The significance of organic separates to carbon dynamics and its modelling in some cultivated soils

Transcription

1 European Journal of Soil Science, December 1996, 47, The significance of organic separates to carbon dynamics and its modelling in some cultivated soils J. BALESDENT Unite de Science du Sol, INRA, 7826 Versailles Cedex, France; and Laboratoire de Bioge ochimie Isotopique, INRA -CNRS, Unniversite P. et M. Curie, ccl2, 4 Place Jussieu, Paris, Cedex 5, France Summary Soil organic matter (SOM) dynamics are usually described by compartmental models. We have sought SOM separates that might be related to SOM dynamic compartments. The turnover of C in various separates from long-term field experiments with maize was measured using the natural 13C labelling technique. The Rothamsted carbon model gave a good prediction of the observed C turnover. Primary particle-size fractions coarser than SO pm had short lives, and could be associated with the plant structural compartment of models. Water-extractable components, are enriched in young C but cannot be associated with labile compartments. None of the chemical separates obtained by acid hydrolysis, wet oxidation, thermic oxidation, pyrolysis or alkaline extraction, were enriched either in young or old C. The results showed neither a sequential relation between fulvic acids and humic acids nor a resistance of nonhydrolysable material. The range of lifetimes of soil C seems to be determined more by physical position and protection than by the chemical nature of SOM. Fractionnements des matizres organiques: apport I Ctude de la dynamique du carbone de sols cultivks et a sa modelisation RCsumC Les modeles de la dynamique du carbon des sols rkpartissent le carbone du sol en compartiments fonctionnels, de durkes de vie tres diffkrentes. On a cherchk B skparer matkriellement les composks de durkes de vie diffkrentes, afin de proposer des estimateurs de ces compartiments. Les vitesses de renouvellement sont mesurkes par la mkthode du marquage nature1 en 13C, dans des sols en culture ckrkaliere (mabs) provenant d expkrimentations de longue durke. Le modtle de Rothamsted donne une bonne reprksentation de la dynamique observke. Les fractions granulomktriques grossitres permettent de quantifier le compartiment structural d origine vkgktale des modtles. Les composks extractibles B l eau sont enrichis en composks jeunes, mais ne sont pas identifiables aux compartiments labiles. Concernant les mati2res organiques < SO pm, aucune des mkthodes classiques kvalukes (hydrolyses acides, oxydations humides, oxydations thermiques, pyrolyses, extraction alcalines) ne permettent de skparer des composks de vitesses de renouvellement diffkrentes. L analyse permet de rejeter l hypothbse de formation des acides humiques par condensation des acides fulviques. La localization et la protection physique des matieres organiques apparaot comme plus dkterminante pour leur vitesse de biodbgradation que leur appartenance B une famille chimique. Introduction The quantitative aspect of soil organic carbon dynamics is described by compartmental models such as those of Jenkinson Correspondence: balesden@versailles.inra.fr Received 18 December 1995; revised version accepted 11 March 1996 & Rayner (1977) and Parton et al. (1987). An important aim of studies of soil organic matter has been to separate the mathematical or conceptual compartments of these models. On one hand such separation could provide a means of evaluating or parameterizing compartment sizes at a given place. On the other hand it could help elucidate the pathways 1996 Blackwell Science Ltd. 485

