Ecosystem multifunctionality enhancement by short-term nitrogen addition in semi-arid saline–alkaline grassland of northern China

Abstract

The vast area of saline–alkaline grasslands in the agro-pastoral ecotone of northern China has important production and ecological functions. Nitrogen (N) deposition changes the function and structure of vulnerable grasslands. However, the impacts of N deposition on ecosystem multifunctionality (EMF) remains unknown. To address this issue, a three-year in-situ study was carried out between 2018 and 2020 to assess the direct impacts of N addition on grassland ecosystem function. Eight N addition levels were applied: 0, 1, 2, 4, 8, 16, 24, and 32 g·N·m⁻²·yr⁻¹. Plant–soil–microbe equilibrium properties, productivity, and plant community structure were monitored, and the impacts of N addition rate (NAR) and year (NAY) on grassland EMF were analyzed. Short-term N addition enhanced multiple individual ecosystem functions such as dominant species, aboveground biomass, soil stoichiometry, and litter, and remarkably decreased the structure of the plant community and soil physical and chemical performance. Furthermore, short-term N addition enhanced grassland aboveground multifunctionality (AGMF) and overall EMF, and had a neutral effect on belowground multifunctionality (BGMF). The primary effect of N addition was the enhancement of AGMF by increasing aboveground biomass, thereby enhancing grassland EMF; however, grassland EMF showed relatively minor fluctuations at N addition rates of >16 g·N·m⁻²·yr⁻¹. The results of this study show that short-term N addition indirectly regulates grassland EMF by increasing aboveground biomass, and provide novel insights into the asynchronous response of AGMF and BGMF of grassland ecosystems to short-term N addition. The addition of an appropriate amount of N is essential to enhance grass yield and maintain grassland EMF in order to manage saline–alkaline grasslands within the agro-pastoral ecotone.