J.A.E. Molina

C and N fluxes between pools of soil organic matter: Model calibration with long-term incubation data

An updated version of the simulation model NCSOIL was calibrated with data from a long-term laboratory incubation (728 days at 28°C) of three cultivated soils amended with K 15NO3 and either [14C]glucose or [14C]cellulose. The kinetics of tracer and non-tracer C and N (CO2-C, inorganic-N and microbial biomass-C) were measured in the three soils on various sampling dates. The new NCSOIL version considers four organic pools: residues, microbial biomass (pool I) with two components (labile and resistant), humads (pool II) which correspond to the active fraction of soil organic matter (SOM), and pool III which is the highly resistant fraction of SOM. To fit the long-term incubation experimental data, it was necessary to reduce all decomposition rates by 60–70% after 35–85 days of incubation, depending on the soil and the treatment. The labile fraction of pool I in non-amended soils was also reduced to 0.20 from the previously-used value of 0.56; the later value, however, was adequate for the glucose and cellulose-enriched soils. Simulations agreed well with experimental data and gave values of pool II which represented about 30% of the SOM, and which had a C:N ratio of 12–13. Results suggested that >60% of the SOM was very resistant.

Assimilation of nitrogen by soil microbial population: NH4 versus organic N

Abstract Nitrogen assimilation by microbial, biomass during the decay of organic material in soil may follow two patterns: (1) direct assimilation of low molecular weight organic N compounds (Direct hypothesis) or (2) immobilization of mineral N, while organic N is completely mineralized (MIT hypothesis). To test these hypotheses equal concentrations of NH 4 -N and alanine-N were added to soils, either one or the other labeled with 15 N, and incubated for 1.2 days. The K 2 SO 4 -extractable organic and mineral N and 15 N and CO 2 release were measured periodically. Experimental results were compared with data computed by two versions of the model NCSOIL, that simulates the C-N turnover and 15 N distribution among soil pools, and is structured to represent either the Direct or the MIT hypothesis. The fitted first order rate constant of mineralization of alanine was 3.2 d −1 , following a delay of 0.25 d. Evolution of CO 2 proceeded at a considerable rate after alanine was decomposed and net N mineralization had ceased, indicating a rapid decomposition of the microbial population that consumed alanine. The isotopic dilution of mineral N proceeded very rapidly and fitted the simulation by MIT better than by the Direct model. The rate of 15 N withdrawal from total extractable N was greater when alanine was labeled and fitted the prediction by the Direct model, but when NH 4 was the source of 15 N, the Direct model failed to predict 15 N consumption. It seemed that both pathways operated concurrently, with the Direct dominating N assimilation by the substrate specific population and the MIT operating at the level of the native soil population.

Modeling the incorporation of corn (Zea mays L.) carbon from roots and rhizodeposition into soil organic matter

Experimental data reported in the literature over the last decennium indicate that roots and rhizodeposition are important sources of carbon for the synthesis of soil organic carbon. Our objective was to verify the capability of the simulation model NCSWAP to reproduce the general conclusions from the experimental literature, and to gain some insight about the processes that control the incorporation of corn belowground production into the soil organic matter. The model was calibrated against the experimental data gathered from a long-term field experiment located near St. Paul, Minnesota. The simulation model updated daily the soil conditions to reproduce over a 13 year period the measured kinetics of seven variables: above-ground corn production, and the total soil organic matter, soil d value, and the soil organic matter derived from corn in the 0‐15 and 15‐30 cm depth. The simulation gave a root-plus-rhizodeposition 1.8 times larger than stalks plus leaves. The translocation efficiency of corn-C into soil organic C at the 0‐15 cm depth gradually decreased to 0.19 of the below-ground deposition. The sensitivity of below-ground photosynthate incorporation into the soil organic matter was analyzed relative to variations in the parameters that control the formation and decay of roots and rhizodeposition. Roots had a greater effect than rhizodeposition on the soil organic matter, though more photosynthates were translocated to rhizodeposition than to roots. q 2001 Elsevier Science Ltd. All rights reserved.

Computer simulation of nitrogen turnover in soil and priming effect.

Abstract Two contrasting hypotheses were stated to describe the How of soil N and C between organic, inorganic and microbial forms: either soil microbes incorporate and assimilate low molecular-weight nitrogenous compounds-direct hypothesis-or they are supplied by inorganic N exclusively: free or bound N organics are mineralized by deaminuses before they penetrate cells-mineralization-immobilization turnover (MIT) hypothesis. Tests of the hypotheses were performed by comparing experimental data describing N turnover and priming effect to those computed by process-oriented models representing the hypotheses. The tests indicated that the direct hypothesis could not account for the observed data. N turnover, under the MIT hypothesis, was higher than the one computed by the Kirkham and Bartholomew equations and remineralization of immobilized N was important.

Optimization of an ecological model with the Marquardt algorithm

Abstract Optimization of parameters in ecological models can present many challenges, including multiple dependent variables on which to calculate fit, slowly converging search algorithms, nonanalytical derivatives, parameters constrained by their physical meaning in the model to a given sign or range, and excessively long run times for optimization calculations. The Marquardt algorithm has been restructured and an appropriate figure-of-merit function formulated to address these problems. As an example, a soil nitrogen and carbon transformation simulation model (NCSOIL) may be optimized for two and three unknown parameters within ten iterations or less, based on initial guesses of parameter values in error by an order of magnitude.

C and N fluxes between pools of soil organic matter: Model calibration with long-term field experimental data

Abstract NCSOIL, an updated simulation model of C and N behaviour in soil, was calibrated with data from published field experiments in which various authors studied the decomposition of C- or N-labelled plant materials under a wide range of climatic and soil conditions. Four organic pools were included in the new version of NCSOIL viz. plant residues (decay rate constant = 0.3 d −1 ); microbial biomass with a labile (0.33d −1 ) and a resistant (0.04 d −1 ) component; humads (0.006 d −1 ); and stable OM (5.5 10 −5 d −1 ). Experimental data were fitted with a reduction factor (RED1), which applied to all decomposition rates and for the whole of the simulated period. This parameter took into account the effect of climatic and soil conditions on microbial decomposition rates. Optimization of this reduction factor gave values which ranged from 0.06 to 1.0 and which were related well to the climatic and edaphic conditions given by the authors. Optimization of the size of the humads pool showed that the active soil organic fraction represented 13–35% of the total soil organic matter.

Analysis of soil microbial biomass nitrogen and 15N with a high background of labeled mineral N

Abstract Microbial biomass is determined from the excess of extractable organic N released from fumigated soil samples. In the presence of relatively high contents of labeled mineral N, small differences in organic N and N may not be detectable. Two approaches were tested to determine organic N content and its N enrichment in the presence of considerably greater concentrations of labeled mineral N: (i) Removal of mineral N from mixed solutions of alanine and NH4 or NO3 by reduction and boiling under alkaline conditions, prior to Kjeldahl digestion. (ii) Including mineral N in Kjeldahl N analysis, by reduction under acidic conditions prior to digestion and calculating organic N and N content by subtracting mineral N and N. The removal of mineral N was either incomplete‐ particularly regarding labeled mineral N, or partly destroyed organic N as well. When mineral N was included in the digest, the recovery of N and 15N was sufficiently accurate to obtain good results of organic N and 15N by subtracting the k...