2 Carbon dynamics in cultivated soil 487 in water with glass beads. The resulting particle-size fractions 2-2 pm and 5-2 pm were separated by wet sieving. From these fractions, light-density organic debris was separated from mineral sand by repeated flotation-panning in water (after Feller, 1979). This senaration gave organic debris and mineral sand fractions. The mineral fractions consisted mainly of quartz. Their carbon content was negligible in most cases ( <.1 % of soil C at Boigneville), but contained some mine-coal at La Minihe. These fractions were not taken into account in the study. The suspension containing the particles finer than 5 pm was then dispersed by ultrasonic treatment and size-separated. The comparison with the particle-size distribution obtained from the reference method demonstrated that dispersion was complete: the weight of inorganic material in each fraction did not differ from the weight obtained by the reference method by more than 1 mg g-' soil. The advantage of the method is that ultrasonic treatment is not applied on particulate organic matter >5pm, because it has been demonstrated that the energy required for complete clay dispersion resulted in the splitting of more than half the organic fraction > 5 pm by weight in C, into finer fractions (Balesdent et al., 1991). The carbon balance of the separation, including solubilized organic carbon, was 99.5 If: 1.%. Chemical separations were done on the -5 pm-sized fraction of five samples. The samples were: La Minihe (4 years), Boigneville (2 years and 2N) and the two C3 references. The -5-pm suspension was flocculated with 1 g 1-' CaC12. The flocs were freeze-dried then passed through a.1 mm sieve before subsampling for further extraction. The supernatant was defined as 'water-soluble' and freeze-dried for isotopic determination. Water extraction at 12 C (Autoclaving) was done as follows. Four-gram samples were added to 5 ml water in centrifuge stainless tubes and maintained at 12 C and.2 MPa in a autoclaving apparatus. Each sample was treated six times to obtain cumulative extraction periods of 1,4, 8, 12, 16 and 2 h. At each time step, the supernatant was centrifuged, re-extracted twice with water, and freeze-dried for analysis. Alkaline extraction was performed as follows. Twenty-gram samples were treated by four repeated extractions with 2 ml.1 M N%P M NaOH. The residue was washed and defined as 'humin'. Fine clay particles in the extract were flocculated by addition of 1 M KCl and added to humin. Extracts were acidified to ph 1.5. The precipitate was defined as 'humic acid'. The supernatant, defined as 'fulvic acid', was briefly dialysed against water in MWCO 1 membranes to remove salts. All fractions were freeze-dried. Hydrolysis, oxidation and pyrolysis were each done successively, up to five or six times, so as to attack approximately 1, 25, 4, 6, 75% of the initial organic carbon, respectively. This was obtained by treating a batch of subsamples with reagents of varying concentration and reactions for varying duration and temperature. Acid hydrolysis was done using the method of hydrolysis described by Cheshire (1979) for total sugar extraction. Some reaction conditions were changed to obtain incomplete hydrolysis. Six different extractions were carried out under the following conditions of time, H2S4 concentration and temperature:.5 h.5 M 2 C;.5 h 12 M 2 C; 2 h 12 M 2 C; 16 h 12 M 2 C; 16 h 12 M 2 C+2 h 1 M 1 C; 16 h 12 M 2 C + 8 h 1 M 1 C. Residues were washed and freeze-dried for mass, carbon and isotope determination. The carbon content in the supernatant was measured for carbon balance then discarded. The carbon was oxidized with hydrogen peroxide oxidation as follows. Five-gram samples were added to 3 ml water and a variable volume of a H22 solution at.27 g g-', left for 16 h at 2 C then heated at 96 C for 4 min and finally at 15 C for 4 h. Five or six separate extractions were done with an increasing quantity of H22 added to the sample, i.e. 3.5, 8.8, 13.2, 26.4 or 44.1 mmol. The residues were freeze-dried, together with solubilized carbon. Thermal oxidation under air was done thermogravimetrically on.5-g samples heated at the rate of 2 C min-' up to 2, 23, 26, 29, 325 or 37 C. Pyrolysis under nitrogen was done similarly up to 27, 3, 38,425 or 6 C. Analysis Total carbon of solids was determined either by dry combustion with the evolved COz measured colometrically, or by dry combustion in an autoanalyser (Erba NA15). Total organic carbon in solution was determined by high temperature catalytic combustion, followed by CO2 NDIR measurement (Dohrmann DC 19). Stable isotope ratios were analysed either in a SIRA 9 (VG-Instruments) on COz obtained by combustion in sealed quartz tubes with CuO at 85"C, or by dry combustion in an autoanalyser (Erba NA15) coupled with a SIRA 1 (VG-Instruments) Isotope Ratio Mass Spectrometer. 