Bernard Nicolardot

Simulation of C and N mineralisation during crop residue decomposition: A simple dynamic model based on the C:N ratio of the residues

C and N mineralisation kinetics obtained in laboratory incubations during decomposition of crop residues under non-limiting nitrogen conditions were simulated using a simple dynamic model. This model includes three compartments: the residues, microbial biomass and humified organic matter. Seven parameters are used to describe the C and N fluxes. The decomposed C is either mineralised as CO2 or assimilated by the soil microflora, microbial decay producing both C humification and secondary C mineralisation. The N dynamics are governed by the C rates and the C:N ratio of the compartments which remain constant in the absence of nitrogen limitation. The model was parameterised using apparent C and N mineralisation kinetics obtained for 27 different residues (organs of oilseed rape plants) that exhibited very wide variations in chemical composition and nitrogen content. Except for the C:N ratio of the residues and the soil organic matter, the other five parameters of the model were obtained by non-linear fitting and by minimising the differences between observed and simulated values of CO2 and mineral N. Three parameters, namely the decomposition rate constant of the residues, the biomass C:N ratio and humification rate, were strongly correlated with the residues C:N ratio. Hyperbolic relationships were established between these parameters and the residues C:N ratio. In contrast, the other two parameters, i.e. the decay rate of the microbial biomass and the assimilation yield of residue-C by the microbial biomass, were not correlated to the residues C:N ratio and were, therefore, fixed in the model. The model thus parameterised against the residue C:N ratio as a unique criterion, was then evaluated on a set of 48 residues. An independent validation was obtained by taking into account 21 residues which had not been used for the parameterisation. The kinetics of apparent C and N mineralisation were reasonably well simulated by the model. The model tended to over-estimate carbon mineralisation which could limit its use for C predictions, but the kinetics of N immobilisation or mineralisation due to decomposition of residues in soil were well predicted. The model indicated that the C:N ratio of decomposers increased with the residue C:N ratio. Higher humification was predicted for substrates with lower C:N ratios. This simple dynamic model effectively predicts N evolution during crop residue decomposition in soil.

Long-term fate of nitrate fertilizer in agricultural soils

Significance Fertilizers are of key importance to sustain modern agriculture, but the long-term fate of fertilizer-derived nitrogen in the plant–soil–water system is not fully understood. This long-term tracer study revealed that three decades after application of isotopically labeled fertilizer N to agricultural soils in 1982, 12–15% of the fertilizer-derived N was still residing in the soil organic matter, while 8–12% of the fertilizer N had already leaked toward the groundwater. Part of the remaining fertilizer N still residing in the soil is predicted to continue to be taken up by crops and to leak toward the groundwater in the form of nitrate for at least another five decades, much longer than previously thought. Increasing diffuse nitrate loading of surface waters and groundwater has emerged as a major problem in many agricultural areas of the world, resulting in contamination of drinking water resources in aquifers as well as eutrophication of freshwaters and coastal marine ecosystems. Although empirical correlations between application rates of N fertilizers to agricultural soils and nitrate contamination of adjacent hydrological systems have been demonstrated, the transit times of fertilizer N in the pedosphere–hydrosphere system are poorly understood. We investigated the fate of isotopically labeled nitrogen fertilizers in a three–decade-long in situ tracer experiment that quantified not only fertilizer N uptake by plants and retention in soils, but also determined to which extent and over which time periods fertilizer N stored in soil organic matter is rereleased for either uptake in crops or export into the hydrosphere. We found that 61–65% of the applied fertilizers N were taken up by plants, whereas 12–15% of the labeled fertilizer N were still residing in the soil organic matter more than a quarter century after tracer application. Between 8–12% of the applied fertilizer had leaked toward the hydrosphere during the 30-y observation period. We predict that additional exports of 15N-labeled nitrate from the tracer application in 1982 toward the hydrosphere will continue for at least another five decades. Therefore, attempts to reduce agricultural nitrate contamination of aquatic systems must consider the long-term legacy of past applications of synthetic fertilizers in agricultural systems and the nitrogen retention capacity of agricultural soils.

Carbon and nitrogen cycling through soil microbial biomass at various temperatures

Abstract C and N cycling were examined in a soil incubated at 4, 12, 20 or 28°C for 140 days. Before incubation the soil was amended with K 15 NO 3 , and either glucose- 14 C or holocellulose- 14 C. The kinetics of tracer and non-tracer C and N from the biomass, mineralized-C and inorganic-N were measured. C and N behaviours in soil were influenced by temperature, substrate and substrate-temperature interactions. Labelled-C mineralization rates after 140 days ranged from 41 to 58% for glucose and from 34 to 65% for holocellulose. Maximal immobilization was 21.8–31.6 mgN kg −1 soil for holocellulose and 24.3–33.5 mg N kg −1 soil for glucose. Re-mineralization began earlier with glucose and at higher temperatures: 6–23% of immobilized-N were re-mineralized for glucose and 0–19% for holocellulose. More labelled C and N were incorporated into the microbial biomass from both carbon sources at lower temperatures. The biomass turnover was highly influenced by temperature: 40–60% of labelled C or N incorporated in the biomass remained in this compartment at 20–28°C, while corresponding values at 4–12°C were only 0–40%. Organic- 14 C mineralization and immobilization rate constants were influenced by temperature, the different trends depending on the carbon source. Thus an overall temperature coefficient (Q 10 ) could not be determined for these complex transformations. Variations in the rate constant with temperature were described using polynomial regressions.

