Nitrate and Ammonium Nitrogen Addition Did Not Alter Nutrient Resorption of Dominant Plant in an Alpine Grassland

Abstract

Worldwide increases in nitrogen deposition rates are influenced by human activities. Although the total amount of N deposition tends to be stable in our country, atmospheric N deposition of the reactive N forms (NO4+-N, NH4+; NO3−-N, NO3−) is remarkably different, and the ratios of NH4+ to NO3− change continuously. Nutrient resorption is a crucial driver of plant nutrient conservation strategies and litter quality. Therefore, the plant nutrient resorption pattern has remarkable ecological significance for nutrient cycling and the community structure of the ecosystem. However, previous studies have found that plants have different preferences in NH4+ and NO3−, and the response of nutrient resorption to different N forms remains unclear. In 2017 and 2018, we conducted field experiments simulating five NO3− and NH4+ addition ratios to examine the responses of the nutrient resorption of the dominant species, Stipa capillata, in the alpine grassland of northwest China. Results showed that N addition treatments did not affect the nutrient resorption efficiencies. The results showed that N application had little effect on soil inorganic N composition, so there was no significant change in the nutrient resorption efficiencies. In addition, the nutrient resorption efficiencies were unaffected across the five different ratios of nitrate and NH4+ addition, suggesting that nutrient resorption was not only controlled by the nutrient supply and nutrient form. Our results have important implications in understanding the significance of the frequency of N addition due to N loss through leaching and denitrification. The results also highlight that low intensity and high frequency N addition are required to investigate the response of plant nutrient resorption to the N supply in the future study.