6I3C was calculated as: where R = 13C+ 12C. The reference is the PDB standard. Calculation of the proportion of new organic carbon in the fractions The proportion of C derived from maize in a soil sample or fraction was calculated according to Balesdent & Mariotti ( 1996): 6 = f. A + 6,f or f = (6-6,f) -F A, (2) where f stands for the proportion of maize-derived C in the sample, 6 for the measured 6I3C of the sample, Greffor the 6I3C of the corresponding sample or fraction separated from the C3 reference soil, and A for the difference between maize plant material and C3 plant material of the reference plot, during the 1996 Blackwell Science Ltd, European Journal of Soil Science, 47,

3 488 J. Balesdent experiment. Equation 2 is unbiased even if I3C enrichment occurs in the soil during C decomposition, provided that the two soils to be compared have similar C dynamics. Such a condition is reached at La Minitre and Boigneville, since maize introduction on these cereal soils induced no quantitative change in C content, and probably no qualitative change regarding decay or the biochemistry of the decay products (Broder & Wagner, 1988). At Auzeville, Equation 2 may be slightly biased due to a possible isotopic difference between reference grassland soil and initial experiment conditions. For the sites of La Minitre and Boigneville, 6,f is the 613C of the equivalent fraction separated from the reference plot. Since some extractions cause isotopic enrichment (i.e. wet oxidation and acid hydrolysis), 6,f was calculated as the 6I3C of the reference sample after the same extend of reaction as in the treatment of the sample with maize. The quantity A was estimated in Boigneville as the difference between maize and wheat material grown in these fields in 1987, i.e (-26.3) = %. At La Minih-e A was estimated from mean soil organic material > 2 mm (dead leaves and roots), i.e (-27.6) = +14.8%. At Auzeville the mean maize value was -12.3% in 1985, and C3 plant material value was estimated from soil material coarser than 2 mm, giving A = (-26.6) = +14.3%. had been estimated previously as 446 g a-i, split into 29 g ax' as shoots returned in October and 156 g a-' incorporated by the roots (Balesdent & Balabane, 1992), regularly from June through September. This set of values leads to an overestimation of total carbon. To fit total carbon (4. kg m-*), I estimated the yearly carbon input to the soil as 368 g mp2 a-i. The model was run with a monthly time-step over 25 years with total OM and with new OM from maize only to compute f. The upper layer with no tillage was simulated assuming an additional carbon input to fit total carbon concentration in the layer at the date of analysis. The time scale throughout this paper started on October preceding the first maize planting. Results and discussions Turnover of particle-size fractions Figure 1 shows the progressive enrichment of particle-size fractions in new C. Since the total C did not vary, this describes the turnover of the fraction. The proportion of residual old, C3-derived, OM would be represented by the inverse curves. The turnover of material coarser than 2 mm was complete in less than 1 year. The turnover rates decreased with decreasing particle-size. The data was first fitted to single exponential kinetics according to the least squares criterion Errors on f Only the residual carbon was analysed after step-by-step exfaction or attack, by hydrolysis, wet oxidation, dry oxidation or pyrolysis. In these cases, the isotopic composition of the extracted or lost material was calculated by mass balance. The systematic error was based on an error of.5x on each 613C used in the calculation off. When f is obtained by direct measurement its error is 2 