C and N fluxes of decomposing 13C and 15N Brassica napus L.: effects of residue composition and N content

Abstract The interactions occurring between biochemical composition and N content of crop residues while decomposing in soil, and the associated N dynamics were assessed by studying the kinetics of C and N biotransformations of different tissues of Brassica napus L. (roots, stems and pod walls). These residues were obtained by growing a rapeseed crop under low and high N nutrition, in a labeling growth chamber with enriched 13 CO 2 atmosphere and a 15 N nutritive solution. The resulting crop residues in which the C-to-N ratio varied between 22 and 135 were homogeneously labeled with 13 C and 15 N. Paired labeled residues ( 13 C 15 N labeled residues with unlabeled soil inorganic N; 13 C 14 N residues with 15 N labeled soil inorganic N) were used to determine net and gross fluxes of immobilization and mineralization. Decomposition was studied during laboratory incubations at 15°C, the initial soil N availability being non-limiting with regard to the rate of C decomposition. The rate of 13 C mineralization from the residues was influenced by the biochemical composition of the tissues and particularly by their soluble C content. The N content of the tissues did not significantly affect the kinetics or the amount of C mineralized, except in the very short-term. Decomposition was rapid and after 168 days of incubation at 15°C, 82% of the C from the stems and pod walls and 69% from the roots at both low and high N contents had disappeared from the soil coarse fraction. Residue decomposition first resulted in net immobilization of soil mineral N for all the residues. The intensity and duration of this immobilization depended on the tissues and the N content of the residues. Compared to the control, the residues with low N content, still induced net N immobilization after 168 days (−22 to −14 mg N g −1 of added C) whereas the high N residues induced little net immobilization or mineralization, at −3 to + 4 mg N g −1 of added C at the same date. The NCSOIL model was used as a tool to calculate, by fitting simulation against the data, the gross N mineralization and immobilization fluxes and also to determine the total N fluxes involved over the 168 days of decomposition. Depending on the tissues and their N content, gross cumulative immobilization ranged from 71 to 113 mg N g −1 of added C and gross mineralization varied from 66 to 123 mg N g −1 of added C. The differences in net mineralization, observed during decomposition of the tissues with low and high N contents, were well explained by the differences between gross mineralization fluxes which were themselves attributable to the different quantities of N mineralized from the residues. The use of modeling to calculate the total gross N fluxes demonstrates that the total amount of N involved in the decomposition of crop residues is much higher than the resulting net fluxes quantified either by N balance or by 15 N tracing.

Simultaneous effects of increasing levels of glucose and oxygen partial pressures on denitrification and dissimilatory nitrate reduction to ammonium in repacked soil cores

Dissimilatory nitrate reduction to ammonium (DNRA) and its importance in comparison to denitrification were studied in soil samples artificially repacked to control water potential and porosity, and incubated for 72 h. Labelled nitrate (100 mg N·kg−1 dry soil, 21.8 % 15N in excess) and increasing levels of glucose-C (250, 500 and 1 000 mg glucose-C·kg−1 dry soil) were initially added to the soil samples to obtain increasing glucose-C/nitrate-N ratios of 2.5, 5 and 10, which were then subjected to different O2 partial pressures (0, 0.5, 1.0 and 2.0 % (v/v)). The results confirmed the good reproducibility of the experimental conditions using this method. Denitrification, rather than DNRA, was the dominant process in all the treatments developed during this experiment: N2O production in the presence of acetylene varied from 4.9 (glucose-C/nitrate-N = 2.5; 2 % O2) to 103.6 % (glucoseC/nitrate-N = 2.5; 0 % O2) of the original nitrate whereas DNRA varied from 1.8 (glucose-C/nitrate-N = 2.5; 2.0 % O2) to 24.6 % (glucose-C/nitrate-N = 10; 1.0 % O2) of the original nitrate. This work demonstrated that under these conditions, DNRA activity was less sensitive than denitrification to an inhibitory effect by O2 and reinforced the idea that carbon is the main driving factor regulating nitrate distribution between denitrification and DNRA.

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.

