Christian Feller

Physical control of soil organic matter dynamics in the tropics

Abstract Management of soil organic matter (SOM) is esential to sustaining the quality and productivity of soils around the globe. This appears to be particularly true in the tropics where there is a greater proportion of nutrient poor, highly weathered soils that are more susceptible to losses of SOM. Developing management practices that promote the maintenance and storage of SOM in the tropics depends on understanding the factors that control SOM dynamics. This paper describes the role that soil physical properties (mineralogy, texture, and structure) play in regulating the accumulation and loss of SOM in tropical soils. Two different approaches are presented here. The first approach explores relationships between total SOM and soil physical properties in the tropics. These include effects of climate and mineralogy on latitudinal gradients in SOM, interactions between texture and mineralogy as determinants of SOM storage and relationships between SOM and the structural stability of soils. The second approach describes characteristics of SOM associated with different physical constituents of the soil, with particular attention to particle-size fractions and aggregated particles of different sizes. In each case we summarise findings on the distribution of SOM among fractions and characterise its biochemical composition, bioavailability and turnover. Evidence for and against the physical protection of organic matter from microbial attack in tropical soils is also given. Wherever possible, we compare and contrast the findings for tropical soils with those of temperate soils. The influence of landuse management on physical control of SOM dynamics is discussed as an overriding factor with each aproach.

Kinetics of added organic matter decomposition in a Mediterranean sandy soil

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.

Spatial carbon, nitrogen and phosphorus budget in a village of the West African savanna—II. Element flows and functioning of a mixed-farming system

Abstract The viability of mixed farming systems in West African savannas relies largely on the management of endogenous organic resources. Assessment of the organic balance at both plot and village territory scales is needed as an indicator of this viability. Distribution of carbon (C), nitrogen and phosphorus in soil and plant biomass was thus quantified for a village in southern Senegal across the different land use systems (LUS) and farm holdings. The village exhibited ring-like organisation, including a compound ring, a bush ring, and lowland paddy fields, with positive gradients of intensification and food production from the savanna to the dwellings. Marked contrasts were found between holdings, especially in livestock availability. Clear relationships were evidenced between the spatial distribution of C and nutrients and the agricultural functions of LUS and holdings, while multi-scale diversity appeared to be the main factor that ensured the functioning of the system. Intensification schemes at the village level aimed at increasing organic resources and their cycling efficiency must thus take into account their impact on this diversity.

Charles Darwin, earthworms and the natural sciences: various lessons from past to future

Abstract In 1881, Darwin (1809–1882) published his last scientific book entitled “The formation of vegetable mould through the action of worms with observations on their habits”, the result of several decades of detailed observations and measurements on earthworms and the natural sciences. The work was considered a “best-seller” at the time, with 3500 copies sold immediately and 8500 in less than 3 years which, at the time, rivaled the sale of his most well known book “On the origin of species”. The book covers the importance of earthworm activity on a variety of topics: pedogenesis and weathering processes, soil horizon differentiation and the formation of vegetable mould (topsoil), the role of earthworm burrowing and casting (bioturbation) in soil fertility and plant growth, the burial of organic materials and soil enrichment with mineral elements, the global cycle of erosion–sedimentation with hydrologic and aerial transfers of fine particles brought up to the soil surface by earthworms and the protection of archaeological remains through their burial. Finally, Darwin also performed a series of original experiments to determine if earthworms possessed, or not, a certain “intelligence”. This part of the book was, among others, one of the main reasons for its success. In this article we analyze the success (past and present) of this book, Darwin’s own opinion of his book and the general contents of the work. Throughout, we discuss the main lessons to be learned from his ‘little’ (as he called it) book and provide brief historic reviews of major literary works on earthworms, both contemporary and posterior to Darwin, emphasizing his role as precursor and/or founder of various scientific disciplines (ethology, soil ecology and pedology). However, despite Darwin’s clear demonstrations of the importance of biological activities (earthworms) in the maintenance of soil fertility, his book on worms has been mostly neglected by agronomists and soil scientists, primarily due to the predominant soil fertility and management paradigms of the 19th and 20th centuries.