x.5ia. When it is calculated by difference the error is larger. The error in A was not taken into account in Table 3 because A is common to all extracts. Rothamsted carbon model To relate the different fractions to current model compartments the Rothamsted model of Jenkinson & Rayner (1977) was used, as described in Jenkinson et af. (1992) (ROTHC-26.3). The soil was estimated to be in steady state on a yearly basis. Monthly parameters to run the model were calculated for the mean site and climate conditions for La Minikre and Boigneville. The DPM: RPM and the BIO : HUM formation ratios were equal to 1.44 and.85, respectively. The ratio of CO2-C to BIO + HUM was set equal 3.71, according to average soil clay content, 18.5%. The 'plant retainment rate modifying factor' was set to.6 in June to August inclusive, and 1.O otherwise. The yearly average decay rates were finally k ~ = p 9.1 ~ a-', k ~ = p.27 ~ a-i, kero =.6a-', =.18a-', k ~ = o ~ a-i. IOM was arbitrary set at 38 g C m-2 (Jenkinson et al., 1992). The carbon returned to the soil Time /years dig. 1 Evolution with time of the proportion of new carbon, as estimated from the I3C natural abundance, in particle-size fractions from French maize-cultivated soils. Fitting procedures are described in text Blackwell Science Ltd, European Journal of Soil Science, 47,

4 Carbon dynamics in cultivated soil 489 Table 2 The turnover of C in the sizefractions. The quantity f is the proportion of new C estimated from I3C abundance, t is time, and k is the reciprocal of the mean age of C in the fraction, as obtained from the regression, followed by 95% confidence interval and number of observations Size Average C Cb C:Nb IPm Ikg rn- Irng g- ratio Regression year Ik-l > a. I I Light fraction. bvalues are means from La Miniere f = I -exp(-kt) f = 1 -exp(-kt) f = 1 -exp(-kt) f = 1 -exp(-kr) f=kt f=kt f=kt f=kt 2.3k.5.32 k.3.54 f.5.16 f f.5.16f.4.12f f.4 (Table 2). This assumes that each compartment is homogenous. The mean turnover times would be.5, 3, 18 and 63 years for fractions > 2, 2-2, 5-2 and -5 pm, respectively. In fact, fractions 2-2 pm and 5-2 pm exhibited little heterogeneity. These fractions could contain some carbon with a slow turnover. The quantities of slow carbon were estimated by fitting the data with a linear combination of two exponentials, one rapid, the other slow, with the same rate constant as fraction -5 pm. The slow material accounted for 15% of fraction 2-2 pm and 35% of fraction 5-2 pm. The results of these adjustments are those shown in Fig. 1. The incorporation of new C in fraction 5-2 pm was slower in the two first years than thereafter. This shows that the fraction is largely fed by the 2-2 pm fraction. The incorporation of new C into -5 pm fraction was progressive, almost linear over the 23 years. Silt- and clay-sized fractions were separated from a few samples, mainly from the site of Auzeville. The incorporation of new C (Table 2) is very similar in the four fractions, with a slightly more rapid incorporation in coarse silts and fine clay. Watersoluble C obtained from particle-size fractionation accounted for approximately 1.5% of soil C (Table 3). This fraction is younger than organo-mineral associations < 5 pm, but contains considerable old C. After 2 years, more than half of the soluble C was still derived from old OM. Turnover of chemical separates from fraction -5 pm (Table 3) Water at 12 C solubilized up to 3% of the C. Extracts were enriched in young C in the first hour, and then the age of the extracted C increased progressively, towards the age of the residue after 24-h extraction. All fractions obtained by alkaline extraction had similar kinetics of enrichment in new C. The youngest components are humic acids; fulvic acids are older. Since 3% was lost (smallest molecules) during dialysis, the bulk fulvic acids might have been younger. This result, nevertheless, does not accord with the theory that postulates fulvic acids are progressively condensed into humic acids. The result agrees better with a theory of humic molecule genesis by simultaneous