Soil Use and Management (2001) 17, 217±221 DOI: 10.1079/SUM200180

Soil particulate organic carbon under different landuse and management

K.Y. Chan

Abstract. Changes in particulate organic carbon (POC) relative to total organic carbon (TOC) were mea-sured in soils from ®ve agronomic trial sites in New South Wales, Australia. These sites covered a widerange of different land use and management practices. POC made up 42±74% of TOC and tended to begreater under pasture and more conservative management than traditional cropping regimes. It was theform of organic carbon preferentially lost when soils under long-term pasture were brought under cultiva-tion. It was also the dominant form of organic carbon accumulating under more conservative managementpractices (direct drilling, stubble retained and organic farming). Across all sites, changes in POC accountedfor 81.2% (range 69±94%) of the changes in total organic carbon caused by differences in land use andmanagement. Signi®cant differences were found between pasture and cropped soils in the carbon content inthe <53 mm fraction, particularly for hardsetting soils. However, even with these, POC was a more sensitiveindicator of change caused by land use and management practices than TOC.

The current method for measuring POC involves dispersion using sodium hexametaphosphate. The dis-persing agent was found to extract 4±19 % of the TOC, leading to a signi®cant under-estimation of POC.

Keywords: Organic carbon, soil, carbon sequestration, land use, land management, hardsetting soils,Australia

I N T R OD U C T I O N

Soil organic matter (SOM) is a heterogeneous mixture oforganic substances with different composition, lability

(turnover time) and functions in soils. Conceptually and formodelling of soil organic carbon dynamics, it has been foundconvenient to partition total carbon content into differentpools (Parton et al. 1987; Jenkinson 1990). The differentforms of organic matter might have different effects on soilquality and respond differently to different managementpractices. The challenge of soil organic matter research hasbeen to develop reliable experimental methods for theidenti®cation of the different pools and to relate these pools(different forms of soil organic matter) to their functionalroles in soil (Stevenson et al. 1989; Dalal & Chan 2001).From the soil management point of view, such knowledge isuseful for the selection of management practices which willachieve the level of the appropriate forms of soil carbon.

Particulate organic matter (POM) is the organic fractionsin 53±2000 mm soil separates (Cambardella & Elliott 1992;Wilson et al. 2001) of which the carbon content is referred toas particulate organic carbon (POC). Recent research incarbon fractionation has indicated that POM is moresensitive to changes in management practices than total

organic matter (Cambardella & Elliott 1992; Franzluebbers& Arshad 1992; Chan 1997; Bowman et al. 1999; Needelmanet al. 1999). Chan (1997) reported that for a Vertisol, 70% ofthe loss of organic carbon as the result of conversion ofpasture to cropping was in the form of POC. Bowman et al.(1999) observed that while the total organic carbon (TOC)content of continuously cropped soil was only 20% greaterthan that of the wheat-fallow soil, the POC content wasdouble that of wheat-fallow soil. Similarly, Needelman et al.(1999) reported that the difference in POC between no-tillsoil and conventionally tilled soil was twice that of thecorresponding difference in total organic carbon (33%versus 15%). Furthermore, there is accumulating evidencesuggesting that this fraction of carbon is a sensitive indicatorof soil quality changes (Franzluebbers & Arshad 1992;Wander et al. 1994; Chan 1997; Wilson et al. 2001). Wanderet al. (1994) studying organic and conventional farmingsystems, concluded that POM was the best index of activeSOM in terms of both quality and quantity. In an AustralianVertisol, Chan (1997) found that the POC level wassigni®cantly correlated with macro-aggregate stability andmineralizable nitrogen. Wilson et al. (2001) also reported astrong correlation between POM and N mineralizationunder different farming systems with varying rotations,forms of tillage and cover crops.

Currently, POC determination often involves usingsodium hexametaphosphate as a dispersant (Cambardella &

Wagga Wagga Agricultural Institute, NSW Agriculture PMB, WaggaWagga, NSW 2650, AustraliaFax number: 2 69381809; E-mail: yin.chan@agric.nsw.gov.au

K.Y. Chan 217

Elliott 1992). The procedure leads to dissolution of soilorganic carbon and can therefore be a source of error. Asreported by Barley (1955 ) for the soil he studied, the carbonmaterial dissolved as a result of sodium hexametaphosphatetreatment was equivalent to 5.6% of total macro-organicmatter. However, the magnitude of error has not beeninvestigated for other soil types.

