Sylvie Recous

Soil inorganic N availability: Effect on maize residue decomposition

Abstract The effect of soil inorganic N availability on the decomposition of maize residues was tested under aerobic conditions in soil samples incubated for 125 days at 15°C. Carbon residue were ground maize shoots applied at 4 g dry matter kg−1 soil. The C-amended soils contained five initial inorganic N concentrations (10, 30, 60, 80 and 100 mg N kg−1 soil). Gross N immobilization was calculated with a 15N tracer, using changes in both the inorganic and organic 15N pools. Inorganic N remained available in those soils having the three highest initial N concentrations. In this case the rates of C mineralization and N immobilization were similar. Soil inorganic N completely disappeared at the beginning of C decomposition in the soil samples with the two lowest N contents, resulting in a marked decrease of C mineralization rate compared to the three highest N contents. Gross N immobilization amounted to 39 mg N g−1 added C after 40 days (end of the net immobilization period) for the three highest N concentrations, indicating that there was no luxury N consumption by the soil microflora. N immobilization was much lower in the two lowest-N treatments because decomposition was slow and microbial N immobilization per unit of mineralized C was reduced. The ratio N immobilized: C mineralized also decreased in all treatments during decomposition due to changes in microbial N demand with time or increasing contributions from other sources of N, such as biomass-N recycling, to microbial N assimilation.

Interactions between decomposition of plant residues and nitrogen cycling in soil

The processes of N mineralization and immobilization which can occur in agricultural soils during decomposition of plant residues are briefly reviewed in this paper. Results from different incubation studies have indicated that the amounts of N immobilized can be very important and that the intensity and kinetics of N immobilization and subsequent remineralization depend on the nature of plant residues and the type of decomposers associated. However, most of the available literature on these processes refer to incubations where large amounts of mineral N were present in soil. Incubations carried out at low mineral N concentrations have shown that the decomposition rate of plant residues is decreased but not stopped. The immobilization intensity, expressed per unit of mineralized C, is reduced and N remineralization is delayed. Nitrogen availability in soil can therefore strongly modify the MIT kinetics (mineralization-immobilization turnover) by a feed-back effect. The mineralization and immobilization kinetics have been determined in a two-years field experiment in bare soil with or without wheat straw. Mineralization in plots without straw seemed to be realistically predicted by accounting for variations in soil temperature and moisture. Immobilization associated with straw decomposition was clearly shown. It was increased markedly by the addition of mineral N throughout decomposition. It is concluded that mineral N availability is an important factor controlling plant residues decomposition under field conditions. A better prediction of the evolution of mineral N in soil may therefore require description and modelling of the respective localization of both organic matter and mineral N in soil aggregates.

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.

Gross nitrogen fluxes in soil : theory, measurement and application of 15N pool dilution techniques

Abstract Isotopic pool dilution using 15 N is proving to be a valuable tool for increasing our understanding of gross N cycling processes and our ability to both model these processes and link them to microbial function. However, not all applications are appropriate. Many of the questions asked by agronomists and soil scientists can often be addressed by simpler experiments in which measurements of the main parameters of inorganic and total N content of soil and plant components would suffice. In addition, the theory, assumptions and techniques associated with the calculation of gross N fluxes can lead to large errors if not applied correctly. Some preliminary assessment of the principle N transformation processes to be studied, followed by an optimisation of the experimental conditions are needed for the effective application of 15 N pool dilution. When applied correctly under carefully controlled laboratory incubations, the technique has been used successfully to quantify gross N fluxes and to understand the fundamental processes that regulate individual microbial N pathways. This has improved our understanding of how C and N cycles are linked, and thus has led us to question the most appropriate structure of C and N cycling models. Field based 15 N pool dilution studies have been used successfully to study the climatic influence on the soil N cycle and also to quantify the impact of external inputs. Further field-based studies are required to aid model development and evaluation. Linking soil microbial/molecular ecology with process-based studies of microbial nutrient cycling presents a new and exciting field of research that will benefit from the further application of isotopic pool dilution techniques for N and other nutrients.

A model for calculating nitrogen fluxes in soil using 15N tracing

ˇ and organic N). This fit validated the compartmental model and enabled calculation of six N fluxes: mineralisation (m), ammonium immobilisation (ia), nitrate immobilisation (in), nitrification (n), volatilisation (v) or denitrification (d) and remineralisation of recently immobilised N (r). Sensitivity analysis indicated that the classical assumptions of exclusive ammonium immobilisation (in=0) and absence of N remineralisation (r = 0) had to be rejected. NH4 immobilisation appeared to be dominant when ammonium and nitrate were both present, but was not exclusive: a Langmuir-type relationship could be established between the immobilisation ratio ia/(ia+in) and the molar ratio of soil N concentrations NH4 + /(NH4 + +NO3 ˇ ). Remineralisation of N occurred simultaneously with immobilisation during wheat straw decomposition and represented 7‐18% of gross immobilisation. Taking into account small gaseous losses, volatilisation or denitrification, allowed a better fit to be obtained between observed and simulated N and 15N pools. Nitrification was better described by first order than by zero order kinetics. The eventuality of direct assimilation of organic N by microbial biomass or N humification could not be determined but had no significant influence on the calculation of other fluxes. When FLUAZ was applied to a single treatment (NH4 labelled), it also gave a good fit but only m, i (=ia+in), n, v or d could be determined. The mineralisation and immobilisation rates were slightly lower than those found with the paired treatments: this diAerence was mainly due to the hypothesis r = 0 and disappeared when r was fixed at the value

