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Decomposability of organic matter in particle size fractions from field soils with straw incorporation
Surl Bwl. Biochrm Vol. IV. No. 4. pp. 4?Y-435. Printed m Great Britain. All nghts reserved
1987
Copyright
0038-0717 97 53.00 + 0.00 C 1987 Pcrgamon Journals Ltd
DECOMPOSABILITY OF ORGANIC MATTER IN PARTICLE SIZE FRACTIONS FROM FIELD SOILS WITH STRAW INCORPORATION Askov
Experimental
BENT T. CHRISTENSEN Vejenvej 55, DK-6600
Station,
Vejen. Denmark
(Accepted 25 October 1986)
Summary-Soils from two field experiments on straw disposal were fractionated according to particle size using ultrasonic dispersion and gravity-sedimentation in water. Samples of whole soils, clay, silt and sand-size fractions were held for 49 days at 2O’C and the CO: evolution measured on 14 dates by gas chromatography. Recovery of soil solids. C and N was 99, 98 and 93%. respectively. Most of the soil C and N was in the clay ( < 2 pm), (loamy sand. 50% C and 56% N: sandy loam, 65% C and 68% N), the silt (2-20 pm) having smaller proportions (loamy sand. 41% C and 38% N; sandy loam. 29% C and 27% N). The sand fraction (20-6000 pm) accounted for 4-7% of the organic matter, and I-24b of the C was water soluble. Straw incorporation generally increased the C and N content of whole soils and size fractions. The decomposition rate constants were higher for the sandy loam than for the loamy sand soil. For both soils. the decomposability of the organic matter decreased in the order: sand > clay > whole soil > silt. Straw incorporation increased the decomposition rate of whole soil and sand organic matter. whereas the effect of straw on clay and silt respiration was small. Between 58 and 73% of the respiration was from clay, 2l-25% from silt and 619% from the sltnd size fraction
size. The CO2 production during incubation of peat size fractions (Williams, 1983) and macro-organic matter isolated from soil (Adams, 1980) has been examined, and Amato and Ladd (1980) studied the biomass carbon of size fractions by the chloroform fumigation method, but no reports on the carbon mineralization from size fractions of arable soils could be traced. Studies on the effect of cultivation (Tiessen and Stewart, 1983; Tiessen et al., 1984), straw incorporation (Christensen, 1986), mineral fertilizer and animal manure addition (Christensen, 1987) on the distribution of organic matter between soil size frdCtions have shown that the various size fractions are affected differently when soil organic matter levels change. These and other studies (Anderson and Paul, 1984; Anderson et al., 1981; Christensen and Sorensen. 1985, 1986) have indicated that different soil size fractions play different roles in soil organic matter stabilization and turnover, the clay bound organic matter probably being more labile than that associated with silt. To determine more directlv the differences in stability of organic matter associated with different soil size fractions, I have examined CO; evolution during incubation of clay. silt, and sand-stzed fractions and whole soil samples. Soil was taken from field experiments on straw disposal (straw burned or incorporated into soil annually for I8 yr). Soil from these experiments have been included in previous studies of soil microbial biomass and activity (Powlson er al.. 1987). and of the organic matter content of macroaggregates and particle size fractions (Christensen. 1986).
Nutrient cycling in soil is closely related to organic matter turnover. carbon metabolism playing a key role in mineralization and immobilization processes of plant nutrients. The saprotrophic soil organisms, acting as a driving force in nutrient cycling, rely on available organic compounds as a source of energy and carbon. Consequently, knowledge about the stabilization and subsequent decomposability of organic matter in soil is indispensable for understanding the fate of plant nutrients added to soil in fertilizers. crop rcsiducs and animal manure. The decomposability of soil organic matter is most often studied by using a “whole soil approach” in which the CO2 production or 0, consumption of unfractionatcd soil samples are assessed. However, additional information may be gained by using soil fractions, provided that the obtained fractions can be related to structural or functional entities in soil and thereby also to biological turnover. The inability to fulfil these conditions is considered as a major drawback of most chemical fractionation schemes applied to soil organic matter. Physical fractionation of soil according to particle sizes has proved to be a useful tool in the study of soil organic matter, revealing differences both in the structural and dynamic properties of organic matter from different soils and size fractions (Christensen, 1987). Cameron and Posner (1979). Chichester (1969, 1970) and Lowe and Hinds (1983) observed that upon incubation in the laboratory, the mineralization of organic nitrogen and sulphur in various soil size fractions tend to increase with decreasing particle 429
430
BEST T. CHRISTESSES \lATERI.ALS
AXD
\lETHODS
Soils Soil was sampled in two long-term field experiments on straw disposal. Both experrments grew continuous spring barley (Hordezmz rulgare L.) and included the treatments: straw and stubble incorporated by ploughing (5 t dry matter ha-’ yr-‘). and straw and stubble burned. The experiments were established in 1966 on a loamy sand soil (Studsgaard site; 6% clay, 7% silt, 4% OM, C to N ratio IS) and on a sandy loam (Ranhave site; 14% clay. 15% silt, 2% OM, C to N ratio IO). Ten soil cores (dia 45 mm) were taken from the O-25 cm layer of each replicate treatment (4 repl.) and bulked to give one sample per replicate. The sampling was carried out in early April 1984 before drilling and fertilization, and followed the equivalent sampling depth concept (Powlson and Jenkinson, 1981). After sieving the moist soils through a 6mm screen, soil samples were air-dried and finally dried at 40°C for 5 days. Isolution of parricle size fractions Particle size fractions wcrc isolated by dispersing 30 g of soil ultrasonically (300 W for IS min) in I50 ml of water using a probe-type disintegrator (Labsonic 1510, B. Braun Mclsungcn, F.R.G.). Claysize ( < 2 jtm) and silt-size (2-20 itm) fractions wcrc obtained by rcpcatcd gravitational sedimentation in water; the sand-size (20-6000 /lrn) fraction was rccovered as the scdimcnt left in the scdimcntation cylinder after the isolation of clay and silt. The procedure is described in detail by Christensen (1985). The supernatant solutions remaining after the clay suspensions had been centrifuged were concentrated on a rotary evaporator and taken as the water soluble fraction of the soil. Soil fractions wcrc dried at 40C for 5 days, weighed and gently crushed, and then bulked to give one clay, silt and sand sample per treatment and soil type. Inchrrion procethtre Whole soils and size samples wcrc incubated at 20 C for 49 days in 120 ml incubation flasks fitted with air-tight septum caps. Bcforc incubation, I ml of a nutrient solution (containing 2.0 mg N, I .6 mg P, I .5 mg K. 0.5 mg Ca. 0.2 mg Mg and 0.3 mg S) was added to each flask together with I ml of a soil inoculum (100 g of fresh soil shaken with 1000 ml of water and left to stand overnight). The nutrient solution was adjusted to pH 7 before use. For whole soil and sand size samples, log was incubated. whcrcas 2g of clay or silt material was used. Moreover, the clay and stlt was mixed with 8 g of puriticd sand (0.060.50 mm) bcforc incubation to improve aeration in thcsc samples. The height of the sample in the flask was about 5 mm. Blanks containing purified sand (log), nutrient solution and soil suspension (I ml of each) were included. All incubations wcrc run in duplicate. The air space of the flasks during the incubation was I I4 ml. assuming a sample density of 2.6 g ml-‘. Septum caps were removed on day I4 and day 30, the atmosphere of the flask rcplaccd by ambient air. and
new caps mounted. Thereby the CO. concentration the flask was kept below 3.54b (v ;).
