Yves-Marie Cabidoche

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.

Long-term pollution by chlordecone of tropical volcanic soils in the French West Indies: A simple leaching model accounts for current residue

Chlordecone was applied between 1972 and 1993 in banana fields of the French West Indies. This resulted in long-term pollution of soils and contamination of waters, aquatic biota, and crops. To assess pollution level and duration according to soil type, WISORCH, a leaching model based on first-order desorption kinetics, was developed and run. Its input parameters are soil organic carbon content (SOC) and SOC/water partitioning coefficient (K(oc)). It accounts for current chlordecone soil contents and drainage water concentrations. The model was valid for andosol, which indicates that neither physico-chemical nor microbial degradation occurred. Dilution by previous deep tillages makes soil scrapping unrealistic. Lixiviation appeared the main way to reduce pollution. Besides the SOC and rainfall increases, K(oc) increased from nitisol to ferralsol and then andosol while lixiviation efficiency decreased. Consequently, pollution is bound to last for several decades for nitisol, centuries for ferralsol, and half a millennium for andosol.

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.

THERESA: I. Matric water content measurements through thickness variations in vertisols

Abstract It is difficult if not virtually impossible to measure the hydraulic characteristics of vertisols. Cracking of the soil makes the usual sensors lose contact. Changes in the water content during the normal shrinkage phase can be calculated from vertical deformation measurements if the ratio between horizontal and vertical deformation components is known. THERESA is a new type of transducer for measuring the thickness of soil layers. It has a small diameter so that bias due to crack induction is delayed to avoid affecting water extraction by roots. Averages can be obtained from multiple sampling when spatial variability of the soil is high. An equidimensional shrinkage model is used for calculating water contents. Water contents during normal shrinkage were accurately estimated under the hypothesis that structural pores (which were assumed to remain rigid during the structural shrinkage phase) become constricted in proportion to shrinkage. Normal shrinkage, which is associated with monotonic drying, seldom lasts very long in the field: it is interrupted by structural swelling at the slightest rainfall event. THERESA only measures the amount of water which is associated with deformation and which is referred to as matric water.

Numerical modelling of water infiltration into the three components of porosity of a vertisol from Guadeloupe

We present a mechanistic model of soil deformation and water infiltration into a Vertisol of Guadeloupe (French West Indies), which accounts for the three components of porosity of this soil (matric, structural and macro-cracks). Time and space scales are respectively of several hours and the prism delimited by macro-cracks. The model accounts for water movements from the structural porosity and from the macro-cracks into the matric porosity. It simulates the non-equidimensional deformations of the prism resulting from water storage in the matric porosity. Inputs of the model, measured in the laboratory, are the shrinkage curve, the retention curve and the hydraulic conductivity of the matric porosity. The anisotropy ratio of the soil deformation was measured in situ. Experiments were conducted in situ to provide structural parameters and data to fit and test the model. It is possible to find a unique set of parameters for each experiment. However, parameters significantly differ from one experiment to another. The model shows that the structural water flow regulates the partition of water infiltrating within the prism and of water flowing in the macro-cracks. The model does not accurately predict water infiltration because of a poor modelling of water flow in the structural porosity.

THERESA: II. Thickness variations of vertisols for indicating water status in soil and plants

Abstract THERESA (Transferts Hydriques Evalues par le REtrait des Sols Argileux) is a method for estimating the water content of vertisols in the field by measuring the vertical deformation of the solid. Deformation is controlled by changes in the water content of the clay matrix, and only the matric water component of the total water content can be determined with this method. The relationship between matric water storage or reserve (Sm and Rm(i)) calculated from deformation data and three indicators of water status in the soil and plants were studied, so as to find out more about the actual meaning of this parameter in terms of water uptake by plants. The three indicators used were: total water storage and reserves (S and R(i)) which control the amount of water available, relative transpiration ( Ep ETM ) which is the outcome of the balance between water availability and plant requirements, and the growth rate of an organ ( ΔL Δt ) which indicates how water stress affects plant growth. Evaluating matric water contents from changes in the thickness of soil layers (measured with THERESA transducers) was tested in three settings: in grassland from 1985 to 1988 and during two sugar cane growth cycles from 1989 to 1991. By measuring vertical solid displacements and the gravimetric water content, the soil shrinkage curve in the field and changes in the thickness of soil layers were available. These were then used for removing the bias in water storage calculation introduced when the changes in bulk density and thickness are ignored. Matric water contents measured with THERESA corresponded to a low availability domain. Relative transpiration ( Ep ETM ) and stem growth rate ( Δ Δt ) decreased significantly as soon as the matric reserve was drawn from. These results give confirmation that matric water contents measured by THERESA are an appropriate indicator for rationing water and for monitoring irrigation in swelling clay soils.