WoodfordIsland sites were analysed by a split-plot analysis of variance,where the main treatment was location and the split-plottreatments were burnt and retained residue management. Theleast significant difference method for comparison of means(P < 0.05) was also used with a ‘treatment � depth’ interaction.The main treatment in the split-plot analysis was location, and thestatistical analysis was not conducted on the S content of theWoodford Island soil, or of the pH and EC of the Mackay soil, due toa high number of missing replicates. Data from the Abergowrie sitewere analysed using a paired T-test (P < 0.05) because the site wasnot replicated.
C fractions identified by the POC-UV method were not able to beanalysed statistically on a site-by-site basis, because soil replicatesfor this method were bulked prior to analysis.
A correlation analysis was conducted on the change (retainedresidue treatment less removed residue treatment values) inreplicate means of the C fractions from both fractionationtechniques to assess the degree of linear association (P < 0.05)between the variables. Woodford Island data were excluded from acorrelation analysis of the changes in C fractions determined by the
two fractionation methods because of the short time over whichthe treatments had been imposed at the site.
All statistical analyses were determined using the analyticalsoftware package Statistix 7.
3. Results
3.1. Total C and N
3.1.1. Total C
The concentration of CT in the soil generally increased whenresidues were retained, but the increase was usually confined tothe upper 20–50 mm of soil (Fig. 1a). There were significantlyhigher concentrations of CT at 0–20 mm depth for the Mackay,Tully and Ayr sites when residues were retained. This trend wasobserved at the Abergowrie site, but at lower significance (P < 0.1)than at the other three sites. One possible explanation for thisresult may be the different trial design and hence the differentsampling procedure at Abergowrie, which may not fully accountfor the spatial variability of the soil. There were no significantdifferences between treatments at any depth at the WoodfordIsland site, although the concentration of CT was slightly higher(0.04%) in the surface soil (0–20 mm) when residues were retained.These differences at the Woodford Island site were small relative toCT. There was no obvious trend for increasing concentrations of CT
in the surface soil (0–20 mm) as the period of residue retentionincreased. The increase in concentration of CT at the oldest site,Abergowrie (17 years), was less than that which occurred at Ayr (9years), Tully (6 years) or Mackay (5 years).
For deeper soil layers, differences between treatments weresignificant only at the Tully and Abergowrie sites. At Tully, therewere significantly higher concentrations of CT in the 20–50 mmand 100–250 mm depths when residues were retained. However,at Abergowrie, the concentration of CT was significantly lower inthe 50–100 mm soil depth when residues were retained. There wasalso a trend to lower concentrations of CT in the 20–250 mm soillayer when residues were retained at the Woodford Island site.
Although there was a general increase in the concentration of CT
in surface soil layers when residues were retained, the total mass ofC to a depth of either 250 mm or 1250 mm was not significantlydifferent between the treatments within each site (results notshown).
3.1.2. Nitrogen
There were generally significant increases in the concentrationof NT at all sites when residues were retained (Fig. 1b), except atWoodford Island where differences were small and inconsistent.The trend to increased concentrations of NT was more pronouncedthan for CT (Fig. 1a), for which results were generally not significantbelow 20 mm soil depth. As for CT, there was no effect of the periodof residue retention on changes in NT.
There were no consistent differences between treatments inNO3–N, NH4–N, potential mineralisation, or microbial biomass(results not shown).
3.2. C fractions measured by the KMnO4 oxidation method
3.2.1. Fraction CL
There was a greater concentration of CL in soils where residueswere retained, especially in the upper soil layers, except at theWoodford Island site (Fig. 2a). Where increases occurred, theywere significant at Abergowrie, Ayr and Mackay. CL results at Tullyfollowed this trend, but with high between-replicate variability sothat differences were not significant. The higher concentration ofCL in soils that had residues retained indicated that, as would beexpected, retaining residues tended to add more labile C into the
P.J. Thorburn et al. / Soil & Tillage Research 120 (2012) 99–111104
system than when residues were removed. The exception observedat the Woodford Island site may be due to the short time overwhich the treatments had been imposed at the site.
When CL was considered as a proportion of CT (Fig. 3), theresults were similar to those of the concentration results (Fig. 2a).However, unlike the concentration results, there was no differencebetween treatments at the Mackay site for the 0–20 mm depth(Fig. 3). Additionally, the depth at which differences weresignificant at Abergowrie changed, occurring in the 20–50 mmand 100–250 mm depths only.