in
Determination of CO2 The CO: concentrations in the flasks were determined on days 0, I, 2. 3, 4, 7. II. 14. IS. 21. 28. 30. 37 and 49. When the atmosphere of the flasks were replaced. the CO? concentration was determined before and after the replacement. The CO: concentration was measured on a gas chromatograph (ML CC 82-22. Mikrolab Aarhus. Denmark) fitted with a thermal conductivity detector and I.4 m glass column (i.d. 3 mm) containing “Chromosorb 101” (8OjlOO mesh. Manville Corp.). The reference column was molecular sieve (5 A. SO/l00 mesh); the carrier gas (He) flow was 27.5 ml min-I. Gas samples were taken from the incubation flasks by inserting a gas-tight syringe with a sideport needle through the septum cap. removing 0.5 ml of air and injecting the sample into the gas chromatograph. Sample CO: concentration was dctermined from peak areas (calculated by a computrrcoupled intergrator). standard CO:-He mixtures providing. the reference areas. Follovvmg gas sampling. the gas volume taken for analysis was replaced by ambient air (0.033% CO:) in order to reduce changes in air pressure within the incubation flask. The percentage of CO, in the flask atmosphere was converted to ilg CO:-C mg-’ sample-C after subtracting blank CO, concentration. knowing the free air space of the flask, the equation of state and the carbon content of the incubated sample. From this the quantity of CO: evolved up to the time of sampling was calculated. Ciremicul unuIw~.s Carbon contents of whole soils and size fractions were determined with a Leco Carbon Analyser coupled to an infrared CO: detector: S contents with a Tecator total-N analyscr using a Kjeldahl method (Hg catalyst). The C content of the water soluble fraction was determined with a Dohrman TOC analyser (DC 54) using unfiltcrcd subsumplcs. The pH of whole soil and size fraction samples was determined I h after mixing the sample with dcmineralized water (sample to water ratio. I to IO).
RESULTS
The pH of the soil size fractions was 6.5-6.6 except for the Studsgaard clay fraction which had a pH value of 5.9 (Table I). Whole soil pH was near neutral. Recovery of soil solids, C and N averaged 99.4. 98.4 and 93.0%. respectively. Most of the recovered soil C (Studsgaard. 48-52%; Ronhavc. 6466%) and N (Studsgaard. 55-57%: Ronhavc, 67-69%) w;ts found in the clay size fraction. while the slit accounted for smaller fractions (Studsgaard. 3942% C and 37-39% N; Ronhavc. 29% C and 2627% N). The sand fraction contained 3.7-7.3% of the C and N. The water soluble fraction of the Studsgaard and Ronhave soil made up 0.9-1.3 and l.6-2.0% of the recovered C. rcspcctivcly.
Ronhwe soil Fraction Clay Sill SwId Water soluble Sum OF lkiclions Whole soil Recovw ?/.
Sludsguard soil Fraction Cluy Silt Sand Water soluble Sum of lraclions Whole soil Recovery %
7.1
6.5 6.5
6.6
7.0
5.Y 6.5 6.6
PII’
14.6(1.3) I s.5 (0.3) 69.3 ( I .h)
9Y.4 (0.2)
5.7 (0.3) 7.6 (0. I) X6. I (0.3)
Y9.3 (0. I)
14.5(I.K) 15.7 (0.4) 69. I (2.2)
Y9.6(0.1)
6.1 (0.2 1.5 (0. I) X6.0 (0.2)
7.43 3.21 0.42 0.18 II.3 Il.7 Y6.b
Il.74 Y.95 1.72 0.22 23.6 23.4 100.9
7.58 3.49 O.b? 0.24 11.1) 12.7 93.9
12.79 9.65 I.80 0.33 24.6 24.1 102.1
mgg ’ soil SKiW Rurned IncorP.
cilrbon
65.8 28.9 3.1 I.6
49.1 42. I 7.3 0.9
63.5 29.3 5.2 2.0
52.1 39.3 1.3 1.3
-___ RelaGvr distribution, %b S&w IncorP. Burned
--_--
0.639
0.281 0.055 ND I.10 I.26 x7. I
0.762
0.454 0.077 ND 1.17 1.16 loo.9
0.767 0.311 0.062 ND I.14 1.27 X9.X
0.714 0.468 0.069 ND I.25 1.33 94.1
69.4 25.6 5.0
54.6 311.8 6.6
67.3 27.3 5.4
57. I 31.4 5.5
Relitlive distribution, %b strvw Incorp. nurr1ell
Nitrogen .-_._ _...~
deviations (n = 4). The water soluble fraction
ml: 1: ’ soil SlrdW Burned Incorn.