Microbial immobilization of ammonium and nitrate in cultivated soils

Abstract The microbial immobilization of ammonium and nitrate was measured by 13 N organic measurements after the application of labelled urea, (NH 4 ) 2 SO 4 , KNO 3 (KN) or NH 4 NO 3 with or without glucose in four different soils. In the soils incubated without glucose, the microbial immobilization of the added ammonium varied between 1.5 and 4 mg N kg −1 soil. No immobilization occurred at the expense of NO 3 when KN was applied. When glucose was added at the rate 500 mg C kg −1 soil, the immobilization was very active between the first and the third day, at 10°C. The maximal amounts of 13 N immobilized were much higher for the [ 15 N]urea, 15 (NH 4 ) 2 SO 4 , 15 NH 4 NO 3 and 15 NO 3 K. treatments than for the NH 4 15 NO 3 application. This preferential immobilization of NH 4 was also observed in pure cultures of bacteria isolated from one of the soils and attributed to the inhibition of nitrate uptake by ammonium. The immobilization ratio, immobilized N: decomposed C, was calculated for glucose, accounting for pool substitution effects and immobilization due to native C. It was independent of the form of N applied and similar between soils, c 45–48 mg N g −1 C.

Decomposition of wheat straw and rye residues as affected by particle size

Effects of contact between the soil and crop residues on the processes of residue decomposition are still poorly understood. The objective of this study was to investigate the effects of residue particle size on the decomposition of wheat (Triticum aestivum L.) straw (C/N=270) and green rye (Secale cereale) residues (C/N=9). Residue particle size was used as a means to vary the contact between crop residues and the soil. Carbon mineralization was measured during 102 d for straw and 65 d for rye, on residues ranging in sizes from laboratory model (0.03 cm) to field-scale (10 cm). The soil was a silt (Typic Hapludalf) and the incubation was performed at 15 °C. The effects of particle size on C mineralization varied for the two residues. In the first two days of incubation, decomposition rate of rye increased with decreasing particle size but thereafter, the trend was reversed. In 65 days, 8% more C was decomposed in the 7-cm residues than in the 0.03-cm ones. For wheat straw, early decomposition (3–17 days) was faster for the small-sized particles (0.06 and 0.1 cm). Thereafter, the largest size classes (5 and 10 cm) decomposed faster. After 102 days, the very fine particles (≤ 0.1 cm) showed the greatest and the intermediate size classes (0.5 and 1 cm), the lowest amount of C mineralized. We hypothesized that greater availability and accessibility of N was responsible for the higher rates of decomposition observed for finely-ground wheat straw while a physical protection of finely ground residues was probably involved in the observed reverse effect for rye.

Mineralisation of C and N from root, stem and leaf residues in soil and role of their biochemical quality

The influence of biochemical characteristics of 15 crop residues on C and N mineralisation in soil was investigated by following the decomposition of roots, stems and leaves of four subtropical species and one temperate species buried into the soil. The C, N and polyphenols contents were measured in different biochemical pools obtained from residues of the different organs. The mineralisation of root C was significantly lower than that of leaves and stems. Chemical analysis showed a higher polyphenol content in the leaves and a higher ligninlike content in the roots. Carbon and N mineralisation were simulated with the STICS decomposition submodel and tested against the data set. The model predicted leaf and stem C mineralisation for all five species fairly accurately, but failed to predict root C mineralisation, indirectly revealing the more complex composition of the root tissue. The results showed the interest of separately considering the different plant parts when studying plant residue decomposition and the need to develop other methods of residue quality characterisation to improve the prediction of residue decomposition.

A review of tillage effects on crop residue management, seedbed conditions and seedling establishment

There is considerable discussion about the influence of soil management techniques on soil erosion, water use and conservation, and more recently carbon dioxide sequestration and waste disposal. The soil–atmosphere interface, particularly the seed bed layer is of particular concern to agronomists and soil scientists because it is the focus of the physical processes affecting crop establishment and biological activity. This paper evaluates the current knowledge (1) in modeling seedling emergence and residue decomposition, (2) seedbed structure and its resulting physical conditions, and (3) tillage operations affect on seedbed structure and residue distribution.

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.