Changes in energy fixation and efficiency of energy capture in above-ground biomass along an environmental gradient in calcareous grasslands

Semiarid calcareous grasslands adjacent to agricultural land are often subject to eutrophication in the border zones. Depending on the extent of the nutrient input, a more or less strongly pronounced nutrient gradient develops in the semiarid grassland, and a change in structure of the plant community occurs. In this study, changes in above-ground energy fixation along nutrient gradients in four semiarid grasslands in the Eifel mountains were investigated. The caloric values of the ash-free material did not differ among the plant groups studied, however the values exhibited pronounced seasonal fluctuations. A relationship was found between the caloric values and changes in the crude protein content over the course of the year for grasses in the border zones of two dry grasslands. In one transect higher caloric values were found in the border zone than in the dry grassland. The seasonal variations in the energy content of the standing crop were determined primarily by the seasonal variations in biomass development. The annual energy fixation and the efficiency of energy capture were markedly lower in the dry grassland sectors than in the more nutrient-rich border zones, and were lower in the border zones than in the adjacent agricultural land. Both exhibited a positive correlation with the annual nitrogen mineralisation. However the utilisation of the nitrogen supply calculated as energy fixation per gram of mineralised nitrogen decreased sharply with increased net nitrogen mineralisation. The energy fixed per unit weight of nitrogen in the above-ground biomass did not vary along the nutrient gradients.