condensation of small molecules and degradation of large molecules (Kogel-Knabner, 1992). Residual humin had the same characteristics as the bulk sample. Humin is classically described as heterogeneous, containing plant and microorganisms debris, as well as strongly humified material (Anderson & Paul, 1984). It is concluded that a scheme that would link humin, HA and FA by any sequential relation would be false if applied to these separates, each taken as a whole, in this soil. Their separation can be of no help in the investigation of C turnover. Acid hydrolysis has often been proposed for concentrating old resistant OM. It is based on the notion that resistance of OM to biological attack is due to chemical resistance. The most biodegradable substrates (i.e. carbohydrates and proteins) are easily hydrolysable (Golchin et al., 1995). In the present study, the differences between fractions are small. First hydrolysates appear enriched in young C, and the residue enriched in old C only in the 2-year-old samples. There is here no concordance between biodegradability and hydrolysability. There are several possible explanations for these findings. Non-hydrolysable material could contain both highly condensed, old humic material and unaltered, relatively young labile lignin components, as well as aliphatic components of various ages (Schulten & Hempfling, 1992; Kogel-Knabner, 1992). In my method, the recondensation of hydrolysabfe material during the extraction was prevented by a two-step extraction. With regard to hydrolysable material, a substantial part of the carbohydrates (cellulose and hemi-cellulose) was first removed by particle-size fractionation before the treatment. Carbohydrate or proteinaceous chains may be protected from biodegradation by association with either humic material or inorganic colloids (Oades, 1995). Wet oxidation has been suggested as a way to attack accessible OM, leaving OM that would be inaccessible to exoenzymes. Interlayer OM would thus resist wet oxidation (Righi et al., 1995). The interpretation of the results is difficult because 5- and 2-year-old samples behave in a contrary way. The method might separate young OM in the early stage of attack. The final residue is enriched in old OM, but is nevertheless far from corresponding to very old or inert OM. The 1996 Blackwell Science Ltd, European Journal of Soil Science, 47,

5 49 J. Balesdent Rothamsted model compartments DPM RPM 136 BIO 2 HUM 748 IOM 96 Particle-size fractions > 5 pm pm 82 I Water soluble 13 Chemical separates of fraction -5 pn Autoclaving (H2, 12 C) -1 h h h h 28 Residue 728 Alkaline extracts Fulvic acids 157 Humic acids 133 Humin 655 Sequential acid hydrolysis (H2SO4 ) Extracted, 2 h 2"Ca 257 Extracted. 8 h 1 C' 36 Residue, 8 h 1 C 437 Sequential wet oxidation (Hz2) Lost, 2 mmoi g-ia 381 Lost, 1 mmol g-la 373 Residue, 1 mmol g-' 247 Age of treatment /years (N) (N) C Proportion of young C 1% ~ - /g kg-i soil C O (4.6) 1.7 (1.8) 5..4 (2.9) 8.7 (2.1 ) ( (1.6) (5.5) 17.7 (1.4) I 48.9 (4.3) 46.6 ( 1.9) (6.3) 34.2 (1.3) 44.1 Table 3 Proportion of young C in particlesize fractions and in chemical subfractions of fraction -5 pm, as estimated from the enrichment in I3C under maize cultivation, and in the Rothamsted model compartments. Sample dated 4.6 years is from La Minibre. Samples dated 2.1 years are from Boigneville. Sample 2.1(N) is the upper layer of a treatment with no tillage, thus relatively enriched in young C. Errors in parenthesis are systematic errors based on a systematic error of.5% on each 6I3C used in the calculation. Error is indicated when the isotope composition was calculated by mass balance. It is otherwise equal to.7% Thermal oxidation (2) Lost, C" 321 Lost, C' 469 Residue 37 C 21 Pyrolysis (N2) Lost, 25-3 C" 25 Lost, 3-5 1"ca 473 Residue 51 C "Isotope composition calculated by difference from mass balance. 2.2 (3.6) 5.4 (1.6) (6.) 6.7 (1.3) (3.3) 29.4 (1.9) (6.3) 27.7 (1.3) ) 46.8 (1.7) (6.6) 45.6 (1.5) Blackwell Science Ltd, European Journal of Soil Science, 47,

