Changes in soil carbon sequestration, fractionation and soil fertility in response to sugarcane residue retention are site-specific
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Soil & Tillage Research 120 (2012) 99111
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Soil & Tillage
jou r nal h o mep age: w ww.e1. 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 (1320 t DM ha1), which containuseful quantities of plant nutrients (0.460.54% N; 0.470.66% K;0.090.17% Mg; 0.180.41% Ca; 0.060.17% S; 0.050.09% P) and
large quantities of C (42%) (de Oliveira et al., 2002; Robertson andThorburn, 2007a). Retaining instead of burning residues providesseveral potential benets (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 benets 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 difcult 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 kg1 (0200 mm) after 6 years (Robertson andThorburn, 2007b), by 5 g kg1 (0200 mm) after 8 years (Galdoset al., 2009), or by 9.2 g kg1 after 55 years (Canellas et al., 2010).However, the amount of C sequestered is affected by climate, soil
A R T I C L E I N F O
Received 7 February 2011
Received in revised form 31 October 2011
Accepted 25 November 2011
Available online 22 December 2011
Green cane management
Particulate organic matter
Soil organic matter
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 ve 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-specic and not in
proportion to the period that residues were retained: for example, soil C (0250 mm) decreased by
0.9 g kg1 and 0.5 g kg1 at sites where residues had been retained for 1 and 17 years, respectively, butincreased by 2.0 g kg1 at a site with residues retained for 6 years. Soil C composition, dened by theKMnO4 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 KMnO4method identied downward movement of C fractions in
the prole 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
Crown Copyright 2011 Published by Elsevier B.V. All rights reserved.
* Corresponding author. Tel.: +61 7 3833 571; fax: +61 7 3833 5505.
E-mail address: Peter.Thorburn@csiro.au (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. All rights reserved.doi:10.1016/j.still.2011.11.009Changes in soil carbon sequestration, frin response to sugarcane residue retent
P.J. Thorburn a,*, E.A. Meier a,b, K. Collins a, F.A. RobaCSIRO Ecosystem Sciences and Sustainable Agriculture Flagship, GPO Box 2583, BrisbabMeier IT Pty Ltd, PO Box 7153, Mt Crosby, Queensland 4306, AustraliacBSES Ltd, 50 Meiers Road, Indooroopilly, Queensland 4068, Australiationation and soil fertilityn are site-specic
Queensland 4001, Australia
l s evier . co m/lo c ate /s t i l l
P.J. Thorburn et al. / Soil & Tillage Research 120 (2012) 99111100texture, 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 signicantlydifferent between sites where sugarcane residues had been burntor retained at Pernambuco State, Brazil after 1 year, or Tully or Ayr,Australia after 710 years (0250 mm; Blair et al., 1998), or at a sitein South Africa after 60 years (0300 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 difcult 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 identied 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 identied 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 4080% 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 signicantly 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 identied consistent trends in soilproperties with the period that residues were applied. This studyaims to address these knowledge gaps by determining themagnitude 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.
Soil samples were taken from ve 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 rst 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 rst 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 rst 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 adjacentstrips (>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 (020 mm, 2050 mm, 50100 mm, 100250 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
P.J. Thorburn et al. / Soil & Tillage Research 120 (2012) 99111 101Table 1Details of experimental sites. Sites are presented in order of increasing duration o
Item Experimental sites
Woodford Island Mackay
Location 153.132 E; 29.487 S 149.119 E; 21.164
Reference Kingston et al. (1998)
Chapman et al. (20
Average rainfall (mm year1) 1020 1670 Irrigation (mm year1) None 1500 None
1990 1988 1980
1991 1989 1981
11/97 11/98 10/98
6 9 17
1 2 3
60 112 120
3 4 4
100 600 100
Silty clay Sandy loam Loam
Brown dermosol Not available Red dermosol
0.31 0.14 0.19
0.19 0.05 0.08
10.7 10.2 10.3
0.63 0.88 0.55
2.08 2.30 0.85
0.13 0.44 0.83fractions 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 denes 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 nal, 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 orga...