3.2.2. Fraction CL1
The C oxidised by the lower KMnO4 concentration, CL1,generally increased when residues were retained, as for CL
(Fig. 2b). The proportion of CT as CL1 was higher in at least thesurface 20 mm of soil at all sites when residues were retained, withthe treatment differences being significant in the 0–20 mm soil
Fig. 2. The differences (retained residue treatment less removed residue treatment
values) in C fractions determined by the KMnO4 oxidation technique: (a) CL, (b) CL1,
(c) CL2, and (d) CNL. The difference (retained residue treatment less removed residue
treatment values) in the lability index, L, is presented in part (e). Sites are presented
in order of increasing periods of residue retention. Significant differences are
indicated by *P < 0.05.
Fig. 1. Difference (retained residue treatment less removed residue treatment
values) in (a) CT and (b) NT, measured for the 0–20 mm, 20–50 mm, 50–100 mm and
100–250 mm soil layers. Experimental sites are presented in the order of increasing
duration of residue retention. Significant differences are indicated by *P < 0.05 and#P < 0.1.
Fig. 3. Total C content for the treatments with residue retained (ret) or removed
(rem) at the five sites, divided into the CL1, CL2 and CNL fractions measured by the
KMnO4 oxidation method. Sites are ordered in duration of residue retention
(Woodford shortest, Abergowrie longest). Symbols , , , indicate significant
differences at P < 0.05 for the CL1, CL2, and CNL fractions, respectively.
P.J. Thorburn et al. / Soil & Tillage Research 120 (2012) 99–111 105
layer at Abergowrie and Mackay, the 0–50 mm soil layers at Tully,and the 0–100 mm layers at Ayr. CL1 was also higher in the 0–20 mm soil layer at Woodford Island when residues were retained,but the difference was not significant.
The proportion of CT that occurred as CL1 also tended to besignificantly greater in the 0–20 mm soil layer when residues wereretained (Fig. 3). These differences were significantly greater in allsoil layers at Ayr when residues were retained, which may be dueto soil type and management differences (such as irrigation).
3.2.3. Fraction CL2
Residue management had little significant effect on theconcentration of CL2 in soils except at Woodford Island andAbergowrie (Fig. 2c). At the Abergowrie site, there were significantlyhigher concentrations of CL2 in the 0–20, 20–50 and 100–250 mmsoil layers when residues were retained rather than removed. Therewas a trend for greater concentrations of CL2 in surface soils at Ayr,Mackay and Tully sites when residues were retained, although thiswas not significant. A contrasting result occurred at the WoodfordIsland site, where there was a significantly greater concentration ofCL2 when residues were removed (as for CL).
Similar differences between treatments occurred when the CL2
fraction was considered as a proportion of CT (Fig. 3). CL2 wassignificantly greater in all soil layers at Abergowrie when residueswere retained, while CL2 was significantly greater in the surface 0–20 mm soil layer at Woodford Island when residues were removed.
3.2.4. Fraction CNL
There was a trend for the remaining C fraction, CNL, to occur ingreater concentrations within the surface soil layer when residueswere retained (Fig. 2d), which was significant at Mackay and Tully.Interestingly, the magnitude of the differences was small ornegligible in the surface soil of the two oldest experiments,Abergowrie and Ayr. Below 20 mm at these sites, the concentrationof CNL tended to be higher when residues were removed.
In contrast to the CNL concentration results, the CNL fraction(expressed as a proportion of CT) was significantly lower at all soildepths sampled at the two oldest sites, Abergowrie and Ayr, whenresidues were retained rather than removed (Fig. 3). CNL as aproportion of CT was not significantly different between treat-ments in the surface 20 mm soil layer at Mackay and Tully whenresidues were retained. By comparison, CNL was greater in all soillayers at Woodford Island when residues were retained, andsignificantly greater in the 0–20 mm soil layer.
3.2.5. Lability index (L)
L was greater in some soil layers at the two oldest sites,Abergowrie and Ayr, when residues were retained (Fig. 2e).Differences were significant to 100 mm at Ayr, but only in the20–50 mm layer at Abergowrie. Treatment effects were generallysmall and variable at the other sites with the exception of WoodfordIsland, where L was significantly greater in the 0–20 mm soil layerwhen residues were removed, consistent with the CL (Fig. 2a) and CNL
(Fig. 2d) results.
