THE INFLUENCE OF SOIL DESICCATION ON PLANT-PRODUCTION, NUTRIENT-UPTAKE AND PLANT NUTRIENT AVAILABILITY IN 2 FRENCH FLOODPLAIN GRASSLANDS

The effects of summer soil desiccation on plant production and plant n utrient availability (determined by wet chemical extraction) in floodp lain grasslands along the rivers Allier and Loire in France were inves tigated. Soil desiccation in these river floodplains is the result of human interference with the natural flooding regime of rivers, such as dam construction and gravel mining. Flooding periods along the Allier have a longer duration (maximum of 202 days as opposed to 38 days for the Loire). The main comparison was between floodplain grasslands alo ng the two rivers. Additional comparisons were made between relatively high lying, wetter areas ('ridges') and low lying, drier areas ('swal es') within both floodplains. Thus, areas with different soil moisture content were examined, independent of river influences. The availabil ity of P was higher in the Allier floodplain than in the Loire floodpl ain, but it was similar between ridges and swales. It was concluded th at P-availability was not related to soil wetness, but to river sedime ntation. Plant production, plant nutrient uptake, and biologically med iated soil processes, such as N-mineralization and nitrification, were all higher on the wetter Allier floodplain and in the wetter swales. These higher process rates were noted where higher amounts of soil bou nd carbon and nutrients were found as well. Plant production, N-minera lization and nitrification were moisture limited at the dry ridge on t he Loire floodplain, as moisture levels were below the wilting point h ere (pF > 4.2). On the wetter parts of the floodplain, plant productio n was N-limited. This was concluded from low tissue N/P ratios (about 10) and a positive relation between plant production and N-mineralizat ion. On the wetter parts, the rate of N-mineralization depended on the size of soil organic-N pools. The size of these pools was positively related to soil wetness, which can be interpreted as a positive effect of river flooding. Reduced flooding lowers the nutrient input to floo dplains and contributes to the occurrence of soil desiccation in summe r, which results in lower nutrient cycling and reduced accumulation of soil organic matter. These effects have negative consequences for imp ortant floodplain functions, such as nutrient retention and biomass pr oduction. (C) 1998 John Wiley & Sons, Ltd.