Determination of total carbon and nitrogen content in a range of tropical soils using near infrared spectroscopy: influence of replication and sample grinding and drying

Near infrared (NIR) reflectance spectroscopy has been receiving increased attention for the rapid and inexpensive determination of soil properties and of total carbon (Ct) and nitrogen content (Nt) in particular. However, methodological aspects such as sample grinding and drying or replication have not been addressed extensively. The objectives of the paper were, thus, to assess how NIR predictions of Ct and Nt were affected by sample grinding (2 mm sieving vs. 0.2 mm grinding), drying (air-drying vs oven-drying at 40°C during 24 h) and replication (use of one to six sub-samples to determine average spectra). This was performed on a range of tropical soils that differed widely in mineralogy (low and high activity clay soils, allophanic soils) and texture (sandy to clayey). The accuracy of the NIR predictions of Ct and Nt was higher with oven-dried compared to air-dried samples and, more markedly, with 0.2 mm ground compared to 2 mm sieved samples. Replication had a positive effect on NIR predictions when 2 mm sieved samples were used, especially for air-dried samples, but this effect was not clear with 0.2 mm ground samples. Thus, the most accurate predictions of Ct and Nt were obtained with oven-dried finely ground samples, with limited response to sample replication. Accurate predictions were, however, also obtained with four replicates on oven-dried 2 mm sieved samples. Acceptable and less tedious results could, thus, be achieved when replacing fine grinding by replication. Even with this procedure, the r2 between predicted (NIR) and measured (reference) values was 0.9 and the ratio of standard error of prediction to mean (CV%) was 20% which can be considered satisfactory for the heterogeneous sample set under study.

Soil organic carbon sequestration in tropical areas. General considerations and analysis of some edaphic determinants for Lesser Antilles soils

Some general notions on soil organic carbon (SOC) sequestration and the difficulties to evaluate this process globally are presented. Problems of time- and space- scales are emphasized. SOC erosion, which is generally difficult to evaluate in relation to land use changes, is discussed in detail. Different aspects of SOC sequestration on the Lesser Antilles are presented for a wide range of soil types. Comparisons between soils revealed that the SOC stocks in the Lesser Antilles are highly dependent upon the mineralogy: higher stocks for allophanic (ALL) soils than for low activity clay (LAC) and high activity clay (HAC) soils. But in terms of potential of SOC sequestration (pSeq-SOC, differences between permanent vegetation and continuous cultivation situations), there are no differences between ALL and LAC soils (22.9 and 23.3 tC. ha−1, respectively). On the other hand, the potentials of SOC sequestration were higher for HAC soils (30.8 – 59.4 tC. ha−1, with the higher levels in the less Mg- and Na-affected Vertisol). Sheet erosion is a serious problem for Vertisol with high Mg and Na on exchange complex, causing high dispersability of fine elements. Thus, the lower SOC levels in these soils may be partly due to erosion losses. Laboratory incubations have shown that 37 – 53% of the protected SOC in these soils was located in aggregates larger than 0.2 mm. The effect of agricultural practices on SOC sequestration was studied for the Vertisols. Intensification of pastures led to higher plant productivity and higher organic matter restitutions and SOC sequestration. The gain was 53.5 and 25.4 tC. ha−1 for the low and high-Mg Vertisol, respectively (0–20 cm layer). SOC sequestration with pastures also depends upon the plot history with lower mean annual increase in SOC for the initially eroded (1.0 gC . kg−1 soil . yr−1) than for the non-degraded (1.5 gC . kg−1 soil . yr−1) Vertisol. Loss of SOC in a pasture-market gardening rotation was 22.2 tC . ha−1 with deep (30–40 cm) and 10.7 tC . ha−1 with surface (10–15 cm) tillage. It was unclear whether the differences in SOC losses were due to mineralization and/or to erosion.