Comparing the effectiveness of radish cover crop, oilseed rape volunteers and oilseed rape residues incorporation for reducing nitrate leaching

The soil water and N dynamics have been studied during two long fallow periods (between wheat or oilseed rape and a spring crop) in a field experiment in Châlons-en-Champagne (eastern France, 48°50′ N, 2°15′ E). The experiment involved frequent measurements of soil water, soil mineral N, dry matter and N uptake by cover crops. Water and N budgets were established using Ritchies model for calculating evapotranspiration in cropped soils and a model (LIXIM) for calculating water drainage, N leaching and N mineralisation in bare soils. During the first autumn and winter, a radish cover crop (grown from September 1994 to January 1995) was compared to a bare soil. During the second period (July 1995 to April 1996), a comparison was carried out between (i) oilseed rape volunteers, (ii) bare soil with two types of oilseed rape residues incorporated into the soil (R0 and R270 residues) and (iii) bare soil without residues incorporation. R0 and R270 residues came from two preceding oilseed rape crops which received two rates of N fertilizer (0 and 270 kg N ha-1).Soil mineral N content was markedly reduced by the presence of radish cover crop or oilseed rape volunteers during autumn. The calculated actual evapotranspiration (AET) did not differ much between treatments, meaning that the transpiration by the cover crop or volunteers was relatively low (100–150 L kg-1 of dry matter). Consequently, nitrate leaching was reduced during the rest of the winter and spring as well as nitrate concentration in the percolating water: 45 vs. 91 mg NO3- L-1 for radish cover crop and bare soil, respectively. The incorporation of oilseed rape residues to soil also exerted a beneficial but smaller action on reducing the nitrate content in the soil. This effect was due to extra N immobilisation which reached a maximum of about 20 kg N ha-1 in mid-autumn for both types of residues. Nine months after the incorporation of the oilseed rape residues, and comparing to the control soil without residues incorporation, N rich residues induced a significant positive N net effect (+ 9 kg N ha-1) corresponding to 10% of N added whereas for N poor residues no net effect was still obtained at the end of experiment (−3 kg N ha-1, not significantly different from 0).To reduce nitrate leaching during long fallow periods, it is necessary to promote techniques leading to decrease mineral-N contents in the soil during autumn before the drainage period, such as (i) residue incorporation after harvest (without fertiliser-N) and (ii) allowing volunteers to grow or sowing a cover crop just after the harvest of the last main crop.

Decomposition de corps microbiens dans des sols fumiges au chloroforme: Effets du type de sol et de microorganisme

Five microbial species (Aspergillus flavus, Trichoderma viride, Streptomyces sp., Arthrobacter sp., Achromobacter liquefaciens) were cultivated in liquid media containing 14C-labelled glucose. The decomposition of these microorganisms was recorded in four different soils after chloroform fumigation by a technique related to that proposed by Jenkinson and Powlson, to determine the mineralization rate of microbial organic matter (Kc coefficient). Three treatments were used: untreated soil, fumigated soil alone and fumigated soil supplied with 14C-labelled cells. Total evolved CO2 and 14CO2 were measured after 7 and 14 days at 28°C. The labelled microorganisms enabled the calculation of mineralization rate Kc (Kc = mineralized microbial carbon/supplied microbial carbon). The extent of mineralization of labelled microbial carbon depended on the type of soil and on the microbial species. Statistical analysis of results at 7 days showed that 58% of the variance is taken in account by the soil effect and 32% by the microorganism effect. Between 35 and 49% of the supplied microbial C was mineralized in 7 days according to the soil type and the species of microorganism. Our results confirmed that the average value for Kc = 0.41 is acceptable, but Kc variability according to soil type must be considered. The priming effect on organic C and native microbial biomass mineralization, due to microbial carbon addition was obtained by comparison between the amount of non-labelled CO2-C produced by fumigated soils with or without added labelled microorganisms: this priming effect was generally negligible. These results indicate that the major portion of the error of microbial biomass measurement comes from the Kc estimation.

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.

Typology of exogenous organic matters based on chemical and biochemical composition to predict potential nitrogen mineralization

Our aim was to develop a typology predicting potential N availability of exogenous organic matters (EOMs) in soil based on their chemical characteristics. A database of 273 EOMs was constructed including analytical data of biochemical fractionation, organic C and N, and results of N mineralization during incubation of soil-EOM mixtures in controlled conditions. Multiple factor analysis and hierarchical classification were performed to gather EOMs with similar composition and N mineralization behavior. A typology was then defined using composition criteria to predict potential N mineralization. Six classes of EOM potential N mineralization in soil were defined, from high potential N mineralization to risk of inducing N immobilization in soil after application. These classes were defined on the basis of EOM organic N content and soluble, cellulose-, and lignin-like fractions. A decision tree based on these variables was constructed in order to easily attribute any EOM to 1 of the 6 classes.