3.3. C fractions measured by the POC-UV scheme
As noted earlier (Table 2; Section 2.5), the replicates for POC-UVmethod were bulked prior to analysis, so the C fractions measured bythis method could not be compared statistically and are provided asa proportion of CT only. The resulting C fractions reported here fromthe POC-UV scheme are therefore indicative rather than conclusive.
3.3.1. POC
The surface soil layers generally contained the greatestproportion of POC, with a general trend across all sites for POC
to decrease with depth (Fig. 4). Treatment effects in the 0–20 mmsoil layer were inconsistent, but there tended to be greateramounts of POC in the 20–100 mm layer when residues wereretained rather than removed. However, overall there werestronger between-site than between-treatment differences.
3.3.2. Humic C
There was a trend for greater humic C fractions to occur in thesurface soil layer when residues were retained, although thedifference between treatments decreased with depth (Fig. 4). This
Fig. 4. The total C content of the retained residue (ret) and removed (rem)
treatments at the five sites, divided into the POC, humic and inert C fractions based
on their readiness to oxidise using the particulate organic carbon-ultraviolet photo-
oxidation method. Sites are ordered in duration of residue retention (Woodford
shortest, Abergowrie longest).
P.J. Thorburn et al. / Soil & Tillage Research 120 (2012) 99–111106
trend was strongest at the Mackay site, but was reversed at theWoodford Island Site, where greater humic C occurred whenresidues were removed.
3.3.3. Inert C
The proportion of C as inert C was greater in the surface soillayers at Tully, Ayr and Abergowrie when residues were removedrather than retained (Fig. 4). In contrast, there was greater inert C insoil at the Woodford Island site when residues were retainedrather than removed. There was a general trend across treatmentsand sites for the proportion of inert C to be lowest in the 0–20 mm
soil layer, but there was no consistent trend with increasing soildepth in deeper layers.
3.4. Correlations between C fractions and with other factors
3.4.1. Correlations between changes in C fractions
There were a number of linear associations between thechanges (retained residues less removed residue treatment values)in different C fractions (summed over 0–250 mm). These associa-tions largely occurred between changes in fractions within thesame fractionation technique. Changes in the KMnO4 fractions, CL1,CL2, and CL, were all highly positively correlated (P < 0.001) witheach other (Table 3), while the change in the CNL fraction washighly negatively correlated (P < 0.001) to these fractions. For theC fractions identified by the POC-UV scheme, the change in POC
was highly negatively correlated (P < 0.001) with changes in boththe <53 mm and humic C fractions, changes in the <53 mm andhumic C fractions were highly positively (P < 0.001) correlated toeach other, and the change in the humic C fraction was negativelycorrelated to change in the inert C fraction.
Significant correlation of variables across the two fractionationtechniques occurred between changes (retained residues lessremoved residue treatment values) in CL1, and changes in the POC
(P = 0.05), <53 mm (P = 0.05), and humic C (P = 0.07) fractions(Table 3). The relationship between CL1 and POC was not surprisingsince both are measures of the more readily oxidisable soil C, andboth were negatively correlated with the <53 mm fraction. Theweaker relationship with humic C is likely a reflection of therelationship between the two UV-POC fractions.
Other significant (P < 0.05) correlations occurred betweenchanges (retained residues less removed residue treatment values)in the CT and C fractions measured with the POC-UV method. Thechange in CT was positively correlated with the change in the POC
fraction and negatively correlated with changes in the <53 mm andinert C fractions (Table 3). There was no correlation betweenchanges (retained residues less removed residue treatment values)in CT and the C fractions measured by the KMnO4 technique.
3.4.2. Correlations between changes in C fractions and other factors
While the period that residues were retained did not have aproportionate effect on the changes (retained residues less removedresidue treatment values) in CT or C fractions in the individual soillayers (Fig. 1), it was significantly positively (P < 0.05) correlatedwith change in the CL2 fraction over the 0–250 mm soil profile (Table3). CL2 and CL also appeared to be positively correlated withcumulative trash input, although the correlations were weaker(P = 0.11 and 0.06, respectively). Significant correlations (P < 0.05)between changes in C fractions and other environmental factorsoccurred between the CL2 fraction and estimated cumulative degreedays (positive), CL2 and change in pH (negative), and CL and change inEC (positive). CL2 was also positively correlated with change in EC,although this relationship was weaker (P = 0.06).