Soil fertility concepts over the past two centuries: the importance attributed to soil organic matter in developed and developing countries

The theories about plant nutrition and soil fertility varied widely from the antiquity to the middle of the nineteenth century, with major (Thaer A. 1809. Grundsätze der rationellen Landwirtschaft (1809–1812). Berlin (Germany): Realschulbuch Ed.) or minor (Liebig J. 1840. Die organische Chemie in ihrer Anwendung auf Agrikultur und Physiologie. Braunschweig (Germany): Vieweg) importance attributed to soil humus or soil organic matter (SOM). The importance assigned to humus over the past two centuries will be developed in this historical paper. Intensification of agriculture in the twentieth century permitted an important increase in cultivated plant yield of food, fiber, wood, and biofuel production, not only in the northern countries, but also in some southern countries (e.g., India and China) with the emergence of the Green Revolution. However, the question of organic restitutions and the maintenance (or increase) of the SOM stock was, at times, not taken into consideration; consequently, there was a general decrease in SOM stock for many tropical soils. It is now widely accepted by scientists that SOM depletion is one of the major factors leading to degradation of ecosystem services and loss of ecosystem resilience: new alternatives are now necessary for the maintenance and/or increase in plant productivity and production of environmental services by agriculture. Therefore, this paper will also present some recent research in different tropical countries with a focus on agroecological management of tropical soils.

Determination of carbohydrates in two ferrallitic soils: analysis by capillary gas chromatography after derivatization by silylation

In order to determine their sugar composition, two tropical ferrallitic soils of contrasting vegetation and organic matter content were subjected to a series of acidic hydrolyses prior to analysis of the extracted monomer sugars by gas chromatography (GC). We tested an alternative method of derivatization (silylation), which appeared more rapid and convenient than the method generally in use for GC analysis of soil carbohydrates (reduction to alditol acetates), and obtained similar results. The former involves the conversion of neutral sugars, amino sugars, and glycuronic acids to their O-trimethylsilyl derivatives (Sweeley et al., 1963), and is preferred in most official methods for GC determination of organic campounds (Barford and Magidman, 1985); yet, its use in soil analysis has only been reported for uronic acids (Mundie, 1976; Casagrande and Park, 1978): Carbohydrate materials in soils have been extensively studied for the past 30y because of their suggested role in soil fertility. Quantitative and qualitative analyses of their composition were first made by paper, column, and gas chromatography combined with colorimetric procedures (Cheshire, 1979). Recent advances in high performance liquid chromatography (HPLC) coupled with fluorescence spectroscopy or refractive index detection (Hamada and Ono, 1984; Angers et al., 1988), and ion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD; Martens and Frankenberger, 199 1; Jsrgensen and Jensen, 1994) allow, then, rapid, sensitive, and detailed determinations of soil saccharides. However, GC, which seems inferior to HPLC and HPAEC-PAD in terms of sample preparation (need of a prior treatment for the formation of alditol acetate derivatives), remains a widely used technique for detection of saccharides in soils (Dalal

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 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.

Soil aggregation under different management systems

Considering that the soil aggregation reflects the interaction of chemical, physical and biological soil factors, the aim of this study was evaluate alterations in aggregation, in an Oxisol under no-tillage (NT) and conventional tillage (CT), since over 20 years, using as reference a native forest soil in natural state. After analysis of the soil profile (cultural profile) in areas under forest management, samples were collected from the layers 0-5, 5-10, 10-20 and 20-40 cm, with six repetitions. These samples were analyzed for the aggregate stability index (ASI), mean weighted diameter (MWD), mean geometric diameter (MGD) in the classes > 8, 8-4, 4-2, 2-1, 1-0.5, 0.5-0.25, and < 0.25 mm, and for physical properties (soil texture, water dispersible clay (WDC), flocculation index (FI) and bulk density (Bd)) and chemical properties (total organic carbon - COT, total nitrogen - N, exchangeable calcium - Ca2+, and pH). The results indicated that more intense soil preparation (M < NT < PC) resulted in a decrease in soil stability, confirmed by all stability indicators analyzed: MWD, MGD, ASI, aggregate class distribution, WDC and FI, indicating the validity of these indicators in aggregation analyses of the studied soil.