Effects of One-Year Simulated Nitrogen and Acid Deposition on Soil Respiration in a Subtropical Plantation in China

Atmospheric nitrogen (N) and acid deposition have become global environmental issues and are likely to alter soil respiration (R-s); the largest CO2 source is from soil to the atmosphere. However, to date, much less attention has been focused on the interactive effects and underlying mechanisms of N and acid deposition on R-s, especially for ecosystems that are simultaneously subjected to elevated levels of deposition of both N and acid. Here, to examine the effects of N addition, acid addition, and their interactions with R-s, we conducted a two-way factorial N addition (control, CK; 60 kg N ha(-1) a(-1), LN; 120 kg N ha(-1) a(-1), HN) and acid addition (control, CK; pH 4.5, LA; pH 2.5, HA) field experiment in a subtropical plantation in China. Our results showed the following: (1) During the one-year observation period, the seasonal dynamics of R-s presented a single peak curve model, which was closely related to the surface soil temperature. (2) The simulated N deposition and acid deposition significantly decreased the R-s in the subtropical plantation. Compared to the CK plots, the LN and HN treatments reduced the annual mean values of R-s by 41% and 56%, and the annual mean values of R-s were inhibited by 26% and 31% in the LA and HA plots. The inhibition of N application on R-s was stronger than that of the simulated acid deposition. (3) Significant interactions between N addition and acid addition on R-s were detected, and R-s was significantly inhibited under four co-addition treatments. (4) The underlying mechanism and main reason for the responses of R-s to simulated N and acid deposition in this study might be the inhibition of soil microbial biomass and soil enzyme activity due to soil acidification under increased N and acid input.