Responses of exchangeable base cations to continuously increasing nitrogen addition in alpine steppe: A case study of Stipa purpurea steppe北大核心CSCD

Aims Soil exchangeable base cations (BCs) play important roles in keeping soil nutrient and buffering soil acidification, which may be disturbed by anthropogenic nitrogen (N) input. Considering relatively limited evidence from alkaline soils, this study was designed to explore the effects of N addition on soil exchangeable BCs in a typical alpine steppe on the Qinghai-Xizang Plateau. Methods From May 2013, eight levels of N addition (0, 1, 2, 4, 8, 16, 24, 32 g·m-2·a-1) in the form of NH4NO3 were added in the alpine steppe, where soil is alkaline. During the following three years (2014-2016), we collected soil samples in mid-August in each year. By measuring the concentrations of exchangeable BCs, we examined their changes along the N addition gradient. We also explored the relationships between BCs and other plant and soil properties. Important findings Continuous N addition resulted in significant loss of exchangeable BCs, especially Mg2+ in all three years and Na+ in two years. The concentrations of BCs were found to be negatively related to above-ground biomass and the concentration of soil inorganic N (p < 0.05). These results indicated that increase in N availability stimulated plant growth, which in turn led to more uptake of BCs by plants. Moreover, enhanced NO3- leaching resulted in the loss of BCs due to the charge balance in soil solution. In addition, increased NH4+ displaced BCs binding to soil surface and made them easy to be leached out of soils. Different from acid soils, soil acidification caused by N deposition in alkaline soils is mainly buffered by calcium carbonate, having less effect on BCs. Our results suggest that N addition results in the loss of exchangeable BCs in alkaline soils, leading to poor buffering capacity and decreased plant productivity over long time period, which needs to be considered during grassland management in the future. © 2018 Editorial Office of Chinese Journal of Plant Ecology. All rights reserved.