Scientific registration n o : 1939 Symposium n o : 7 Presentation : poster. MOLINA Jean-Alex E. (1), NICOLARDOT, Bernard (2), CHENG, H. H.

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1 Scientific registration n o : 1939 Symposium n o : 7 Presentation : poster Influence of clay content and time on soil organic matter turnover and stabilization Influence de la teneur en argile et du temps sur le taux de renouvellement et de stabilisation de la matière organique du sol MOLINA Jean-Alex E. (1), NICOLARDOT, Bernard (2), CHENG, H. H. (1) (1) Department of Soil, Water, and Climate, University of Minnesota, St. Paul, MN, U.S.A (2) INRA - Unité d'agronomie de Châlons-Reims, Reims, France Introduction Soil clay content and total soil organic matter (SOM) are positively correlated. The incorporation and accumulation of added C into the SOM is more influenced by clay than by the soil ph, the nature of plant cover, or the amount of organic residues added. Several mechanisms have been documented to account for higher SOM in clay-rich soils: Adsorption of organic matter between layers of expanding clays; ion exchange, van de Waals forces, and H bonding with clay surfaces; metal-organic complexes bonded to clays - all processes that tend to increase the stability of the SOM. The influence of clay on the dynamics of organic matter decay and humus formation is more complex. Clay can increase microbial biomass and O2 uptake, accelerate the synthesis of humic acid-type polymers, but also decrease the release of CO2 by increasing the fraction of decayed C stabilized into humus (e.g. the review by Martin and Haider, 1986; Franzluebbers, et al., 1996). The objective of this study was to quantify changes in the mineralizationimmobilization turnover (MIT) during the incubation of soils with various clay contents. In a previous study, the MIT for soils with clay contents ranging from 14.6 to 25.1 percent was documented (Nicolardot et al., 1994). In this report, we contrasted these MIT with those obtained with sandy and clay soils. Material and Method Experimental data The MIT of five soils was measured: (1) Ouroux, (2) Gréoux-les-Bains, (3) Mons-en- Chaussée, (4) Louvain,, and (5) Dijon (Table 1). The soils were incubated at constant temperature (28 o C) and water content set for optimum conditions of microbial activity and growth in the presence of organic-c and NO3 -N tracers. The first three soils were incubated with glucose or cellulose for 728 days. Procedural details have been reported previously (Baugnet, 1989; Nicolardot et al., 1994). A variation of the same method was used for the sandy and clay soils. They were incubated with 1500 mg 14 C-barley straw.kg - 1

2 1 soil for 140 days. The water soluble compounds were washed away from the straw before addition to soil. Tracer 15 N-KNO3 was added to adjust the C/N of the added material to 15. The kinetics of total and tracer microbial biomass-c, CO2, and inorganic N were monitored periodically, as previously described (Nicolardot al., 1994). Computed data Two versions of the model NCSOIL were used to obtain simulated data. Version A assumed that the decay rates of the added compounds (glusose, cellulose, or barley straw), and of the SOM pools (Pool I, Pool II, and Pool III) were constant. In version B, the decay rates of the SOM pools, but not of added compounds, were adjusted by a clay reduction factor µcl = EXP(-D * t), where D is a constant parameter obtained by calibration, and t is the time of incubation, expressed in days, and set to zero at the beginning of the incubation. Values for D and Pool II (initial concentration) were obtained for each soil by calibration against the kinetics of total and tracer C and N to optimize the performance of each version of the model. Results The model with the clay reduction factor (version B) provided a better fit of the data than version A with constant decay rates. An illustration of the data fit obtained is shown in Figures 1 and 2. Specific mineralization-immobilization rates which combine the mineral and the biomass dynamics are sometimes referred to as metabolic quotient (mg C or N.mg -1 biomass.kg -1 soil.day -1 ). Average rates were computed between time of sampling. The negative values correspond to net N immobilization. The specific rates were not used to optimize the model performance. The simulated effect of the clay reduction factor increased with the clay concentration (Fig. 3). The sandy soil (Ouroux) had retained 90 % of its activity after 140 days of incubation, but the SOM pools of the clay soil (Dijon) had stabilized within 100 days. Calibrated D values for the soils with intermediate clay content (Gréoux-les-Bains, 25.1 %; Mons-en-Chaussée, 19.5 %; and Louvain, 14.6 % clay) fell between those of the sandy and clay soils. Optimum values for the initial concentration of Pool II (version B) ranged from 10 to 33 % of the total organic-c (Table 2), the highest percentage coresponding to the calcareous soil (Gréoux, 34.8 % CaCO3). Discussion Models have been effective in simulating the effect of clay at steady state by various relations which are time-independent. Sørensen (1972) observed that the release of CO2 from cellulose or glucose in the presence of montmorillonite was not only reduced but delayed for 90 days. This work showed that the stabilization of the SOM pools during incubation was simulated by a clay reduction factor exponentially related to time: µcl = EXP(-D * t). The parameter D was constant for each soil under the experimental conditions (constant temperature and water content), but decreased with increasing clay content. This exponential function with only one parameter provided for a gradual decrease in the decay rates. It gave a better fit than that obtained with 2 parameters representing a sudden drop of the decay rate after some time of incubation (Nicolardot et al., 1994). 2

