Aims Nitrogen (N)-fixing legumes, despite being highly phosphorus (P)-demanding, constitute an important plant functional group and play key roles in N-poor ecosystems such as alpine grasslands. However, legume performance, including biomass, abundance and species richness, is expected to change, because anthropogenic activities have drastically increased soil N and P availability worldwide. We conducted a field experiment to assess the effects of N and P addition, alone and in combination, on legume performance in an alpine grassland, and identified and clarified the mechanisms underlying these changes.
Methods A three year field experiment of N addition (10g N m ?2 year-1), P addition (5g P m ?2 year-1), and N + P combined addition (both N and P, same amounts as solo treatments) was conducted in an alpine grassland on the Tibetan Plateau in China from 2011 to 2013. Effects of nutrient addition were assessed at the community level (above-ground net primary production (ANPP), height and light intensity), functional group level (biomass, species richness, relative height, relative coverage and relative density of legumes) and species level (foliar N, P concentration of two legumes).
Important findings Overall, adding N alone significantly increased ANPP by 20.82%, but adding P alone did not; whereas, addition of N and P together resulted in a large increase in ANPP (+37.03%) than addition of either alone, indicating potential co-limitation of alpine grasslands. In contrast, adding P alone significantly promoted legume performance as measured by 65.22% increase in biomass and 58.45% increase in relative abundance, while adding N alone reduced legume performance as measured by 39.54% decrease in biomass and 50.36% in relative abundance. Combining P and N addition did not mitigate the negative effect of N addition on legume performance and, surprisingly, suppressed legume biomass by 53.14% and relative abundance by 63.51%. N and P addition altered the balance of light competition between grasses and legumes as indicated by the changes in light levels, plant heights and litter accumulation. However, there were no obvious changes in legume species richness in response to N and P within our experimental timeframe. This study provides further evidence of the importance of P as a co-limiting nutrient in alpine grasslands, contrary to the traditional view that N limitation predominates in such regions. The contrasting effects of N and P addition on legume performance provide important insights into potential changes in legume performance in nutrient-limited grasslands following N and P enrichment under climate change, with implications for nutrient management in alpine grasslands.