SOIL N DYNAMICS IN A NATURAL CALCAREOUS GRASSLAND UNDER A CHANGING CLIMATE

This paper reports the results from a medium-term field scale investig ation into the effects of simulated climate change on soil N mineralis ation in a semi-natural calcareous grassland in southern England. The experiment utilised soil warming cables, automatic rainshelters and a watering system to examine two climate change scenarios: warmer winter s with summer drought and warmer winters with enhanced summer rainfall . Gross N mineralisation rates in treated plots were determined, using N-15 pool dilution techniques, at 6-weekly intervals over a 3-year pe riod. Results from control plots showed a strong seasonality of minera lisation with highest rates in autumn and winter and lowest rates in s ummer. They suggest that water availability is the main constraint on microbial processes and plant growth. Unexpectedly, additional summer rainfall had no direct effect on N mineralisation at the time of appli cation (summer). The treatment did, however, significantly (<0.05%) re duce rates in subsequent autumn and winter months. In contrast, summer drought significantly increased N mineralisation rates in autumn and winter. Winter warming similarly had no direct effect on N mineralisat ion in winter but decreased rates in spring. We hypothesise that the o bserved treatment effects result from changes in organic C and N input , in plant litter, resulting from the direct impact of climatic manipu lation on perennial plant growth, death and senescence. This paper com pares and contrasts the response to climate manipulation in the grassl and sq stem with results from other ecosystem types such as northern f orests.