Table I. Dry weight, pH, carbon and nitrogw content of whole roils and soil fractions. Yalues in parenthesis are aaxid was noi an&zed for nitrogen content (ND)
BENT T. CHRISTESSES
Fig. I. Cumulative CO, evolution from whole soil samples taken from the straw burned and stram incorporated trcttments (Studsgaard and Ronhave). Lines are fitted by linear rcgrcssion (see Table 2).
Stravv incorporation. compared with straw burning. increased the C and N content of nearly all soil fractions. The increases were however moderate as dcmonstratcd earlier by Christensen (1986) and Powlson rf al. (1957). Straw incorporation had a small intlucncc on the relative distribution of C and N bctu-ccn soil fractions. but this could not be tested statistically with the present experimental design. The CO2 evolution from incubated whole soils and size fractions is shown in Figs I. 2 and 3. The decomposition rate constant for the various samples
Fig. 2. Cumulative incorporated
(Table I) was calculated by simple linear rcgrcssion applied to the cumulative CO, yield for the period day 7 to day 49. The initial Hush of CO, was thcrcby excluded. The regression equation was: J’ = (1 + k.Y. where J is cumulative CO2 yield (icg CO,-C mg-’ sample-C), .Y is incubation period (days). k is the decomposition rate constant ()lg CO,-C mg ~’ sample-c per day) and N is a constant. For all incubated samples. the decomposition rate constant was higher for the Ronhave than for the Studsgaard samples. The decomposability of the
CO, evolution from soil size fractions isolated from the straw burned and >trau treatments (Studsgaard). Lines are fitted by linear regression (see Table 2).
CO: evolution from size fractions
s
RBCHAK
Fig. 3. Cumulative CO2 evolution from soil size fractions isolated from the straw burned and incorporated treatments fRm~havt$. Lines are fitted by linear regression (see Table 2).
soil organic matter decreased in the order: sand > clay 2 whole soil > silt. Straw incorporation increased the decomposition rate of whole soil and sand organic matter, whereas only minor efkts were noted for clay and silt. The correlation coefficients Table 2. The decomposition rate conr~ant k (pg CO& mg ’ fra&on-Cday ‘) for the739 day period. k was obtained from the regression equation ,r = ti+ kx, where ,r is the eumu~ative CO, production, .Y is the incubation period and CIis a constant. I is the SWW
Studsgaard k r
trcntmcnl
Clay SilI Sand Whole soil
&IS 0.17 0.08 0.09 0.19 0.40 0.11 O.f7
Burned Incocpornted Burned Incorporated Burned Incorporated Burned Incoroorated
Ronhave k r
0.996 0.998 0.99x 0.997 0.986 0.983 0.996 0.995
0.35 0.39 0.23 0.28 0.49 1.30 0.28 0.38
0.994 0.993 0.999 0.987 0.994 0.995 0.996 0.995
straw
(Table 2) between cumulative COz yield and decomposition period were high (0.9834999). Table 3 shows the calculated respiration tosses after 50 days of incubation. using the decomposition rate constants in TabIc 2. The CO2 yield was calculated as pg CO,-C g-’ soil to allow comparison of whole soil CO, production with the sum of CO2 evolved from the different size fractions. During a 50day incubation, 0.6 and 0.9% of the Studsgaard whole soi{ C is respired by the straw burned and straw incorporated soils, respectively. Corresponding values for Rernhave are 1.4 and 1.9%. Losses of CO,-C based on the addition of contributions from size Fractions are 0.7% (straw burned) and 0.8% (straw incorporated) for the Studsgaard site, and 1.6% (straw burned) and 2.0% {straw incorporated) for the Rnnhave site. Thus differences between sum of fractions and respiration of the intact soil are relatively
small.
The clay site fraction of Studsgaard
made up
Table 3. The amount of CO: respired from Studsgaard and llrrnhave whole soils and particlesizefracrionrafter SOdays of incubation at ?O C. calculated by using the decomposition rate constants k given in Table 2 Straw
treatment Clay silt Sand
Burned Incorporated Burned Incorporated Burned IZ?COrpOf&d
Sum of fractians Whole soil
Burned Incorporated Burned incorporated
Caktrlared as (fraction CO&’
Calculated respirarion lossesafter 50 days Relative dist~bu~~on. %’ % of sample-C Ronbave Studsgaard Ronhavc Sttidsgaard Rsnhavt
~g CO,-Cg-‘soil
Studsgarrcl 106 109 40 43 16 36 162 I88 I?9 202
sum of fraction CO/Z> * ilx).