3.5. Other soil chemical properties
Differences in soil nutrients (retained residues less removedresidue treatment values) were significant at different sites and atdifferent soil depths (Table 4). However, there was no consistenttrend for nutrient concentrations to increase with greater periodsof residue retention, nor for pH, CEC or EC to change in response tothe duration of residue retention. For example, there was littlesignificant difference between treatments in the concentration ofCa, K, Mg and Na at Ayr and Abergowrie, where residue retentionhad been practiced for the longest period, while the concentrationof P was greater at these sites when residues were removed insteadof retained. There was also no trend for greater concentrations of
Table 4Summary of the differences between treatments (retained residue treatment less removed residue treatment values) for the soil nutrients and properties analysed. Sites are
presented in order of increasing duration of residue retention. Negative values indicate where the nutrient concentration in the treatment with residues removed was greater
than in the retained residue treatment. Significant (P < 0.05) differences are indicated by *.
Chemical property Soil depth (mm) Woodford Island Mackay Tully Ayr Abergowrie
Ca (cmolc kg�1) 0–20 �0.40* �0.59* 0.06 �0.16 0.15*
20–50 �0.74* 0.26 �0.24* �0.32 0.05
50–100 �0.62* 0.25 �0.32* �0.78* �0.00
100–250 �0.87* 0.18 �0.26* �0.50* �0.02
K (cmolc kg�1) 0–20 �0.25* �0.11 �0.10* 0.01 �0.08
20–50 �0.07 0.07 �0.06 0.01 �0.02
50–100 �0.13 0.06 �0.04 0.01 0.00
100–250 �0.10 0.03 0.02 0.00 �0.02
Mg (cmolc kg�1) 0–20 �0.57* 0.04 0.00 0.08 0.02
20–50 �0.06 0.25* �0.08* 0.01 0.04
50–100 0.06 0.26* �0.11* �0.16* 0.02
100–250 �0.03 0.17 �0.05 �0.04 �0.06
Na (cmolc kg�1) 0–20 �0.02 0.01 0.01 0.00 0.00
20–50 0.03 0.01 0.00 0.00 0.00
50–100 0.07 0.01 0.00 �0.01 0.00
100–250 0.15* 0.02* 0.00 �0.04* 0.00
P (mg kg�1) 0–20 �15.58* �5.25 7.67* �23.8* �35.25*
20–50 �13.0 1.0 3.0* �16.4* �20.75
50–100 �11.75 4.5 �0.33 �9.4* �16.75
100–250 �5.25 12.0 1.33 �12.6* �7.75
S (mg kg�1) 0–20 �27.83 �4.0* 7.67* 0.87 1.42*
20–50 4.25 �1.75 5.67* 1.11 2.25
50–100 0.25 1.42 4.0 0.95* 2.75
100–250 �2.25 1.0 �1.0 0.02 0.5
pH 0–20 �0.35 �1.3 �0.3* �0.10 0.31
20–50 0.04 1.12 �0.25* �0.16 0.37
50–100 �0.01 �0.22 �0.28* �0.05 �2.18
100–250 0.12 �0.06 �0.27* �0.11 �0.07
CEC (cmolc kg�1) 0–20 �0.74* �0.95* 0.81* 0.3 0.08
20–50 �0.25 0.17 0.24* 0.48 0.24
50–100 �0.19 0.26 0.08 �0.02 0.15
100–250 �0.6 0.41 0.19 �0.59 �0.19
EC (mS cm�1) 0–20 �72.1* – 6.1* 7.8 15.5*
20–50 �5.9 �36.0 0.9 8.9 �28.1
50–100 �16.2 �0.8 �0.6 3.2 �2.2
100–250 �16.8 �1.8 3.3 1.5 8.9
P.J. Thorburn et al. / Soil & Tillage Research 120 (2012) 99–111 107
nutrients in the surface soil at Mackay and Tully where cultivationhad occurred least recently, compared to other sites. However,exchangeable Ca was significantly higher in some or all soil depthswhen residues were removed, at all sites except at Abergowrie.Greater exchangeable Ca in soils from the burnt residue treatmentcoincided withhigher pHin these soils, which was significant at Tully.