My aim in this paper was to see if changes in POC couldbe used as an indicator of changes in soil quality caused byland use and management. To do this, I measured changesin POC under different systems of land use and manage-ment at a number of contrasting sites in New South Wales,Australia. The changes in POC were compared to those inTOC as well as to those in the fraction of carbon associatedwith the soil mineral fraction.

M A T E R I A L S A N D M E TH O DS

Soil sampling and preparationComposite soil samples (0±10 cm) were collected from ®veagronomic trial sites throughout New South Wales,Australia (Table 1). Signi®cant differences in soil propertiesunder different land use and management practices at thedifferent sites have been documented earlier (Table 1). Ateach site, soil samples were collected from at least twomanagement treatments, one of which had been under long-

term pasture/native vegetation. The soil samples were driedat 40 °C, ground to pass through a 2 mm sieve and stored.

Particulate organic carbon (POC) determinationParticulate organic carbon was determined followingCambardella & Elliot (1992). Twenty grams of soil wasdispersed by shaking for 15 h on an end-over-end shaker in100 ml of 5 g l±1 sodium hexametaphosphate solution. Thedispersed soil suspension was passed through a 53 mm sieveand after rinsing several times with distilled water, thematerial retained on the sieve was dried at 105 °C. The soilsuspension that had passed through the sieve was made upto 250 ml in a 500 ml measuring cylinder. The suspensionwas thoroughly mixed by end-over-end shaking and a 25 mlsample centrifuged at 12 100g for 1 h. Carbon was measuredin the supernatant using a ShimadzuR Dissolved CarbonAnalyser. The residue after centrifuging was returned to the<53 mm suspension which was then evaporated to dryness at105 °C, weighed, ground with a mortar and pestle (to<0.5 mm) and analysed for C and N using a LECO CNSAnalyser (Nelson & Sommers 1982). The total C and Ncontents present in the <53 mm fraction were calculated aftercorrecting the soil weight for the amount of sodiumhexametaphosphate present in the extractant. POC wascalculated as the difference in C passing the 53 mm sieve andthat obtained from the corresponding whole soil sample,expressed on an oven-dried whole soil basis. Particulate

Table 1. History and basic soil properties at ®ve different sites in New South Wales, Australia.

Location Land use History Annualrainfall,(mm)

Soil type Sand(%)

Silt(%)

Clay(%)

Textureclass

Reference

Nyngan 310 34¢S,1470 12¢E

nativevegetation

undisturbed woodland 431 Al®sol 54 13 32 clay loam Chan et al. 2001

CT/SB cropped for 50 yearsunder conventional tillageand stubble burning and

fallowing

54 13 32

4 yr pasture after 4 year of lucernepasture on cropped soil

54 13 32

Wagga Wagga350 05¢S, 1470

20¢E

DD/SR under direct drilled andstubble retention for 10 years

550 Al®sol 63 12 23 clay loam Chan et al. 1992

CT/SB under conventional tillageand stubble retention for 10 years

63 12 23

Cowra350 50¢S, 1480

41.5¢E

pasture over 20 years under pasture 564 Al®sol 77 11 13 sandy loam Chan & Mead 1988

DD/SR under DD and SR for 3 years 77 11 13CT/SB under CT and SB for 3 years 77 11 13

Walgett300 01¢S, 1480

07¢E

pasture undisturbed native pasture 474 Vertisol 22 18 58 clay Chan 1989

CT/SB under CT and SB for 50 years 22 18 58

Somserby330 23¢S , 1510

21¢E

Organic under organic system for 5 years 1300 Ferrasol 74 7 16 sandy loam Wells et al. 2000

Conventional under conventionalsystem for 5 years

77 6 15

(CT/SB ± cropping under conventional tillage and stubble burning; DD/SR ± cropping under direct drilling and stubble retention)

Soil organic carbon under different land use and management218

organic nitrogen (PON) was calculated similarly. All thedeterminations were made in triplicate.

RE S UL TS A N D DI S CU S S I O NS

The sites spanned a wide range in annual rainfall and thesoils varied greatly in texture, from sandy loam (13% clay)to clay (58%) (Table 1). At all ®ve sites, signi®cantlydifferent levels of TOC were found amongst the differentland use and management practices (Table 2). For the threesites including a pasture, namely, Nyngan, Cowra andWalgett, TOC of pasture soil was 2.63, 2.72 and 1.94 timesthat of the corresponding cropped soils, respectively. Therewere also signi®cant differences in TOC between thecropped soils under different management practices, withhigher levels found in the soil under more conservativemanagement, e.g. DD/SR (cropping under direct drillingand stubble retention) versus CT/SB (cropping underconventional tillage and stubble burning) and organic versusconventional systems.