6 Carbon dynamics in cultivated soil 491 results are compatible with a relation between resistance to H22 and resistance to biodegradation, but further investigations with other tracers are required. It was difficult to analyse the extracted material by mass balance because of an isotope fractionation by oxidation, the extent of which is greater than the effect being studied. The two thermal attacks, oxidation and pyrolysis, gave similar results: the younger products were always found in products released at intermediate temperatures, but the age differentiation was weak. Thermal decomposition of soil carbon has been related to structure, with for instance some decarboxylation occurring at low temperature, and aromatic material generally oxidized at higher temperature than aliphatic carbon, but the temperature of decomposition is also affected by inorganic bonding (Schnitzer & Kahn, 1972). The results from the sequential thermal degradation of whole soil carbon are poorly related to soil carbon age. Relating separates to compartments of Rothamsted model The Rothamsted carbon model was run for the conditions of the sites. Total soil carbon content in the model was first fitted to actual content (4 kg C m-2) by adjustment of the carbon input to soil. Under this condition the model gave an excellent prediction of the proportion of new C in the soil (Fig. 2). Data for the first 4 years were a little less than predicted by the model. This could be due to dead plant material such as coarse stems and root crowns not being taken into account in our measurement of total C. The enrichment of size fractions in new C was further compared with the enrichment of model compartments (Figs 3 and 4). According to their physical nature, fractions >5 pm might contain the RPM compartment and fraction -5 pm might contain BIO+HUM+IOM. The location of the microbial biomass in the -5 pm fraction is supported by experiments of inorganic N immobilization at La Minitre. Microbially immobilized N was almost absent from the fractions >5 pm (Balabane & Balesdent, 1995). Figure 3 shows the correspondence between new C in fractions >5 pm and new C in RPM. The rather good agreement of the shape of the kinetics indicated a good correspondence of the life-time of fraction >5 pm and of RPM. The similar plateau value indicated that most of RPM was in the fraction >5 pm. Figure 4 shows the proportion of new C in the separates compared to that in the model compartments. The fraction > 5 pm also contains some BIO + HUM + IOM, as discussed above. This is also in agreement with the observation that the fraction > 5 pm contains more carbon (.65 kg C m-2) on the average than RPM (.56 kg C m-2). In Fig. 3 we see that fraction -5 pm agrees well with.2 -c :.4 2 z.3 3 z" c 5.2 E R Timebears Timebears Fig. 2 The proportion of new C in total soil carbon, as measured in maize-cultivated soils (m) and as predicted by the Rothamsted carbon model (--). C input in the model was adjusted to fit soil total soil C content. Fig. 3 The incorporation of new C in particle-size fractions compared to that in compartments of the Rothamsted carbon model. (a) Fraction >SO bm (m) and RPM (--). (b) Fraction <5 pm (m) and HUM + IOM + B1 (--) Blackwell Science Ltd, European Journal of Soil Science, 47,

7 492 J. Balesdent C ). L- al d.c.6 z.4 C.- I2 8.2 n Timebears Fig. 4 The turnover (proportion of new C) in particle-size fractions compared to that in compartments of the Rothamsted carbon model. Fraction >5 pm (m) and RPM (--). (b) Fraction <5 pm () and HUM + IOM + BIO (- - -). BIO + HUM + IOM. Fraction -5 pm may nevertheless contain some RPM (on the average.1 kg C m-*). To summarize, the fraction >5 pm contains.45 kg RPM and.2 kg BIO + HUM + IOM, and fraction -5 pm.1 kg RPM and 3.3 kg BIO + HUM + IOM. In the Rothamsted model HUM + BIO is almost equally supplied by DPM and RPM. The observed incorporation of new C in the first years in fraction -5 pm (Fig. 3) is in good agreement with such a hypothesis: HUM + BIO supplied either by RPM alone or by DPM alone would not tally with my findings. The chemical separates, also, were compared to