Responses of plant 15N natural abundance and isotopic fractionation to N addition reflect the N status of a temperate steppe in China

Abstract

Elevated anthropogenic nitrogen (N) deposition could alter N status in temperate steppe. However, threshold observations of N status change from N limit to N saturation by far are not conclusive in these ecosystems. Research on the natural abundance of ¹⁵N (δ¹⁵N) could greatly help provide integrated information about ecosystem N status. The goal of this study was to investigate the suitability of measurements of δ¹⁵N of major ecosystem N pools and several key species, plant ¹⁵N fractionation, together with key vegetation and soil indicators in response to N fertilization as a tool to identify the N status in a temperate steppe in Inner Mongolia. We carried out a N addition experiment during 2011–14 on a Stipa krylovii steppe in Inner Mongolia, Northern China. We investigated the response of several key N transformation processes, vegetation and soil properties to N addition. Aboveground biomass and belowground biomass (BGB) δ¹⁵N, root and foliar δ¹⁵N of three dominant species (Artemisia frigida, S. krylovii and Leymus chinensis), δ¹⁵N of soil total N and soil KCl-extractable NO₃⁻-N were determined. The responses of isotope fractionation during plant N uptake and reallocation to N addition were also determined. Our results suggest that the N addition rate of 5g N m⁻² yr⁻¹ could be regarded as threshold of early N saturation in this S. krylovii steppe as indicated by an increase in plant fractionation and a decrease in plant δ¹⁵N. When N input rate is >10 g N m–² yr–¹, increased N deposition can lead to an apparent reduction in species richness and BGB as well as an increase in NO₃⁻ in extractable soil pools <30-cm soil profile. With N addition, S. krylovii and A. frigida undergo earlier N status shift from N limitation toward N excess compared with L. chinensis, contributing to L. chinensis out-competing other species. Overall, this study provides a better understanding of N status change in temperate steppe based on isotope evidence and several other functional variables and contributes to predicting the responses of temperate steppe to future global N deposition scenario.