4. Discussion
The absence of a unifying relationship between the period thatresidues were retained and changes in CT (Fig. 1a), NT (Fig. 1b) orother soil nutrients (Table 4), was consistent with other studies insugarcane production systems (e.g. Blair et al., 1998; Graham et al.,2002b; Prasad and Power, 1991; Schomberg et al., 1994). Whenresidues were retained for very short periods, such as at WoodfordIsland (1 year), there were no measurable increases in theconcentration of CT (Fig. 1a), as also found by Ball-Coelho et al.(1993) and Meier et al. (2006). This result would be expected fromthe small mass of C returned to the soil from just one sugarcanecrop, especially given the amount of C mineralised during residuedecomposition. Longer periods of retaining residues increased theconcentration of CT in the surface soil (Fig. 1a), consistent withprevious research (e.g. Blair et al., 1998; Johnston, 1986; Ladd et al.,1994). Longer periods of retaining residues were also linked togreater values of L, and this index has been linked to the continuity
of C supply to the soil and size of the C pool (Blair et al., 1995).However, neither the concentration of CT (Table 3), various Cfractions or other nutrients (Table 4) were significantly correlatedto the period that residues were retained (Fig. 1a and b; Table 4).Similarly, there was no significant difference between treatmentsin the mass (as opposed to concentration) of CT when summed to250 mm or 1250 mm at any site (data not presented). A lack ofresponse in CT to longer-term residue retention is consistent withfindings from other sugarcane studies conducted for periods of upto 60 years (e.g. Blair et al., 1998; Graham et al., 2002b).
These findings for sugarcane residues contrast markedly withthose from other cropping systems, where comparisons of CT and Cfractions between different crops, cropped/uncropped compar-isons or contrasting crop residue management generally revealclear treatment differences (e.g. Lefroy et al., 1993; Dalal et al.,2011). A contributing factor to the different effect of crop residueretention on soil C accumulation in sugarcane systems could be therelatively slow decomposition of sugarcane residues. Sugarcaneresidues decompose considerably more slowly than residues witha similar biochemical composition from other crops (Thorburnet al., 2001). The long time (e.g. a year) for which sugarcaneresidues are on the soil surface may provide a greater opportunityfor soil fauna to incorporate residues (and hence the nutrients theycontain) from both the soil surface and shallow soil layers deeperinto the soil. The contrasting interactions between residue
P.J. Thorburn et al. / Soil & Tillage Research 120 (2012) 99–111108
management and soil C in sugarcane productions in contrast toother cropping systems clearly require further investigation.
A relationship between the number of crops for which residuesare retained and changes in soil C concentrations was not evident isthis study (Fig. 1a). This result could have been affected byvariations between the sites and/or treatments in factors such aspesticide management, compaction, etc. However, CT responded tosite differences in cultivation, climate, and soil texture in expectedways which are likely to have shaped the overall experimentalresults. Firstly, in terms of the effect of cultivation, the greatestincreases in the concentration of CT of the 0–20 mm soil layeroccurred at Mackay and Tully (Fig. 1a). At these sites, the residueretained plots would not have subjected to cultivation andassociated mixing of C within the soil profile for 5–6 years.[Because these plots were under zero tillage and the sites were intheir first crop cycle (Table 1), so had not undergone the cultivationassociated with destroying the crop at the end of the crop cycle andestablishing a plant crop.] Zero tillage in other cropping systemshas similarly stratified C at the top of the soil profile (Baker et al.,2007; Yang et al., 2008), but resulted in the same overall Csequestration when the whole soil profile has been compared withconventionally tilled soils. By comparison, stratification of CT wasweaker at Abergowrie (Fig. 1a), where residues had been retainedfor the longest period and the site had undergone cultivation forthe process of crop destruction-establishment the greatest numberof times. The lack of tillage in the residue removed treatment atWoodford Island (unlike other sites with residues removed, whichwere tilled) may have contributed to the contrasting results at thissite (Fig. 1a). Secondly, soil texture may have made a smallcontribution to the retention of relatively high concentrations of CT
in the surface soils of Mackay and Tully (Fig. 1a), since these twosites had the highest clay content of the four ‘northern’ sites (Table1) and therefore had more potential to retain organic C (Hartemink,2008). By comparison, the soils at Ayr and Abergowrie had coarsertextures (Table 1), which may have provided less protection to theC added when residues were retained (Fig. 1a). Finally, in terms ofthe climate, Woodford Island was substantially cooler and drierthan the other four sites (Table 1), which may have affected therate of soil C cycling and residue decomposition at that site, and ofits contribution to CT. The complexity of these factors determiningthe relationship between sugarcane residue management and soilC sequestration suggest that a modelling approach (Thorburn et al.,2001, 2002, 2005; Robertson and Thorburn, 2007b; Galdos et al.,2010) may be a useful approach to understanding the site-specificimpacts of sugarcane residue retention.
