Effects of nitrogen addition amount and frequency on soil respiration and its components in a temperate semiarid grassland

Abstract

Aims Increasing global nitrogen (N) deposition has exerted significant influences on productivity and carbon cycle of terrestrial ecosystems. More than 90% of the carbon in grasslands is stored in the soil, therefore any changes in soil total respiration (Rs) might have a vital impact on the carbon balance and the stability of soil carbon pool of grassland ecosystems. Most of our understanding about the responses of Rs to N deposition was based on N deposition manipulative experiments with short-term (<5 years) and low frequency (1-2 times per year) N addition treatments. It is still unclear how the long term N addition and different N addition frequency will affect Rs and its components in semiarid grasslands.Methods Our study is based on a long term N addition manipulative experiment platform conducted in a typical temperate semiarid steppe, Nei Mongol. The experimental treatment consisted of six N addition amounts and two N addition frequencies. N addition treatments began at 2008. Soil respiration and its components were measured every two weeks during the growing season in 2018 and 2019.Important findings 1) Rs significantly decreased with increasing N addition amount. The negative impact of N addition on Rs was mainly resulted from the inhibition of heterotrophic respiration (Rh). 2) No significant differences were observed in responses of Rs and its components to low and high frequency N addition treatments. 3) Soil acidification caused by long term N addition inhibited soil microbial activity and changed soil microbial community composition, consequently decreased Rs and Rh. Our results suggested that the negative effect of N addition on soil carbon release still lasted after a decade of N addition treatment. In particular, the decrease of Rh would enhance the stability of soil carbon pool. No significant differences in the two N addition frequency treatments indicated that the potential impacts caused by simulated N addition with different frequencies would be diminished with prolonged treatment period. Therefore, the results of long-term (>10 years) simulated N addition experiments can provide reliable references for evaluating the responses of natural ecosystems to atmospheric N deposition.