If0 148 38 4P 10 I: 237 164 241
0.9 a.9 0.4 0.S 0.9 2.0 0.7 0.8 0.6 0.9
1.8 2.0 1.2 1.4 2.4 6.5 1.6 2.0 I.4 I.9
65 58 25 23 50 19
73 62 ?I ?I 6 II
BEST T. CHRISTENSEN
424
SO-52% of the soil C (Table I) and B-65% of the respiration loss (Table 3). Silt accounted for 3942% of the soil C and -‘3-‘5% _ of the soil respiration loss; values for sand were 746 of the soil C and IO-19% of the respiration. A similar pattern was observed for the Rtanhave soil. DISCL’SSION
DaLaune et al. (1981) showed that the pH and redox potential (aeration status) of incubated soilwater suspensions strongly influenced CO2 production during the decomposition of added plant material, indicating the importance of minimizing pH differences when comparing CO2 productions from incubated soil samples. The pH of the samples employed in this study ranged from 6.5 to 7.1, the Studsgaard clay fraction being somewhat more acid. However, these relatively small differences in sample pH are not sufficient to explain the observed differences in respiration rates. The distribution of C and N between soil particle size fractions is in accordance with earlier results from a number of arable soils (Christensen. 1985, 1957). The wutcr-soluble fraction contained between I and 2% of the rccovcred C. For soils incubated with ‘“C-labelled organic substrates for 5-6 yr, Christcnsen and Sorensen (1955) found that 4-9% of the labellcd C was water-soluble. Studies on the distribution of labelled and native C (Christensen and Sorensen, 1985) and N (Christensen and Sorensen, 1986) demonstrated that even after 5-6 yr of incubation, clay contained higher proportions of labelled than of nattve organic matter; the opposite vvas true for silt. The present study indicates that the native organic matter is less soluble in water than that originating from labelled substrates incubated with soil for 5-6 yr. The decomposability of the soil organic matter decreased in the order: sand > clay > silt. Thus siftassociated organic matter was found to be more resistant to microbial attack than that of clay. The sand fraction showed least stability, the organic matter of this fraction being dominated by particulate plant remains (“free organic matter”). Despite the higher decomposability of organic matter in the Ronhave soil compared with the Studsgaard soil, the relative decomposition pattern was similar for the various size fractions of the two soils. Straw incorporation also affected the decomposition rate constants of the size fractions in a similar way; clay and silt was only slightly affected while straw incorporation increased the decomposability of the sand organic matter. Models of the turnover of organic matter in soil often include pools of physically and chemically stabilized organic matter (e.g. Anderson, 1979; Jenkinson and Rayner, 1977; Parton er al.. 1983; van Veen et al., 1985). Christensen (1985) suggested that changes in the stability of physically protected soil organic matter may occur during the dispersion that preceed the isolation of soil size fractions, but the quantitative significance of physically-protected organic matter in soil is not known in detail. It could be argued that the CO: evolution measured during incubation of soil size fractions reflects the chemical
stability of the organic matter, and that the difference between the sum of CO2 from size fractions and COfrom whole soil represents the decomposition of physically-stabilized organic matter, released during the fractionation procedure. Inspection of Table 3 shows however that this difference is small and sometimes negative. Consequently, this study does not provide evidence for the existence of a large pool of organic matter stabilized in aggregates. Alternatively, the pool of phystcally-protected organic matter may be envisaged as composed of organic matter that becomes stabilized by its intimate association with the primary soil particles. and that this association is not broken down by the dispersion treatment applied. Finally. it may be argued that the comparison of incubated size fractions and whole soils is not entirely valid since the physical and chemical conditions of the various samples during the incu~tion are not exactly identical, despite the addition of nutrients and the admixture of purified sand to the clay and silt fractions. For whole soils as well as size fractions, an initial flush of CO! was observed during the G-7 day period (Figs 1-3). This CO: flush may reflect the decomposition of organic matter that has been made more available by soil drying. including the decomposition of lysed microbial biomass. For the soil size fractions, however, part of the biomass ceil content is probably removed in the water soluble fraction. The studies of Chichester (1969 and 1970) and Cameron and Posncr (1979) demonstrated that increasing proportions of the N in a given particle size class were mineralized with decreasing particle sizes. In contrast, Lowe and Hinds (1983) stated that percentages of total fraction N and S released during incubation did not show any systematic changes with decreasing particle sizes, although, on average, silt (2-50 11m) released less mineral N and S than the clay fractions (I-2pm, 0.2-l pm. and <0.2pm). The soil fraction > SOltm was not examined in their study. In the present study, the highest CO2 production rate was from the sand size fractions. This observation does not conflict with the results cited above showing that the sand fraction had the lowest mineralization rate of N. The high CO, evolution from the sand fraction indicates a high decomposability of the organic matter which may be coupled with an immobilization of N. Adams (1980) showed that the addition of macro-organic matter (organic materials separated by water flotation and retained on a 250pm sieve) to a soil lead to an increased CO2 production and a decreased N mine~li~ation. Most of the organic matter separated out with the sand size fraction is probably comparable with the macroorganic matter fraction of Adams (1980).
REFERENCES
Adams T. McM. (1980) Macro organic matter content of some Northern Ireland soils. Research
Record
of Agricuirural
28, I-1 I.
Amato M. and Ladd J. N. (1980) Studies of nitrogen immobilization and mineralization in calcareous soils-V. Formation and distrjbution of isotope&belled biomass during decomposition of W- and ‘JN-labelled plant material. Soil Bic~lqqyI_?ffioc~wtentisrr~ 12, 105-41 I.