Although no relationship was identified between any of the Cfractions in any soil layer and the period that residues wereretained (Figs. 2–4), the fractions were more sensitive than CT
(Fig. 1a) to the short term changes in residue management atWoodford Island (Figs. 2c and 3) as found by others (Galdos et al.,2009; Graham and Haynes, 2006). Further, POC (Fig. 4) appeared tobe more sensitive to short-term changes in residue managementthan fractions determined using the KMnO4 method (Fig. 3) as alsofound in other studies (Skjemstad et al., 2006). However, Cfractions measured by both methods provided useful and,sometimes, contrasting (as implied by the general lack ofcorrelation between fractions from the two methods) insightsinto the effect of treatments on soil C. The benefit of using multiplemethods even extends to using different concentrations of KMnO4
to fractionate soil C.We found that a greater number of C fractions from the KMnO4
method were useful for identifying changes in soil C thanconcluded by others. Previous research found that changes in Cfractions across different concentrations of KMnO4 (33, 167 and333 mM) were correlated (Blair et al., 1995; Lefroy et al., 1993). Asa result, later studies concentrated on the C fraction oxidised by
333 mM KMnO4 (CL; Blair et al., 1995). However, our resultssuggest that use of both the 33 mM and 333 mM concentrationsmay enable a greater understanding of the changes in C sub-poolsdue to retaining sugarcane residues. For example, we found moresignificant differences between treatments in the soil organic Clevels measured by layer at the lower KMnO4 concentration (CL1,Fig. 2b) than detected by the 333 mM concentration (CL, Fig. 2a).Also, in the surface soil layer at all sites (except Woodford Island),residues significantly increased the concentration of CL1 (Fig. 2b)and altered the proportion of CT that occurred as CL1 (Fig. 3). Incontrast, the CL sub-pool within each soil layer did not display asimilar consistent trend across all sites (Fig. 2a), but altered theproportion of CL within CT only at the Abergowrie and Ayr sites(Fig. 3). However, when changes in the C fractions by layer werepooled over the 0–250 mm profile, the strongest correlations withvarious factors occurred with the CL and CL2 fractions (Table 3).
At least two other insights into changes in soil C were gainedfrom measuring both the CL and CL1 fractions with the KMnO4
method. Firstly, one possible explanation for the lack of a strongincrease in soil C at the sites where residues had been retained forthe longest period (Abergowrie and Ayr; Fig. 1a) is that it may takea considerable length of time for the CL fraction of CT to build upwhen residues are retained, whereas the CL1 content alters morerapidly after residues are retained. Bell et al. (1999) showed thatthe CL1 fraction has a role in aggregate stability which may be onepossible contributing factor to the more rapid change in thisfraction compared to CL, as it may be more physically protectedbecause of its binding role in aggregation. However, it could also bea function of the ‘quality’ of C (i.e. organic functional groups) insugarcane residues. Sugarcane, and consequently sugarcaneresidues, have high concentrations of soluble C (Rayment, 2003;Robertson and Thorburn, 2007a), so soluble C would probablyincrease the CL1 fraction within a relatively short period ofretaining residues (e.g. 1 year), compared to CL. Secondly, the Cfractions were useful for explaining why the effect of retainingresidues on the C fractions was largely confined to the surface soillayers, except at Ayr and Abergowrie where the effects wereobserved to greater depths (Figs. 2–4). Apart from differences inthe distribution of C due to the timing of cultivation discussed interms of CT above, the differences in C fractions at Ayr observed to100 or 250 mm could be due to site-specific characteristics such assoil type and irrigation. For example, the low clay content at theAyr site (Table 1) implies a less ‘protective’ capability of the soilwhich, when combined with irrigation, has likely resulted in thetranslocation of soluble organic functional groups down the soilprofile causing the observed treatment differences in the CL, CL1
and CL2 fractions at the greater depths (Fig. 2a–c). The presence ofCL below the soil surface has led other researchers to conclude thatthe fraction is mobile, and may represent the C source required todrive denitrification and methanogensis processes (Blair et al.,1995). Leaching may also be evident at the Tully site whichreceived highest average rainfall (Table 1), and where a relativelylarge proportion of CL occurred at the 20–50 mm depth (Fig. 2a).
