Bernard Nicoullaud

Influence of rock fragments on the water retention and water percolation in a calcareous soil

Abstract The water retention properties of a calcareous soil containing rock fragments have been determined in the laboratory thanks to pressure plate measurements done on both the fine earth and the rock fragments from the soil. The available water content (AWC) has been calculated from these data. We have shown that when the rock fragments are neglected, the AWC can be overestimated by 39%. When we do not neglect their volume but when their hydraulic properties are not considered, the AWC can be underestimated by 34%. By using a reservoir model, we have also calculated the effect of rock fragments on water percolation to groundwater. Depending of the climatic characteristics of the year, the underestimation of percolation when we neglect the rock fragments can reach up to 14.9% and the overestimation when we neglect their hydraulic properties can be equal to 15.8%. These findings emphasise the role of the rock fragments on the water supply in stony soils.

Exploring the Spatial Relationships Between Some Soil Properties and Wheat Yields in Two Soil Types

A field study was conducted to quantify spatial soil variability and to analyze correlations among soil properties at different spatial scales. Soil samples from 0 to 30 cm depth were collected from two adjacent fields in the southwestern Beauce Plain (France) which consisted of Haplic Calcisols and Rendzic Leptosols. Factorial kriging analysis (FKA) was used to describe the co-regionalization of nine soil properties. A linear model of co-regionalization including a nugget effect, and two spherical models were fitted to the experimental data direct and cross-variograms of the topsoil layer properties which were previously estimated. Co-kriged regionalized factors, related to short and long-range variation, were then mapped to characterize soil variation across the two fields. The potential value of ancillary sampled variables, such as yield data, to provide information on soil properties was tested. The relation between yield and measured soil properties appeared to be weak in general. However, the structures of the variation in yield appeared to be relatively stable for two years and showed similar patterns as the co-kriged soil factors. This suggests that information on the scale of variation of soil properties can be derived from yield maps, which can also be used as a guide to suitable sampling interval for soil properties and as a basis for managing fields in a precise way.

Effect of topography on nitrous oxide emissions from winter wheat fields in Central France.

We assessed nitrous oxide (N(2)O) emissions at shoulder and foot-slope positions along three sloping sites (1.6-2.1%) to identify the factors controlling the spatial variations in emissions. The three sites received same amounts of total nitrogen (N) input at 170kgNha(-1). Results showed that landscape positions had a significant, but not consistent effect on N(2)O fluxes with larger emission in the foot-slope at only one of the three sites. The effect of soil inorganic N (NH(4)(+)+NO(3)(-)) contents on N(2)O fluxes (r(2)=0.55, p<0.001) was influenced by water-filled pore space (WFPS). Soil N(2)O fluxes were related to inorganic N at WFPS>60% (r(2)=0.81, p<0.001), and NH(4)(+) contents at WFPS<60% (r(2)=0.40, p<0.01), respectively. Differences in WFPS between shoulder and foot-slope correlated linearly with differences in N(2)O fluxes (r(2)=0.45, p<0.001). We conclude that spatial variations in N(2)O emission were regulated by the influence of hydrological processes on soil aeration intensity.

A portable infrared laser spectrometer for flux measurements of trace gases at the geosphere–atmosphere interface

A portable infrared laser absorption spectrometer named SPIRIT (SPectrometre Infra-Rouge In situ Tropospherique) has been set up for the simultaneous flux measurements of trace gases at the geosphere–atmosphere interface. It uses a continuous wave distributed feedback room temperature quantum cascade laser and a patented new optical multi-pass cell. The aim of SPIRIT field studies is to get a better understanding of land and water bodies to atmosphere exchange mechanisms of greenhouse gases (GHG). The analytical procedures to derive concentrations and fluxes are described, as well as the performances of the instrument under field conditions. The ability of SPIRIT to assess space and time dependence emissions of two GHG—nitrous oxide (N2O) and methane (CH4)—for different types of ecosystems is demonstrated through in situ measurements on peatland, on fertilized soil, and on water body systems. The objectives of these investigations and preliminary significant results are reported.

A crop model for land suitability evaluation a case study of the maize crop in France

Abstract A crop model to evaluate land suitability is described. It has been devised to study spatial variation and uses readily available input data. The case study described is for the maize crop and uses a simple growth model for this crop. The model is incorporated within procedures that allow the descrip tion of crop environment variability both in space and time and the model is run under a Geographical Information System. Input data are stored in soil, climate and crop management data bases, for 20 × 20 km areas and constitute the basic information for crop growth simulation. From the network of synoptic meteorological stations, climatic variables are spatially interpolated to give predicted values for each elementary area. The model computes every ten days : i) potential crop productivity in the absence of any stress ; ii) productivity in limited-water situation. The modelling principles for the soilplant-atmosphere system are simple : development depends on thermal time, growth depends on energy use efficiency and the calculated water balance uses a reservoir model. Because of the ten-day time step, particular attention was given to the way in which water stress affects the growth-development functions. A study proved the model to be reliable for estimating maize productivity in various locations although some discrepancies between measurements and simulations can occur for intermediate variables in extreme environmental conditions. As illustrations of the model performance, map outputs of land suitabilities over France for maize growing are presented.

Changes in Field Soil Water Tracked by Electrical Resistivity

Recently, geophysical methods have been developed that can monitor soil characteristics spatially at high resolution. However, interpreting electrical measurements is difficult because geophysical data can be influenced by many soil variables, some of which vary over time. Our objective here was to use spatial measurements of electrical resistivity to define zones of homogeneity, to interpret them in terms of changing water contents, and to compare them with a soil map. Our underlying assumption was that the time variation of electrical resistivity at the field scale was only due to the dynamics of soil moisture in our studied field. Monitoring of soil electrical resistivity and soil moisture was performed at four dates during 2006 by two methods: by the use of the MUCEP (MultiContinuous Electrical Profiling) device, which gives measurements over a whole field, and by local gravimetric measurement of soil water content. Homogeneous zones were defined directly from measurements of the electrical resistivity and after ordinary kriging of the water content. Our analysis of spatial and temporal variability has permitted us to discriminate three temporally homogeneous zones, in terms of both electrical resistivity and water content, which were broadly related to the soil map. The use of electrical measurements enabled us to directly describe spatial and temporal changes in soil water content at the field scale and to describe some hydraulic processes, like lateral flows or upward capillary flows, that would be difficult to derive from soil maps.