Consequences of elevated temperatures on legume biomass and nitrogen cycling in a field warming and biodiversity experiment in a North American prairie

Increases in global temperature are likely to have effects on the nitrogen cycle, including those mediated through effects on legumes, which have a role in the N cycle by fixing N-2. These effects may alter plant functioning and community structure, especially in N-limited ecosystems. We manipulated temperature and plant diversity in the field to investigate the effects of elevated temperature on aboveground biomass, shoot N concentration ([N]), and reliance on N-2 fixation of four prairie legumes (Amorpha canescens Pursh., Dalea purpurea Vent., Lespedeza capitata Michx. and Lupinus perennis L.) planted in plots of varying species numbers. We monitored the effect of warming on soil microclimate and net N mineralisation rates, as these variables may mediate the effect of warming on legumes. Warming decreased soil moisture and increased soil temperature, but had no effect on net N mineralisation. Warming increased the aboveground biomass of D. purpurea and L. perennis, but decreased shoot [N] for all species in one year. Though the data were not optimal for quantifying N-2 fixation using stable isotopes, they suggest that warming did not affect the reliance on N-2 fixation. Species diversity did not have strong effects on the response to warming. These results suggest that legume-mediated effects of temperature on N cycling will arise from changes in biomass and tissue chemistry, not N-2 fixation. We observed strong interannual variation between a wet and dry year for N mineralisation, shoot [N] and reliance on N-2 fixation, suggesting that these may be more responsive to precipitation changes than elevated temperature.