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 thosemediated through effects on legumes, which have a role in the N cycle by fixing N2. These effects may alter plant functioning and community structure, especially in N-limited ecosystems.Wemanipulated temperature and plant diversity in the field to investigate the effects of elevated temperature on aboveground biomass, shoot N concentration ([N]), and reliance on N2 fixation of four prairie legumes (Amorpha canescens Pursh., Dalea purpurea Vent., Lespedeza capitataMichx. 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 variablesmaymediate the effect ofwarming on legumes.Warming decreased soilmoisture 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 N2 fixation using stable isotopes, they suggest that warming did not affect the reliance on N2 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 N2 fixation. We observed strong interannual variation between awet and dry year for Nmineralisation, shoot [N] and reliance onN2 fixation, suggesting that these may be more responsive to precipitation changes than elevated temperature. Additional keywords: Amorpha canescens, Dalea purpurea, grassland, Lespedeza capitata, Lupinus perennis, Petalostemum purpureum. Received 17 November 2012, accepted 17 May 2013, published online 28 June 2013