3 It is difficult to distinguish experimentally if clay acts directly on microbial growth and activity, or indirectly by protecting substrates or deactivating the degradative exoenzymes (Stotzky, 1986). A sensitivity analysis was performed to identify the combination of pools that had to be controlled by µcl to optimize the model. A good fit of experimental to computed data was obtained when the clay reduction factor modified the decay rates of the SOM pools (Pool I, Pool II, and Pool III), or, to a lesser extent, of Pool I only. The model performed poorly with any other combination of pools: Pool II or Pool III only, or even the SOM pools when µcl was also controlling the decay rates of the added chemicals (glucose, cellulose, or barley straw). Analysis of the data by the model would thus indicate that clay did not have any impact on the decay of added compounds. Instead, it limited the transfer of C from Pool II into Pool I, and internally within Pool I, the latter being the process that simulates the feeding of microbes on microbes and microbial successions. This resulted in an accumulation of C and N in Pool I and Pool II. Validation of this effect would require a comparison of experimental to simulated concentrations of Pool II. Optimum initial concentrations of Pool II fell within the range of values observed for the potentially mineralizable N (No) (Stanford and Smith, 1972). This concordance does not constitute a validation since the decay rate constant of Pool II was set to correspond to that of No. Similarly, validation on the basis of the kinetics of Pool I is inappropriate since it was already used to calibrate the model. Validation, therefore, will have to wait for the results of an experimental approach that gives concentrations of some soil organic fraction similar to those obtained by calibration of the dynamic Pool II. Literature Baugnet, M Suivi à l'aide de traceurs isotopiques des évolutions du carbone et de l'azote dans deux soils après incorporation de résidus végétaux. Mémoire de fin d'études. ENITA, Dijon. Franzluebbers, A. J., Haney, R. L., Hons, F. M. and Zuberer, D. A Active fractions of organic matter in soils with different texture. Soil Biol. Biochem. 28: Martin, J. P. and Haider, K Influence of mineral colloids on turnover rates of soil organic carbon. In P. M. Huang and M. Schnitzer (ed.) Interactions of soil minerals with natural organics and microbes. SSSA Special Publication Number 17. Nicolardot, B., Molina, J. A. E. and Allard, M. R C and N fluxes between pools of soil organic matter: Model calibration with long-term incubation data. Soil Biol. Biochem. 16: Sørensen, L. H Stabilization of newly formed amino acid metabolites synthesized in soil by clay minerals. Soil Sci. 114:5-11. Stanford, G. and Smith, S. J Nitrogen mineralization potentials of soils. Soil Sci. Soc. Am. Pro. 36: Stotzky, G Influence of soil mineral colloids on metabolic processes, growth, adhesion, and ecology of microbes and viruses. In P. M. Huang and M. Schnitzer (ed.) Interactions of soil minerals with natural organics and microbes. SSSA Special Publication Number 17. Keywords: Model simulation, NCSOIL, bioreactive SOM, texture Mots clés : modèle, simulation, matière organique du sol, réactivité, texture 3

4 FIGURE 1. SPECIFIC C MINERALIZATION 0,18 0,16 mg C-CO2 / mg C-BIOMASS / kg SOIL / DAY 0,14 0,12 0,1 0,08 0,06 0,04 LOUVAIN, 14.6 % CLAY measured ON OFF 0, DAY 4

5 FIGURE 2. SPECIFIC NET N MINERALIZATION AND IMMOBILIZATION 0,004 0,002 mg N-INORGANIC / mg C-BIOMASS / kg SOIL / DAY -0,002-0,004-0,006-0, LOUVAIN, 14.6 % CLAY measured ON OFF -0,01-0,012 DAY 5

6 FIGURE 3. CLAY REDUCTION FACTOR: EXP(-D * DAY) 1,2 1 CLAY REDUCTION FACTOR 0,8 0,6 0,4 D=7.0E-4, Ouroux (6.7 % clay) D=4.0E-3, Greoux-les- Bains (25.1 % clay) D=4.1E-3, Mons-en- Chaussee (19.5 % clay) D=3.4E-3, Louvain (14.6 % clay) D=7.0E-2, Dijon (32.2 % clay) 0, DAY 6

7 Table 1. Soil properties Ouroux Soil Gréoux-les- Bains Mons-en- Chaussée Louvain Dijon Clay (%) Organic-C (%) C:N ratio ph Biomass (mg C.kg -1 ) (set as initial value for Pool I) Table 2. Optimum values for the clay reduction factor µ cl = EXP(-D * day), and the initial concentration of Pool II Ouroux Soil Gréoux-les- Bains Mons-en- Chaussée Louvain Dijon D 7.0E-4 4.0E-3 4.1E-3 3.4E-3 7.1E-2 Pool II (mg C.kg-1) (% organic-c)