Marc Pansu

Handbook of Soil Analysis

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Kinetics of added organic matter decomposition in a Mediterranean sandy soil

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Modelling the effect of active roots on soil organic matter turnover

Abstract Carbon mineralization kinetics of 17 organic materials were studied in a Mediterranean sandy soil. These added organic matters (AOM) used in the organic fertilizer industry differed in their origin and composition: plant residues from the agri-food industry, animal wastes, manures (plant and animal origin), composts at different composting times and organic fertilizers. The mixtures AOM-soils were incubated under aerobic conditions at 28°C during 6 months. Soil moisture was maintained at 75% water holding capacity and respired-CO2 was regularly trapped into alkali media in closed chambers, then checked by HCl titration. Analyses of CO2 were performed in triplicate at 17 sampling occasions. The mineralized AOM fraction (MAOMF) varied according to the AOM origin: from 12–33% of added C for composts, to 65–90% for animal-originated AOM, with many intermediate patterns for plant-originated AOM. Seven decomposition models from the literature were fitted to actual MAOMF: (a) three consecutive models with two 1st-order-kinetic compartments and three parameters (m1, humification; m2, exchange; m3, decomposition), (b) three parallel models (m4, with two compartments and three parameters; m8, a 1st-order plus 0-order model with three parameters; m5, a three-compartment model with four parameters), and (c) m7, a model with one 2nd-order-kinetic compartment and two parameters. Additionally, m6, a simplified version of m5 was proposed. Models m2 and m7 did not match with actual data or gave a poor fit. By the correlation parameters, the most simple model m4 was chosen instead of the consecutive models m1 and m3. Residual sums of squares were always greater—but not significantly—in m8 than in m4, which confirmed the superiority of the models with two 1st-order compartments against 1st-order plus 0-order models for incubation times higher than 100 days. Model m5 (most of its parameters being not correlated) gave the best predictions of our data. The proposed m6 version gave predictions with similar precision as m4 and appeared powerful with only two parameters (very labile and stable fractions of the AOM). A compromise between the precision of the predictions and the simplicity of the formulae allowed the recommendation of the well-known m4 model, and above all the simpler m6 model.

Prediction by near infrared spectroscopy of the composition of plant raw materials from the organic fertiliser industry and of crop residues from tropical agrosystems

The aim of this experiment was to study the effect of living roots on soil carbon metabolism at different decomposition stages during a long-term incubation. Plant material labelled with 14C and 15N was incubated in two contrasting soils under controlled laboratory conditions, over two years. Half the samples were cropped with wheat (Triticum aestivum) 11 times in succession. At earing time the wheat was harvested, the roots were extracted from the soil and a new crop was started. Thus the soils were continuously occupied by active root systems. The other half of the samples was maintained bare, without plants under the same conditions. Over the 2 years, pairs of cropped and bare soils were analysed at eight sampling occasions (total-, plant debris-, and microbial biomass-C and -14C). A five compartment (labile and recalcitrant plant residues, labile microbial metabolites, microbial biomass and stabilised humified compounds) decomposition model was fitted to the labelled and soil native organic matter data of the bare and cropped soils. Two different phases in the decomposition processes showed a different plant effect. (1) During the initial fast decomposition stage, labile 14C-material stimulated microbial activities and N immobilisation, increasing the 14C-microbial biomass. In the presence of living roots, competition between micro-organisms and plants for inorganic N weakly lowered the measured and predicted total-14C mineralisation and resulted in a lower plant productivity compared to subsequent growths. (2) In contrast, beyond 3–6 months, when the labile material was exhausted, during the slow decomposition stage, the presence of living roots stimulated the mineralisation of the recalcitrant plant residue-14C in the sandy soil and of the humified-14C in the clay soil. In the sandy soil, the presence of roots also substantially stimulated decomposition of old soil native humus compounds. During this slow decomposition stage, the measured and predicted plant induced decrease in total-14C and -C was essentially explained by the predicted decrease in humus-14C and -C. The 14C-microbial biomass (MB) partly decayed or became inactive in the bare soils, whereas in the rooted soils, the labelled MB turnover was accelerated: the MB-14C was replaced by unlabelled-C from C derived from living roots. At the end of experiment, the MB-C in the cropped soils was 2.5–3 times higher than in the bare soils. To sustain this biomass and activity, the model predicted a daily root derived C input (rhizodeposition), amounting to 5.4 and 3.2% of the plant biomass-C or estimated at 46 and 41% of the daily net assimilated C (shoot + root + rhizodeposition C) in the clay and sandy soil, respectively.