As with the C fractions identified by the KMnO4 method, the Cfractions measured with the POC-UV technique were moresensitive than CT to changes in residue management (Fig. 4).Firstly, the fractions identified by these methods changed in ageneral way that reflected the duration of residue retention. POC
was more sensitive than CL1 or CL in identifying early changes inlabile C at Woodford Island, where residues had been retained foronly one year, and where the proportion of CT as POC was muchgreater in the surface soil when residues had been retained. Thisgreater sensitivity of POC than CL to short-term changes inmanagement was also observed by Skjemstad et al. (2006). Theproportion of CT as inert C was also lowest at the Abergowrie andAyr sites when residues were retained, which was also where
P.J. Thorburn et al. / Soil & Tillage Research 120 (2012) 99–111 109
residues had been retained instead of burnt for the longestperiods (Fig. 4). Secondly, the POC fraction provided additionalinformation about the distribution of C in the soil profile. Thepotential for leaching of the CL1 fraction at Ayr, demonstrated bysignificantly greater concentrations at most depths whenresidues were retained (Fig. 2b), was supported by POC
measurements from the POC-UV methods (these fractions werelinearly correlated) (Fig. 4). Relatively high proportions of POC
were measured at all depths in Ayr when residues were retained(Fig. 4), supporting the proposition that leaching had occurred inthis relatively sandy soil (Table 1). POC occurred in greatestproportions at the surface of the other, less sandy soils (Table 1;Fig. 4), where its physical size may have prevented it fromleaching to lower depths to the same extent. Although someleaching of the CL, CL1, and CL2 fractions at Tully is implied(Fig. 2), this was not consistent with the low proportions of POC
in all soil layers at Tully (Fig. 4). This may imply rapiddecomposition and conversion of the POC fraction to humic Cat this wet tropical site, rather than losses due to leaching ofPOC. The limited potential for the POC fraction to leach may haveadded to the success of the POC-UV fractionation scheme usedto measure C fractions for modelling changes in soil organic C bySkjemstad et al. (2004), since the C transformation routines usedalso did not include C leaching.
5. Conclusions
By considering a greater range of sites and periods of residueretention than previously reported, this study confirmed that,
Appendix A
Details of selected soil properties by depth increments for the coincreasing duration of residue retention.
Chemical property Soil depth (mm) Woodford Island
C (g kgS1)a 0–20 28.15
20–50 25.69
50–100 25.34
100–250 24.20
CL1 (g kgS1)a 0–20 1.64
20–50 1.45
50–100 1.34
100–250 1.26
CL (g kgS1)a 0–20 4.53
20–50 2.99
50–100 3.00
100–250 2.63
POC (g kgS1)a 0–20 3.40
20–50 3.50
50–100 1.50
100–250 2.00
Humic C (g kgS1)a 0–20 11.00
20–50 11.00
50–100 11.70
100–250 11.40
N (g kgS1) 0–20 2.66
20–50 2.06
50–100 2.00
100–250 1.88
P (mg kgS1) 0–20 61.33
20–50 51.50
50–100 52.75
100–250 32.00
unlike other cropping systems, there is no consistent, generalrelationship across sites between changes in soil C or nutrientconcentrations in response to the duration of sugarcane residueretention. Changes in the concentration of CT and NT in differentsoil layers only become apparent after around 5 years of retainingresidues, but the extent of the changes is highly site-specific due tomultiple influences on soil organic C (e.g. soils, climate, crops,management practices, soil fauna) at each site. Changes in the Cfractions determined by both the KMnO4 oxidation and POC-UVmethods appear to be more responsive to the duration and totalamount of residues returned to the soil than CT, although changeswere, once again, site-specific. Consequently, a modelling ap-proach may be useful to integrate the different factors affectingeach site to determine the extent of C sequestration in response toretaining sugarcane residues.