Rothamsted model compartments (Table 3). Water-soluble C may contain one part of DPM, but from its kinetics of enrichment in new C we can conclude that at least 7% of water soluble C is extracted from the HUM + IOM compartments. Most of the young compartments (DPM, BIO, part of RPM) may be found in the C extracted by autoclaving (Table 3), but this extract is essentially derived from HUM + IOM. Conclusions Physical fractionation of SOM in primary particle-size or density separates fractions has been of interest for SOM dynamics studies for decades (Christensen, 1992). The strong correlation observed between particle-size and age demonstrates the usefulness of separating the coarsest fractions such as Free OM (HCnin et al., 1959) or Particulate Organic Matter (Cambardella & Elliott, 1992). Separated particulate organic matter coarser than 5 pm by mild agitation without ultrasonic treatment agrees well with the compartment of plant structural material, named RPM by Jenkinson & Rayner (1977), and it would probably do for the STRUC compartment of Century (Parton et al., 1987). Organic matter extracted by cold water or autoclaving contains some of the plant metabolic components and the microbial biomass, but it also contains such a large amount of old organic carbon that its separation appears useless for quantifying these compartments. None of the other chemical separation methods I used allowed the concentration of either the young metabolic material or the old inert OM. Resistance to wet oxidation may reflect resistance to biodegradation. Alkaline extraction and acid hydrolysis cannot be recommended for investigating soil C turnover. On the one hand, the kinetics of enrichment of new C confirms that most of soil OM can be represented as being very homogeneous so far as C dynamics are concerned. Thus, the HUM compartment of Jenkinson & Rayner (1977) appears to be supplied both by the progressive alteration of plant debris and by rapid incorporation of soluble or microbial C. On the other hand, the results of these rough chemical separations agree with those obtained by more sensitive methods of chemical characterization of OM (Capriel et al., 1992; Schulten & Hempfling, 1992). They support the view that variations in rate of C turnover in soils may be less due to chemical composition of OM than to the localization and the physical protection of OM (Oades, 1995; Golchin et al., 1994). Acknowledgements Long-term field experiments are essential for such studies. I thank the Domaine ExpCrimental de La Miniere (INRA), the Station d Agronomie de Toulouse (INRA) and the Institut Technique des CCrCales et de Fourrages for their contributions, and J. P. PCtraud, C. Picot, M. Grably for their friendly help in physical fractionation, chemical extraction and isotope analysis, respectively. References Anderson, D.W. & Paul, E.A Organo-mineral complexes and their study by radiocarbon dating. Soil Science Socieg of America Journal, 48, Balabane, M. & Balesdent, J Input of fertilizer-derived labelled N to soil organic matter during a growing season of maize in the field. Soil Biology and Biochemistry, 24, Balabane, M. & Balesdent, J Medium-term transformations of organic N in a cultivated soil. European Journal of Soil Science, 46, Balesdent, J The turnover of soil organic fractions estimated by radiocarbon dating. Science ofthe Total Environment, 62, Balesdent, J. & Balabane, M Maize root-derived soil organic carbon estimated by natural I3C abundance. Soil Biology and Biochemistry, 24, Balesdent, J., Mariotti, A. & Boisgontier, D Effect of tillage on soil organic carbon mineralization estimated from I3C abundance in maize fields. Journal of Soil Science, 41, Blackwell Science Ltd, European Journal of Soil Science, 47,