Evaluation of three incubation designs for mineralization kinetics of organic materials in soil

The dynamics of carbon (C) and nitrogen (N) of plant residues and organic fertilisers are of great interest for agricultural and global warming studies. The proportion of the fractions obtained from biochemical analyses (fibres by sequential Van Soest analysis) can be used for predicting both C and N transformation of organic materials in soils. Considering the expensive and time-consuming Van Soest method, the principal aim of this study was to elaborate near infrared (NIR) calibrations for fibres, in order to use them for consecutive studies (for example, our works on transformation of added organics or TAO model). A wide set of organic fertilisers and their raw materials was sampled, including plant materials originating from temperate (especially Mediterranean) and tropical regions. The particular objective of this work was to build NIR calibrations for fibre fractions, along with C and N content, in plant materials used in the organic fertiliser industry and green house gases mitigating strategies. The second particular objective was to test for two levels of validation of the equations previously elaborated: (1) validation with a set of randomly chosen samples that was not considered during the calibration step, (2) extrapolation of the predictive capacity of the equations when applying them to outliers that were previously discarded. The fibres were the best predicted parameters, as R² = 0.95, 0.91, 0.97, 0.97 for neutral detergent soluble, hemicelluloses, cellulose and lignin, respectively, whereas the characteristics of total organic matter had R² varying from 0.87 (N Kjeldahl) to 0.94 (C Dumas). The accuracy of the calibrations developed for fibres was confirmed by the first level of validation, since the standard errors of prediction were close to the corresponding standard errors of cross-validation and the standard errors of calibration. Nevertheless, the calibrations developed for ash and C Dumas were not so good. Surprisingly, at the second level of validation, some outliers were not so badly predicted. This can illustrate the robustness of the calibrations for cellulose, lignin and, to a lesser extent, N Dumas which are key parameters for our modelling works on C and N transformation of added organics in soils.

Near infrared reflectance spectroscopy applied to model the transformation of added organic materials in soil

Abstract Carbon (C) mineralization was assessed during incubations of a Mediterranean sandy soil amended with various organic by‐products covering a wide range of C and nitrogen (N) contents. The laboratory incubation systems consist in measuring continuously the soil respiration (as CO2‐C) in closed chambers, or less current, in pre‐storing soil containers in semi‐open chambers until transferred and measured for CO2‐C evolved in closed ‘measuring‐jars’. The latest were improved, the new designs permitting to test a much greater number of by‐products with a minimum handling. No significant differences were found between the results obtained by the different incubation systems. The storage systems using pre‐storage of soils gave reproducible cumulative CO2‐C curves. Results obtained with the pre‐storage systems could be compared confidently to C mineralization data from studies using permanent closed chambers. One of them was specially reliable and can thus be recommended for long‐term incubation experiments.

Modelling the transformation of organic materials in soil with nuclear magnetic resonance spectra

Raw, mixed and composted organic materials (OM) from agricultural and urban wastes were subjected to biochemical analyses, near infrared (NIR) reflectance spectroscopy and laboratory incubations. Respiration during incubations was accurately predicted using a decomposition model [transformation of added organic materials, (TAO)] of very labile, intermediary resistant, and stable OM fractions. Calibrations using NIR spectra were developed to determine the very labile and stable fractions of OM used to predict three-month OM mineralisation in soil. This study has confirmed that OM decomposition is mainly driven by OM quality on a short-term basis. The wavelengths contributing heavily to the prediction of very labile and stable OM components and molecular functions of these fractions were identified. The resulting TAO–NIR spectroscopy model is an efficient tool to study the degradation of natural molecules and its management for plant growth and sustainability of ecosystems. As a sub-model of a more complex C cycle model, it can instantaneously simulate labile and stable fractions of various organic inputs in soil and, as a non-destructive and easily portable spectroscopic method, could be used to assess C dynamics on a regional scale.

Handbook of soil analysis : mineralogical, organic and inorganic methods

Changes in the carbon (C) and nitrogen (N) compartments that result from the addition of organic material (OM) to the soil are predicted by the transformation of added OM (TAO) model with three parameters: very labile (P′L) and stable (PS) fractions of the OM and the rate of remineralization (kᵣₑₘᵢₙ) of nitrogen immobilized by microorganisms. We propose relations between P′L, PS, kᵣₑₘᵢₙ and various chemical groups in the OM identified by their ¹³C nuclear magnetic resonance (NMR) spectra. The aromatic content increased the predicted PS in accordance with published results. The O‐aromatic content also increased PS, but much less so than the aromatic content. The carboxyl content decreased PS and increased P′L as in the TAO model based on infrared spectrometry. The carbonyl content decreased P′L, whereas di‐O‐alkyl increased P′L. The chemical composition of the population of decomposer organisms did not appear to be homeostatic, but was related rather to the composition of the substrate: kᵣₑₘᵢₙ was positively correlated with the carboxyl and di‐O‐alkyl content and negatively correlated with the alkyl content. Solid state ¹³C NMR spectroscopy gave better predictions of the transformations that resulted from adding OM than biochemical fractionation and near infrared reflectance spectrometry (NIRS). It is fast and non‐destructive and provides new insights into the processes that control decomposition for research into waste recycling, agro‐ecology and climate change. HIGHLIGHTS: Linking decomposition of organic materials in soil to NMR measurements. First mathematical model of decomposition based on NMR spectra. Stability of the OM depends on the chemical groups and the inorganic N supply. NMR is a promising tool for monitoring ecosystem changes and soil–air exchanges.