The Effect of Phosphorus Addition and Cutting Date on the Nutrient Dynamics and Species Composition of Floodplain Meadows

Many studies have investigated the impact of nitrogen (N), phosphorus (P) and potassium (K) on grassland in terms of vegetation production, composition and species richness, however few have investigated the impact of P addition on floodplain meadows and none have addressed the effect of P availability on N mineralization and N uptake. It has been suggested by several researchers that net N mineralization increases with increasing P availability in the soil, which implies the effects of P on vegetation could be in part mediated by N, however this has never been tested directly. This thesis tests the hypothesis that net nitrogen mineralization increases with increasing phosphorus availability in the soil. A replicated P addition experiment in the field has been used in conjunction with performing multiple cuts throughout the growing season to investigate the allied effect of nutrient removal. The biomass harvested was analysed for N and P content to estimate N and P uptake by the vegetation. A laboratory incubation experiment was also used to investigate the effect on N-mineralization rate of adding P to soil cores. The field experiment revealed that the addition of P in combination with N produced a significant increase in vegetation N uptake, and that P was also significant in explaining plant species composition. However, addition of nutrients N and P had no effect on species richness and vegetation community change was driven by temporal changes in water regime. The incubation experiment demonstrated that P addition increased N mineralization in the soil cores where significantly increased levels of NH4+ were detected with increasing P concentration. Above-ground biomass was primarily limited by N, although the increase in N offtake in the treatment that added both P and N indicates co-limitation. The cutting trial revealed that peak nutrient offtake occurred later in the growing season with the addition of P; maximum nutrient removal was mid-June for unfertilized plots and mid-July for P fertilized plots indicating that cutting date may need to be flexible on this vegetation type