8 Carbon dynamics in cultivated soil 493 Bdesdent, J. & Mariotti, A Measurement of soil organic matter turnover using I3C natural abundances. In: Mass Spectrometry of Soils (eds T.W. Boutton & S. Yamasaki), pp Marcel Dekker Inc., New York. Buesdent, J., Mariotti, A. & Guillet, B Natural I3C abundance as a tracer for studies of soil organic matter dynamics. Soil Biology and Biochemistry, 19, Balesdent, J., Pbtraud, J.-P. & Feller, C Effets des ultrasons sur \ la distribution granulombtrique des matitres organiques des sols. Science du Sol, 29, Bomand, M., Dejou, J., Robert, M. & Roger, L Composition mintralogique de la phase argileuse des terres noires de Limagne (Puy-de Dame). Le probleme des liaisons argiles-matitre organique. Agronomie, 4, Broder, M.W. & Wagner, G.H Microbial colonization and decomposition of corn, wheat, and soybean residue. Soil Science Society of America Journal, 52, Cambaradella, C.A. & Elliott, E.T Particulate soil organicmatter changes across a grassland cultivation sequence. Soil Science Society of America Journal, 56, Capriel, P., Harter, P. & Stephenson, D Influence of management on the organic matter of a mineral soil. Soil Science, 153, Cerri, C., Feller, C., Balesdent, J., Victoria, R. & Plenecassagne, A Application du traqage isotopique nature1 en I3C B I btude de la dynamique de la matitre organique dans les sols. Comptes-Rendus de I Acadimie des Sciences, Paris, 3, Cheshire, M.V Nature and Origin of Carbohydrates in Soils. Academic Press, London. Christensen, B.T Physical fractionation of soil and organic matter in primary particle size and density separates. Advances in Soil Science, 2, 1-9, Feller, C Une mtthode de fractionnement granulomttrique de la matiere organique des sols: application aux sols tropicaux i texture grossitre, tres pauvres en humus. Cahiers ORSTOM, sirie Pddologie, Paris, 17, Golchin, A., Oades, J.M., Skjemstad, J.O. & Clarke, P Soil structure and carbon cycling. Australian Journal of Soil Research, 32, Golchin, A., Oades, J.M., Skjemstad, J.O. & Clarke, P Structural and dynamic properties of soil organic matter as reflected by I3C natural abundances, pyrolysis mass spectrometry and solid- state I3C NMR spectroscopy in density fractions of an oxisol under forest and pasture. Australian Journal of Soil Research, 33, Htnin, S., Monnier, G. & Turc, L Un aspect de la dynamique des matitres organiques du sol. Comptes-Rendus de I Acadimie des Sciences, Paris, 248, Jenkinson, D.S. & Rayner, J.H The turnover of soil organic matter in some of the Rothamsted classical experiments. Soil Science, 123, Jenkinson, D.S., Harkness, D.D., Vance, E.D., Adams, D.E. & Harrison, A.F Calculating net primary production and annual input of organic matter to soil from the amount and radiocarbon content of soil organic matter. Soil Biology and Biochemistry, 24, Kogel-Knabner, Biodegradation and humification processes in forest soils. Soil Biochemistry, 8, Motavalli, P.P., Palm, C.A., Parton, W.J., Elliott, E.T. & Frey, S.D Comparison of laboratory and modeling simulation methods for estimating soil carbon pools in tropical forest soils. Soil Biology and Biochemistry, 26, Oades, J.M An overview of processes affecting the cycling of organic carbon in soils. In: The Role of Non-Living Organic Matter in the Earth s Carbon Cycle (eds R. G. Zepp & C. Sonntag), pp John Wiley & Sons, New York. Parton, W.J., Schimel, D.S., Cole, C.V. & Ojima, D.S Analysis of factors controlling soil organic matter levels in Great Plains Grasslands. Soil Science Sociely of America Journal, 51, Righi, D., Dinel, H., Schulten, H.-R. & Schnitzer, M Characterization of clay-organic-matter complexes resistant to oxidation by peroxide. European Journal of Soil Science, 46, Scharpenseel, H.W The search for biologically inert and lithogenic carbon in recent soil organic matter. In: Soil Organic Matter Studies, Vol. 2, pp International Atomic Energy Agency, Vienna. Schnitzer, M & Kahn, S.U Humic Substances in the Environment. Marcel Dekker, Inc., New York. Schulten, H.-R. & Hempfling, R Influence of agricultural soil management on humus composition and dynamics: classical and modern analytical techniques. Plant and Soil, 142, Trumbore, S.E., Vogel, J.S. & Southon, J.R AMS-14C Measurements of fractionated soil organic matter: an approach to decipher the soil carbon cycle. Radiocarbon, 31, Blackwell Science Ltd, European Journal of Soil Science, 47,