The fractionation procedure showed that the C:N ratiosof POM were consistently higher than those of the organicmatter in the <53 mm fraction (a mean 17.5 versus 10.0)(Table 2). According to Oades et al. (1987), organic matterwith C:N ratios close to 20 is mostly composed of plant

material at an early stage of decomposition whereas organicmatter in the <53 mm fraction with C:N ratios close to 10 ismostly made up of more humi®ed materials, namelyresistant portions of lignin plus aromatics from microbesassociated with clay in small aggregates. The fractionationprocedure was therefore successful in separating totalorganic carbon into two fractions of different chemicalconstitution and degree of decomposition.

Across all sites, POC made up 42 to 74% of TOC andtended to be greater under pasture than under cropping(Table 2), so that changes in POC at the different sitesfollowed the trend of TOC. However, when the POC levelof the pasture and cropped soils at the different sites werecompared, the losses in POC accounted for most (69.1±81.0%) of the decline in TOC as a result of conversion ofpasture to crops (Table 3). Similarly, the higher TOC foundin the more conservative cropping treatment was duepredominantly to POC (80.9±94.3%). These results there-fore con®rmed earlier ®ndings that POC is a labile carbonfraction (Chan 1997; Bowman et al. 1999; Needelman et al.1999).

Similarly, Hassink (1997) reported that TOC in Dutcharable soils was lower than in the corresponding pasture soilsand the loss of carbon came mainly from the >20 mmfraction, which is not associated with the mineral particles ofthe soil and is therefore closely related to POC of the presentstudy. He showed that the amounts of OC associated withclay and silt (<20 mm) were similar between the arable andthe corresponding pasture soils. He attributed this to thecapacity of a soil to preserve C by association with clay andsilt particles and supported his argument by demonstrating apositive relationship between the proportion of primaryparticles <20 mm in a soil and the amounts of C that wereassociated with this fraction in the top 10 cm layer of a rangeof soils from both the temperate and tropical regions.

Figure 1 shows no relationship between the amount ofcarbon in the <53 mm fraction and the amount of soilmineral particles <53 mm for the different New South Walessoils under investigation. Furthermore, for the Cowra and

Table 2. Total organic carbon, carbon fractions and C:N ratios of soils under different land use and management practices at ®ve different sites,New South Wales, Australia.

Sites History TOCa

(g kg±1)C<53mm(g kg±1)

POCb

(g kg±1)Soluble Cc

(g kg±1)C:N <53mm C:N>53mm

Nyngan Native pasture 21.4(0.6) 9.1(0.6) 12.3(0.6) 2.5(0.2) 11.2(0.03) 21.0(1.2)CT/SB 7.5(0.3) 4.7(0.2) 2.7(0.2) 0.9(0.1) 9.13(0.23) 16.9(0.5)

4 yr pasture 8.8(0.2) 5.2(0.2) 3.7(0.2) 1.0(0.1) 8.5(0.09) 15.3(0.3)Wagga DD/SR 17.5(0.3) 8.7(0.3) 8.8(0.2) 2.2(0.1) 10.4(0.1) 14.3(0.1)

CT/SB 14.8(0.2) 8.2(0.3) 6.7(0.2) 1.9(0.1) 10.4(0.1) 17.8(0.7)Cowra Pasture 32.2(0.4) 8.3(0.4) 23.9(0.4) 2.4(0.2) 9.0(0.0) 11.5(0.1)

DD/SR 16.8(0.2) 5.4(0.1) 11.4(0.1) 1.7(0.1) 9.6(0.0) 15.8(0.2)CT/SB 6.9(0.1) 3.5(0.2) 3.4(0.2) 1.3(0.1) 10.3(0.0) 15.3(0.2)

Walgett CT/SB 9.3(0.1) 5.6(0.1) 3.7(0.1) 0.4(0.0) 8.5(0.2) 23.1(4.6)Pasture 18.0(0.1) 7.3(0.1) 10.7(0.1) 0.8(0.0) 7.7(0.2) 20.3(1.4)