Acknowledgements
We thank Dr Graham Kingston, Alan Hurney, Les Chapman andGary Ham for allowing this study to be superimposed on their fieldexperiments and providing supporting data. We also thank DrAndrew Wood for facilitating access to the Abergowrie site, andDrs Peter Larsen and Ross Mitchell for their contributions toplanning this study. We acknowledge Dr Phil Moody (QueenslandDepartment of Environment and Resource Management) and DrJan Skjemstad (CSIRO Land and Water) for providing analyses oflabile and POC-UV C fractions, respectively, and the AustralianGovernment and sugar industry through the Sugar Research andDevelopment Corporation for financial support.
ntrol (burnt) experimental sites. Sites are presented in order of
Mackay Tully Ayr Abergowrie
16.10 14.05 13.41 13.84
13.10 12.22 12.17 11.19
11.60 11.31 10.45 10.92
10.08 9.71 8.90 9.42
1.06 1.09 1.43 0.97
0.95 0.83 1.18 0.64
0.80 0.66 1.12 0.57
0.58 0.52 0.67 0.47
2.74 3.67 3.32 2.23
2.16 1.64 3.19 1.55
1.57 2.20 2.34 1.44
1.29 1.92 1.98 1.25
5.70 1.10 1.10 2.1
2.90 0.00 1.40 0
1.80 0.20 0.30 0
1.30 0.00 0.20 1.1
4.40 4.70 5.80 4.40
4.00 3.80 5.20 4.30
4.70 3.30 5.20 3.20
4.20 2.80 4.40 3.10
0.91 1.13 0.82 0.99
0.81 1.07 0.78 0.79
0.76 1.04 0.70 0.76
0.66 0.97 0.63 0.65
69.75 25.00 150.00 160.00
64.50 19.67 141.80 124.75
87.50 17.67 135.40 107.25
50.25 15.67 134.60 61.00
Appendix A (Continued )
Chemical property Soil depth (mm) Woodford Island Mackay Tully Ayr Abergowrie
Ca (cmolc kgS1) 0–20 4.49 2.89 1.98 8.36 1.21
20–50 4.61 1.66 1.97 8.56 0.80
50–100 4.61 1.54 2.14 8.74 0.82
100–250 4.86 1.64 2.45 9.05 1.28
Mg (cmolc kgS1) 0–20 4.88 1.45 0.89 2.66 0.77
20–50 4.21 0.91 0.75 2.55 0.44
50–100 4.18 0.72 0.68 2.48 0.38
100–250 4.72 0.67 0.49 2.03 0.53
K (cmolc kgS1) 0–20 0.89 0.53 0.43 0.26 0.46
20–50 0.34 0.18 0.21 0.18 0.22
50–100 0.31 0.11 0.14 0.13 0.13
100–250 0.26 0.08 0.07 0.09 0.09
pH 0–20 5.71 5.30 5.47 6.19 4.43
20–50 4.99 4.94 5.35 6.14 4.31
50–100 5.03 4.85 5.48 6.05 4.44
100–250 5.18 4.82 5.57 6.22 4.59
CEC (cmolc kgS1) 0–20 13.73 6.02 4.50 9.73 4.29
20–50 12.40 4.95 4.50 9.80 3.62
50–100 12.25 4.72 4.45 9.81 3.70
100–250 12.89 4.69 4.43 10.22 4.21
EC (mS cmS1) 0–20 281.18 78.20 44.60 55.74 176.63
20–50 128.28 79.90 33.53 42.22 102.55
50–100 79.93 34.00 25.67 36.18 52.05
100–250 64.60 31.73 19.60 32.56 30.43
Na (cmolc kgS1) 0–20 0.31 0.11 0.09 0.10 0.04
20–50 0.25 0.08 0.08 0.12 0.03
50–100 0.24 0.09 0.07 0.16 0.04
100–250 0.34 0.10 0.06 0.28 0.04
a Defined in Table 2.
P.J. Thorburn et al. / Soil & Tillage Research 120 (2012) 99–111110
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