CO, evolution Anderson D. W. (1979) Processes of humus formation and transformation in soils of The Canadian Great Plains. Journul oi Soil Science 30, 77-84. Anderson D. W. and Paul E. A. (1984) Organo-mineral complexes and their study by radiocarbon dating. Soil Science Society of America Journal 18, 298-301. Anderson D. W.. Saggar S., Bettany J. R. and Stewart J. W. B. (1981) Particle stze fractions and their use in studies of soil organic matter: I. The nature and distribution of forms of carbon. nitrogen and sulfur. Soil Science .Socief.t of .4merica Journal 45. 767-772. Cameron R. S. and Posner A. M. (1979) Mineralisable organic nitrogen in soil fractionated according to particle size. Journul of Soil Science 30. 565-577. Chichester F. W. (1969) Nitrogen in soil organo-mineral sedimentation fractions. Soil Science 107. 356-363. Chichester F. W. (1970) Transformations of fertilizer nitrogen in soil II. Total and ‘“N-labelled nitrogen of soil organo-mineral sedimentation fractions. Plant and Soil 33, 437356. Christensen B. T. (1985) Carbon and nitrogen in particle size fractions isolated from Danish arable soils by ultrasonic dispersion and gravity-sedimentation. Acfa Agriculwoe Scandinarica 35. I75- 187. Christensen B. T. (1986) Straw incorporation and soil organic matter in macro-aggregates and particle size separates. Journal oj’ Soil Science 37, 125-135. Christensen B. T. (1987) The use of particle size fractions in soil organic matter studies. In Soil Organic Matter and Soil Productkicy (J. Cooley. Ed) (Intecol symposium. Ekcnas. Flen, Swcdcn. June 56. 1986). lntecol Bulletin (in press). Christensen B. T. and Sorensen L. H. (1985) The distribution of native and labelled carbon bctwccn soil particle size fractions isolated from long-term incubation experiments. Journrrl of Soil Science 36. 2 19-229. Christensen B. T. and Sorensen L. fl. (1986) Nitrogen in particle size fractions ofsoils incubated for five years with Journal of Soil “N-ammonium and “C-hcmiccllulosc. .%irnw 37, Z-1I 147. DeLaune R. D.. Reddy C. N. and Patrick W. H. Jr (1981) Organic matter decomposition in soil as influenced by pH
fr‘om size fractions
435
and redox conditions. Soil Biology & Biochemislry 13, 533-53-I. Jenkinson D. S. and Rayner J. H. (1977) The turnover of soil organic matter in some of the Rothamsted classical experiments. Soil Science 123, 298-305. Lowe L. E. and Hinds A. A. (1983) The mineralization of nitrogen and sulphur from particle-size separates of gleysolic soils. Canadian Journal of Soil Science 63, 761-766. Parton W. J.. Persson J. and- Anderson D. W. (1983) Simulation of organic matter changes in Swedish soils. In Anul.vsis of Ecological Systems: Slate-of-the-Arr in Ecological Modelling (W. K. Lauenroth et al.. Eds). pp. 5 I I-5 16. Elsevier. Amsterdam. Powlson D. S. and Jenkinson D. S. (1981) A comparison of the organic matter. biomass. adenosine triphosphate and mineralizable nitrogen contents of ploughed and directdrilled soils. Journal of Agricultural Science, Cambridge 97. 713-721. Powlson D. S.. Brookes P. C. and Christensen B. T. (1987) Measurement of soil microbial biomass provides an early indication of changes in total soil organic matter due to straw incorporation. Soil Biology & Biochemistry 19, 159-164. Tiessen H. and Stewart J. W. B. (1983) Particle-size fractions and their use in studies of soil organic matter. Il. Cultivation effects on organic matter composition in size fractions. Soil Science Socki! of America Journul 47. 509-5 14. Tiessen H.. Karamanos R. E.. Stewart J. W. B. and Selles F. (1984) Natural nitrogen-15 abundance as an indicator of soil organic matter transformations in native and cultivated soils. Soil Science Sociery of Americu Journul 48, 312-315. van Veen J. A., Ladd J. N. and Amato M. (1985) Turnover of carbon and nitrogen through the microbial biomass in a sandy loam and a clay soil incubated with [“C(U)]glucose and (“N](NH+)$O, under different moisture regimes. Soil Biology & Biochemisrry 17, 741-756. Williams 8. L. (1983) The nitrogen content of particle size fractions separated from peat and its rate of mineralization during incubation. Journui of Soil Science 34, 113-125.
1987
Copyright
0038-0717 97 53.00 + 0.00 C 1987 Pcrgamon Journals Ltd
DECOMPOSABILITY OF ORGANIC MATTER IN PARTICLE SIZE FRACTIONS FROM FIELD SOILS WITH STRAW INCORPORATION Askov
Experimental
BENT T. CHRISTENSEN Vejenvej 55, DK-6600
Station,
Vejen. Denmark
(Accepted 25 October 1986)
Summary-Soils from two field experiments on straw disposal were fractionated according to particle size using ultrasonic dispersion and gravity-sedimentation in water. Samples of whole soils, clay, silt and sand-size fractions were held for 49 days at 2O’C and the CO: evolution measured on 14 dates by gas chromatography. Recovery of soil solids. C and N was 99, 98 and 93%. respectively. Most of the soil C and N was in the clay ( < 2 pm), (loamy sand. 50% C and 56% N: sandy loam, 65% C and 68% N), the silt (2-20 pm) having smaller proportions (loamy sand. 41% C and 38% N; sandy loam. 29% C and 27% N). The sand fraction (20-6000 pm) accounted for 4-7% of the organic matter, and I-24b of the C was water soluble. Straw incorporation generally increased the C and N content of whole soils and size fractions. The decomposition rate constants were higher for the sandy loam than for the loamy sand soil. For both soils. the decomposability of the organic matter decreased in the order: sand > clay > whole soil > silt. Straw incorporation increased the decomposition rate of whole soil and sand organic matter. whereas the effect of straw on clay and silt respiration was small. Between 58 and 73% of the respiration was from clay, 2l-25% from silt and 619% from the sltnd size fraction
size. The CO2 production during incubation of peat size fractions (Williams, 1983) and macro-organic matter isolated from soil (Adams, 1980) has been examined, and Amato and Ladd (1980) studied the biomass carbon of size fractions by the chloroform fumigation method, but no reports on the carbon mineralization from size fractions of arable soils could be traced. Studies on the effect of cultivation (Tiessen and Stewart, 1983; Tiessen et al., 1984), straw incorporation (Christensen, 1986), mineral fertilizer and animal manure addition (Christensen, 1987) on the distribution of organic matter between soil size frdCtions have shown that the various size fractions are affected differently when soil organic matter levels change. These and other studies (Anderson and Paul, 1984; Anderson et al., 1981; Christensen and Sorensen. 1985, 1986) have indicated that different soil size fractions play different roles in soil organic matter stabilization and turnover, the clay bound organic matter probably being more labile than that associated with silt. To determine more directlv the differences in stability of organic matter associated with different soil size fractions, I have examined CO; evolution during incubation of clay. silt, and sand-stzed fractions and whole soil samples. Soil was taken from field experiments on straw disposal (straw burned or incorporated into soil annually for I8 yr). Soil from these experiments have been included in previous studies of soil microbial biomass and activity (Powlson er al.. 1987). and of the organic matter content of macroaggregates and particle size fractions (Christensen. 1986).