Somersby Organic 21.9(0.2) 8.9(0.2) 13.0(0.2) 2.1(0.1) 13.4(0.1) 21.3(0.4)Conventional 18.4(0.1) 9.7(0.1) 9.7(0.1) 2.1(0.1) 12.3(0.3) 17.0(0.6)

Range 6.9±32.2 3.5±9.7 2.7±23.9 0.4±2.5 7.7±13.4 11.5±23.1Mean 16.2 7.0 0.92 1.6 10.0 17.5

Numbers within the brackets are standard errors. (aTOC is total organic carbon; bPOC is particulate organic carbon; csoluble C is the C extracted bysodium hexametaphosphate)

Table 3. Changes in POC as a percentage of total change in organiccarbon between pasture and cropping and of cropping under differentmanagement practices.

Land use Sites Changes in POC(as % changes in TOC)

Pasture vs cropping Nyngan 69.1Cowra 81.0

Walgett 80.5

Cropping under different Wagga 81.5management Cowra 80.9

Somersby 94.3

K.Y. Chan 219

Nyngan soils, the carbon contents in the <53 mm fractiondeclined signi®cantly under cropping, to only half of thoseunder pasture (Table 2). For the Wagga and Somersby sites,the changes in this fraction of organic carbon were muchsmaller. Nevertheless, changes in carbon of the <53 mmfraction were much smaller than those of POM, accountingfor only 5.7 to 30.9% of the total changes in TOC (Table 2).Therefore, POC was still a more sensitive indicator ofchange than TOC.

Assuming that the <53 and <20 mm fractions are similarwith respect to their abilities to stabilize C, our resultssupport Hassink's earlier ®nding that Australian soils have amuch lower capacity to retain carbon in their clay and siltfractions compared to soils from other temperate andtropical regions (Hassink 1997). The reasons for thediscrepancy are not known with certainty but could berelated to the differences in soil mineralogy and climatebetween Australia and the other parts of the world. ManyAustralian soils are highly weathered and dominated by clayminerals such as kandites and illites. The lower cationexchange capacity and surface area might both limit thecapacity of the Australian soils to retain OC. Anotherplausible explanation is the combination of low precipitationand high temperature, both are characteristics of large partsof Australia, leading to low inputs of OC (Hassink 1997).

Our results suggest a tendency for some of the Australiansoils to lose mineral-associated soil organic carbon (<53 mmfraction) under cropping. It is interesting to note that bothof the soils that exhibited this behaviour to the largestextent, the Cowra and Nyngan soils, were hardsetting soils,a major soil type in Australia (Mullins et al. 1990). Theremight be a relationship between the smaller capacity of theclay plus silt fraction to preserve/protect C and hardsettingsoil properties.

Large amounts of organic carbon were present in thesolution phase of the sodium hexametaphosphate extracts(Table 2). Amounts of soluble organic carbon extracted bysodium hexametaphosphate, varied between 0.4±2.4 g kg±1,averaged 1.6 g kg±1. These represented 4.4 to 18.8% of theTOC in the range of soils under investigation. The lowest

percentage was found in the Vertisol and the highest in theAl®sol at Cowra, with the others being close to 10%. Underthe current experimental procedure, all the soluble carbon isincluded in the <53 mm fraction, i.e. C <53 mm which tendsto result in the under-estimation of POC. Assuming all thesoluble C came from the POM fraction, the highest under-estimation of POC was 28% in the case of the Cowra ±CT:SB soil.

CO NC L U S I O NS

POC was the form of organic carbon preferentially lost whensoils under long-term pasture were brought under cultiva-tion. It was also the form of soil organic carbon thataccumulates under more conservative management prac-tices, such as direct drilling, stubble retained and organicfarming. Across all sites, POC accounted for 81.2% (69.1±94.3%) of the changes in TOC arising from different landuse and management.

Due to its greater lability compared to the OC in the<53 mm fraction, POC was a more sensitive indicator ofchanges due to land use and management than TOC. Forsoil management research comparing different managementregimes, it is therefore more appropriate to measure changesin POC rather than TOC. The uncertainty in thedetermination of POC due to organic carbon dissolutionby sodium hexametaphosphate warrants further investiga-tion.

A CK N O W L E D G E M E N TS

I would like to thank Albert Oates and R. Ashley fortechnical assistance.

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Received January 2001, accepted after revision April 2001.

# British Society of Soil Science 2001

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