Bruno Mary

Stability of organic carbon in deep soil layers controlled by fresh carbon supply

The world’s soils store more carbon than is present in biomass and in the atmosphere. Little is known, however, about the factors controlling the stability of soil organic carbon stocks and the response of the soil carbon pool to climate change remains uncertain. We investigated the stability of carbon in deep soil layers in one soil profile by combining physical and chemical characterization of organic carbon, soil incubations and radiocarbon dating. Here we show that the supply of fresh plant-derived carbon to the subsoil (0.6–0.8 m depth) stimulated the microbial mineralization of 2,567 ± 226-year-old carbon. Our results support the previously suggested idea that in the absence of fresh organic carbon, an essential source of energy for soil microbes, the stability of organic carbon in deep soil layers is maintained. We propose that a lack of supply of fresh carbon may prevent the decomposition of the organic carbon pool in deep soil layers in response to future changes in temperature. Any change in land use and agricultural practice that increases the distribution of fresh carbon along the soil profile could however stimulate the loss of ancient buried carbon.

An overview of the crop model stics

Abstract stics is a model that has been developed at INRA (France) since 1996. It simulates crop growth as well as soil water and nitrogen balances driven by daily climatic data. It calculates both agricultural variables (yield, input consumption) and environmental variables (water and nitrogen losses). From a conceptual point of view, stics relies essentially on well-known relationships or on simplifications of existing models. One of the key elements of stics is its adaptability to various crops. This is achieved by the use of generic parameters relevant for most crops and on options in the model formalisations concerning both physiology and management, that have to be chosen for each crop. All the users of the model form a group that participates in making the model and the software evolve, because stics is not a fixed model but rather an interactive modelling platform. This article presents version 5.0 by giving details on the model formalisations concerning shoot ecophysiology, soil functioning in interaction with roots, and relationships between crop management and the soil–crop system. The data required to run the model relate to climate, soil (water and nitrogen initial profiles and permanent soil features) and crop management. The species and varietal parameters are provided by the specialists of each species. The data required to validate the model relate to the agronomic or environmental outputs at the end of the cropping season. Some examples of validation and application are given, demonstrating the generality of the stics model and its ability to adapt to a wide range of agro-environmental issues. Finally, the conceptual limits of the model are discussed.

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.

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.

Nitrogen in the Environment

When leafy vegetables such as spinach and lettuce are grown in greenhouses during winter and early spring, i.e. at low light intensity and short day length, they may accumulate a high amount of nitrate in the leaves (Corre and Breimer 1979). A high nitrate content in vegetables is undesirable, because it may be harmful for the consumer. Nitrate itself is not toxic, but it is easily reduced to the toxic compound nitrite. Reduction to nitrite can occur during postharvest storage of vegetables (Aworth et al. 1980), as well as after ingestion as food in saliva and in the gastrointestinal tract (Maynard et al. 1976; Walters and Walker 1979). Acute nitrite toxicity causes a respiratory dysfunction called methaemoglobinaemia. By the oxidation of the ferrous iron of haemoglobin to the ferric form, methaemoglobin is formed which cannot transport oxygen, thereby causing tissue asphyxia. Chronic nitrite poisoning may result in the formation of carcinogenic nitrosamines. These N-nitroso compounds can be formed from nitrite and secondary amine compounds, which often occur in food (Walters and Walker 1979; Vermeer et al. 1998). As yet, the occurrence of (gastric) cancer has not been directly related to the consumption of nitrate, but it is generally accepted that a high nitrate intake should be prevented (Forman et al. 1985; Westgeest 1989).

C and N cycling during decomposition of root mucilage, roots and glucose in soil

C and N dynamics were followed during decomposition of root mucilage, roots and glucose in soil, incubated for 6 months at 25°C. Each of the substrates, derived from maize plants, was added at two rates, which ranged from ca 100 to 450 mg C kg−1 soil. Carbon mineralization from each substrate was determined from 13C variations at natural abundance. Nitrogen fluxes were calculated by artificial 15N tracing of soil nitrate-N. The kinetics of C and N fluxes, expressed per unit of added C, were almost independent of the C rate for each of the three substrates. The calculated true mineralization rate of root mucilage-C was comparable to glucose mineralization; mineralization of roots-C was slower. A positive ‘priming effect’ was found and was proportional to the amount of substrate-C added. The priming increased throughout the incubation and was more pronounced for glucose than for mucilage or roots. In contrast, there was no priming effect for N during glucose decomposition. Gross immobilization of labelled nitrate-N reached a maximum of 72, 19 and 61 mg N g−1 added C for mucilage, roots and glucose, respectively. This immobilization was obtained when the true mineralization rates of carbon were 35, 38 and 40%. Gross N immobilization and true C mineralization were highly correlated during root decomposition. Total N assimilation, i.e. gross immobilization of NO3-N plus microbial assimilation as NH4-N or organic N, was estimated by applying the isotope dilution method to biomass-N. It reached 88, 66 and 61 mgNg−1 added C for mucilage, roots and glucose, respectively. The higher N assimilation for mucilage was probably due to exclusive bacterial decomposition, whereas fungi were involved for the two other substrates. The remineralization of N and the decline in biomass-N during the second part of the incubation were markedly faster and more complete for mucilage than for roots and glucose.

Use ofr 13C variations at natural abundance for studying the biodegradation of root mucilage, roots and glucose in soil

Variations in the natural abundance of 13C in the CO2 evolved during the biodegradation of three substrates derived from maize plants provide a way of studying the decomposition of plant material in soil. The δ13C isotopic composition of the CO2 evolved (called δC) was followed during the decomposition of root mucilage, roots and glucose (initial composition δS), all from maize plants, in sand inoculated with a soil extract. Results indicate that a negative isotopic enrichment (ϵ = δC − δS < 0) occurred at the beginning and end of the decomposition process, whatever the substrate. For the intermediate stages of mineralization, the isotopic enrichment was small (δC slightly less than δS), or negligible. It is possible to take this enrichment effect into account in biodegradation studies, by introducing a correction factor in the calculation of the true mineralization rate. During the decomposition of the three substrates in soil, the correction factor varied between 1.04 and 1.08 for root mucilage, 1.13 and 1.17 for roots and 1.13 and 1.27 for glucose.