Nutrient cycling in soil is closely related to organic matter turnover. carbon metabolism playing a key role in mineralization and immobilization processes of plant nutrients. The saprotrophic soil organisms, acting as a driving force in nutrient cycling, rely on available organic compounds as a source of energy and carbon. Consequently, knowledge about the stabilization and subsequent decomposability of organic matter in soil is indispensable for understanding the fate of plant nutrients added to soil in fertilizers. crop rcsiducs and animal manure. The decomposability of soil organic matter is most often studied by using a “whole soil approach” in which the CO2 production or 0, consumption of unfractionatcd soil samples are assessed. However, additional information may be gained by using soil fractions, provided that the obtained fractions can be related to structural or functional entities in soil and thereby also to biological turnover. The inability to fulfil these conditions is considered as a major drawback of most chemical fractionation schemes applied to soil organic matter. Physical fractionation of soil according to particle sizes has proved to be a useful tool in the study of soil organic matter, revealing differences both in the structural and dynamic properties of organic matter from different soils and size fractions (Christensen, 1987). Cameron and Posner (1979). Chichester (1969, 1970) and Lowe and Hinds (1983) observed that upon incubation in the laboratory, the mineralization of organic nitrogen and sulphur in various soil size fractions tend to increase with decreasing particle 429
430
BEST T. CHRISTESSES \lATERI.ALS
AXD
\lETHODS
Soils Soil was sampled in two long-term field experiments on straw disposal. Both experrments grew continuous spring barley (Hordezmz rulgare L.) and included the treatments: straw and stubble incorporated by ploughing (5 t dry matter ha-’ yr-‘). and straw and stubble burned. The experiments were established in 1966 on a loamy sand soil (Studsgaard site; 6% clay, 7% silt, 4% OM, C to N ratio IS) and on a sandy loam (Ranhave site; 14% clay. 15% silt, 2% OM, C to N ratio IO). Ten soil cores (dia 45 mm) were taken from the O-25 cm layer of each replicate treatment (4 repl.) and bulked to give one sample per replicate. The sampling was carried out in early April 1984 before drilling and fertilization, and followed the equivalent sampling depth concept (Powlson and Jenkinson, 1981). After sieving the moist soils through a 6mm screen, soil samples were air-dried and finally dried at 40°C for 5 days. Isolution of parricle size fractions Particle size fractions wcrc isolated by dispersing 30 g of soil ultrasonically (300 W for IS min) in I50 ml of water using a probe-type disintegrator (Labsonic 1510, B. Braun Mclsungcn, F.R.G.). Claysize ( < 2 jtm) and silt-size (2-20 itm) fractions wcrc obtained by rcpcatcd gravitational sedimentation in water; the sand-size (20-6000 /lrn) fraction was rccovered as the scdimcnt left in the scdimcntation cylinder after the isolation of clay and silt. The procedure is described in detail by Christensen (1985). The supernatant solutions remaining after the clay suspensions had been centrifuged were concentrated on a rotary evaporator and taken as the water soluble fraction of the soil. Soil fractions wcrc dried at 40C for 5 days, weighed and gently crushed, and then bulked to give one clay, silt and sand sample per treatment and soil type. Inchrrion procethtre Whole soils and size samples wcrc incubated at 20 C for 49 days in 120 ml incubation flasks fitted with air-tight septum caps. Bcforc incubation, I ml of a nutrient solution (containing 2.0 mg N, I .6 mg P, I .5 mg K. 0.5 mg Ca. 0.2 mg Mg and 0.3 mg S) was added to each flask together with I ml of a soil inoculum (100 g of fresh soil shaken with 1000 ml of water and left to stand overnight). The nutrient solution was adjusted to pH 7 before use. For whole soil and sand size samples, log was incubated. whcrcas 2g of clay or silt material was used. Moreover, the clay and stlt was mixed with 8 g of puriticd sand (0.060.50 mm) bcforc incubation to improve aeration in thcsc samples. The height of the sample in the flask was about 5 mm. Blanks containing purified sand (log), nutrient solution and soil suspension (I ml of each) were included. All incubations wcrc run in duplicate. The air space of the flasks during the incubation was I I4 ml. assuming a sample density of 2.6 g ml-‘. Septum caps were removed on day I4 and day 30, the atmosphere of the flask rcplaccd by ambient air. and
new caps mounted. Thereby the CO. concentration the flask was kept below 3.54b (v ;).
in
Determination of CO2 The CO: concentrations in the flasks were determined on days 0, I, 2. 3, 4, 7. II. 14. IS. 21. 28. 30. 37 and 49. When the atmosphere of the flasks were replaced. the CO? concentration was determined before and after the replacement. The CO: concentration was measured on a gas chromatograph (ML CC 82-22. Mikrolab Aarhus. Denmark) fitted with a thermal conductivity detector and I.4 m glass column (i.d. 3 mm) containing “Chromosorb 101” (8OjlOO mesh. Manville Corp.). The reference column was molecular sieve (5 A. SO/l00 mesh); the carrier gas (He) flow was 27.5 ml min-I. Gas samples were taken from the incubation flasks by inserting a gas-tight syringe with a sideport needle through the septum cap. removing 0.5 ml of air and injecting the sample into the gas chromatograph. Sample CO: concentration was dctermined from peak areas (calculated by a computrrcoupled intergrator). standard CO:-He mixtures providing. the reference areas. Follovvmg gas sampling. the gas volume taken for analysis was replaced by ambient air (0.033% CO:) in order to reduce changes in air pressure within the incubation flask. The percentage of CO, in the flask atmosphere was converted to ilg CO:-C mg-’ sample-C after subtracting blank CO, concentration. knowing the free air space of the flask, the equation of state and the carbon content of the incubated sample. From this the quantity of CO: evolved up to the time of sampling was calculated. Ciremicul unuIw~.s Carbon contents of whole soils and size fractions were determined with a Leco Carbon Analyser coupled to an infrared CO: detector: S contents with a Tecator total-N analyscr using a Kjeldahl method (Hg catalyst). The C content of the water soluble fraction was determined with a Dohrman TOC analyser (DC 54) using unfiltcrcd subsumplcs. The pH of whole soil and size fraction samples was determined I h after mixing the sample with dcmineralized water (sample to water ratio. I to IO).
RESULTS
The pH of the soil size fractions was 6.5-6.6 except for the Studsgaard clay fraction which had a pH value of 5.9 (Table I). Whole soil pH was near neutral. Recovery of soil solids, C and N averaged 99.4. 98.4 and 93.0%. respectively. Most of the recovered soil C (Studsgaard. 48-52%; Ronhavc. 6466%) and N (Studsgaard. 55-57%: Ronhavc, 67-69%) w;ts found in the clay size fraction. while the slit accounted for smaller fractions (Studsgaard. 3942% C and 37-39% N; Ronhavc. 29% C and 2627% N). The sand fraction contained 3.7-7.3% of the C and N. The water soluble fraction of the Studsgaard and Ronhave soil made up 0.9-1.3 and l.6-2.0% of the recovered C. rcspcctivcly.
Ronhwe soil Fraction Clay Sill SwId Water soluble Sum OF lkiclions Whole soil Recovw ?/.
Sludsguard soil Fraction Cluy Silt Sand Water soluble Sum of lraclions Whole soil Recovery %
7.1
6.5 6.5
6.6
7.0
5.Y 6.5 6.6
PII’
14.6(1.3) I s.5 (0.3) 69.3 ( I .h)
9Y.4 (0.2)
5.7 (0.3) 7.6 (0. I) X6. I (0.3)
Y9.3 (0. I)
14.5(I.K) 15.7 (0.4) 69. I (2.2)
Y9.6(0.1)
6.1 (0.2 1.5 (0. I) X6.0 (0.2)
7.43 3.21 0.42 0.18 II.3 Il.7 Y6.b
Il.74 Y.95 1.72 0.22 23.6 23.4 100.9
7.58 3.49 O.b? 0.24 11.1) 12.7 93.9
12.79 9.65 I.80 0.33 24.6 24.1 102.1
mgg ’ soil SKiW Rurned IncorP.
cilrbon
65.8 28.9 3.1 I.6
49.1 42. I 7.3 0.9
63.5 29.3 5.2 2.0
52.1 39.3 1.3 1.3
-___ RelaGvr distribution, %b S&w IncorP. Burned
--_--
0.639
0.281 0.055 ND I.10 I.26 x7. I
0.762
0.454 0.077 ND 1.17 1.16 loo.9
0.767 0.311 0.062 ND I.14 1.27 X9.X
0.714 0.468 0.069 ND I.25 1.33 94.1
69.4 25.6 5.0
54.6 311.8 6.6
67.3 27.3 5.4
57. I 31.4 5.5
Relitlive distribution, %b strvw Incorp. nurr1ell
Nitrogen .-_._ _...~
deviations (n = 4). The water soluble fraction
ml: 1: ’ soil SlrdW Burned Incorn.
Table I. Dry weight, pH, carbon and nitrogw content of whole roils and soil fractions. Yalues in parenthesis are aaxid was noi an&zed for nitrogen content (ND)
BENT T. CHRISTESSES
Fig. I. Cumulative CO, evolution from whole soil samples taken from the straw burned and stram incorporated trcttments (Studsgaard and Ronhave). Lines are fitted by linear rcgrcssion (see Table 2).
Stravv incorporation. compared with straw burning. increased the C and N content of nearly all soil fractions. The increases were however moderate as dcmonstratcd earlier by Christensen (1986) and Powlson rf al. (1957). Straw incorporation had a small intlucncc on the relative distribution of C and N bctu-ccn soil fractions. but this could not be tested statistically with the present experimental design. The CO2 evolution from incubated whole soils and size fractions is shown in Figs I. 2 and 3. The decomposition rate constant for the various samples
Fig. 2. Cumulative incorporated
(Table I) was calculated by simple linear rcgrcssion applied to the cumulative CO, yield for the period day 7 to day 49. The initial Hush of CO, was thcrcby excluded. The regression equation was: J’ = (1 + k.Y. where J is cumulative CO2 yield (icg CO,-C mg-’ sample-C), .Y is incubation period (days). k is the decomposition rate constant ()lg CO,-C mg ~’ sample-c per day) and N is a constant. For all incubated samples. the decomposition rate constant was higher for the Ronhave than for the Studsgaard samples. The decomposability of the
CO, evolution from soil size fractions isolated from the straw burned and >trau treatments (Studsgaard). Lines are fitted by linear regression (see Table 2).
CO: evolution from size fractions
s
RBCHAK
Fig. 3. Cumulative CO2 evolution from soil size fractions isolated from the straw burned and incorporated treatments fRm~havt$. Lines are fitted by linear regression (see Table 2).
soil organic matter decreased in the order: sand > clay 2 whole soil > silt. Straw incorporation increased the decomposition rate of whole soil and sand organic matter, whereas only minor efkts were noted for clay and silt. The correlation coefficients Table 2. The decomposition rate conr~ant k (pg CO& mg ’ fra&on-Cday ‘) for the739 day period. k was obtained from the regression equation ,r = ti+ kx, where ,r is the eumu~ative CO, production, .Y is the incubation period and CIis a constant. I is the SWW
Studsgaard k r
trcntmcnl
Clay SilI Sand Whole soil
&IS 0.17 0.08 0.09 0.19 0.40 0.11 O.f7
Burned Incocpornted Burned Incorporated Burned Incorporated Burned Incoroorated
Ronhave k r
0.996 0.998 0.99x 0.997 0.986 0.983 0.996 0.995
0.35 0.39 0.23 0.28 0.49 1.30 0.28 0.38
0.994 0.993 0.999 0.987 0.994 0.995 0.996 0.995
straw
(Table 2) between cumulative COz yield and decomposition period were high (0.9834999). Table 3 shows the calculated respiration tosses after 50 days of incubation. using the decomposition rate constants in TabIc 2. The CO2 yield was calculated as pg CO,-C g-’ soil to allow comparison of whole soil CO, production with the sum of CO2 evolved from the different size fractions. During a 50day incubation, 0.6 and 0.9% of the Studsgaard whole soi{ C is respired by the straw burned and straw incorporated soils, respectively. Corresponding values for Rernhave are 1.4 and 1.9%. Losses of CO,-C based on the addition of contributions from size Fractions are 0.7% (straw burned) and 0.8% (straw incorporated) for the Studsgaard site, and 1.6% (straw burned) and 2.0% {straw incorporated) for the Rnnhave site. Thus differences between sum of fractions and respiration of the intact soil are relatively
small.
The clay site fraction of Studsgaard
made up
Table 3. The amount of CO: respired from Studsgaard and llrrnhave whole soils and particlesizefracrionrafter SOdays of incubation at ?O C. calculated by using the decomposition rate constants k given in Table 2 Straw
treatment Clay silt Sand
Burned Incorporated Burned Incorporated Burned IZ?COrpOf&d
Sum of fractians Whole soil
Burned Incorporated Burned incorporated
Caktrlared as (fraction CO&’
Calculated respirarion lossesafter 50 days Relative dist~bu~~on. %’ % of sample-C Ronbave Studsgaard Ronhavc Sttidsgaard Rsnhavt
~g CO,-Cg-‘soil
Studsgarrcl 106 109 40 43 16 36 162 I88 I?9 202
sum of fraction CO/Z> * ilx).
If0 148 38 4P 10 I: 237 164 241
0.9 a.9 0.4 0.S 0.9 2.0 0.7 0.8 0.6 0.9
1.8 2.0 1.2 1.4 2.4 6.5 1.6 2.0 I.4 I.9
65 58 25 23 50 19
73 62 ?I ?I 6 II
BEST T. CHRISTENSEN
424
SO-52% of the soil C (Table I) and B-65% of the respiration loss (Table 3). Silt accounted for 3942% of the soil C and -‘3-‘5% _ of the soil respiration loss; values for sand were 746 of the soil C and IO-19% of the respiration. A similar pattern was observed for the Rtanhave soil. DISCL’SSION
DaLaune et al. (1981) showed that the pH and redox potential (aeration status) of incubated soilwater suspensions strongly influenced CO2 production during the decomposition of added plant material, indicating the importance of minimizing pH differences when comparing CO2 productions from incubated soil samples. The pH of the samples employed in this study ranged from 6.5 to 7.1, the Studsgaard clay fraction being somewhat more acid. However, these relatively small differences in sample pH are not sufficient to explain the observed differences in respiration rates. The distribution of C and N between soil particle size fractions is in accordance with earlier results from a number of arable soils (Christensen. 1985, 1957). The wutcr-soluble fraction contained between I and 2% of the rccovcred C. For soils incubated with ‘“C-labelled organic substrates for 5-6 yr, Christcnsen and Sorensen (1955) found that 4-9% of the labellcd C was water-soluble. Studies on the distribution of labelled and native C (Christensen and Sorensen, 1985) and N (Christensen and Sorensen, 1986) demonstrated that even after 5-6 yr of incubation, clay contained higher proportions of labelled than of nattve organic matter; the opposite vvas true for silt. The present study indicates that the native organic matter is less soluble in water than that originating from labelled substrates incubated with soil for 5-6 yr. The decomposability of the soil organic matter decreased in the order: sand > clay > silt. Thus siftassociated organic matter was found to be more resistant to microbial attack than that of clay. The sand fraction showed least stability, the organic matter of this fraction being dominated by particulate plant remains (“free organic matter”). Despite the higher decomposability of organic matter in the Ronhave soil compared with the Studsgaard soil, the relative decomposition pattern was similar for the various size fractions of the two soils. Straw incorporation also affected the decomposition rate constants of the size fractions in a similar way; clay and silt was only slightly affected while straw incorporation increased the decomposability of the sand organic matter. Models of the turnover of organic matter in soil often include pools of physically and chemically stabilized organic matter (e.g. Anderson, 1979; Jenkinson and Rayner, 1977; Parton er al.. 1983; van Veen et al., 1985). Christensen (1985) suggested that changes in the stability of physically protected soil organic matter may occur during the dispersion that preceed the isolation of soil size fractions, but the quantitative significance of physically-protected organic matter in soil is not known in detail. It could be argued that the CO: evolution measured during incubation of soil size fractions reflects the chemical
stability of the organic matter, and that the difference between the sum of CO2 from size fractions and COfrom whole soil represents the decomposition of physically-stabilized organic matter, released during the fractionation procedure. Inspection of Table 3 shows however that this difference is small and sometimes negative. Consequently, this study does not provide evidence for the existence of a large pool of organic matter stabilized in aggregates. Alternatively, the pool of phystcally-protected organic matter may be envisaged as composed of organic matter that becomes stabilized by its intimate association with the primary soil particles. and that this association is not broken down by the dispersion treatment applied. Finally. it may be argued that the comparison of incubated size fractions and whole soils is not entirely valid since the physical and chemical conditions of the various samples during the incu~tion are not exactly identical, despite the addition of nutrients and the admixture of purified sand to the clay and silt fractions. For whole soils as well as size fractions, an initial flush of CO! was observed during the G-7 day period (Figs 1-3). This CO: flush may reflect the decomposition of organic matter that has been made more available by soil drying. including the decomposition of lysed microbial biomass. For the soil size fractions, however, part of the biomass ceil content is probably removed in the water soluble fraction. The studies of Chichester (1969 and 1970) and Cameron and Posncr (1979) demonstrated that increasing proportions of the N in a given particle size class were mineralized with decreasing particle sizes. In contrast, Lowe and Hinds (1983) stated that percentages of total fraction N and S released during incubation did not show any systematic changes with decreasing particle sizes, although, on average, silt (2-50 11m) released less mineral N and S than the clay fractions (I-2pm, 0.2-l pm. and <0.2pm). The soil fraction > SOltm was not examined in their study. In the present study, the highest CO2 production rate was from the sand size fractions. This observation does not conflict with the results cited above showing that the sand fraction had the lowest mineralization rate of N. The high CO, evolution from the sand fraction indicates a high decomposability of the organic matter which may be coupled with an immobilization of N. Adams (1980) showed that the addition of macro-organic matter (organic materials separated by water flotation and retained on a 250pm sieve) to a soil lead to an increased CO2 production and a decreased N mine~li~ation. Most of the organic matter separated out with the sand size fraction is probably comparable with the macroorganic matter fraction of Adams (1980).
REFERENCES
Adams T. McM. (1980) Macro organic matter content of some Northern Ireland soils. Research
Record
of Agricuirural
28, I-1 I.
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