Nitrogen Addition Decreases Soil Respiration without Changing the Temperature Sensitivity in a Semiarid Grassland

The mechanisms underlying the response of soil respiration (Rs) to nitrogen (N) addition remain to be explored in semiarid ecosystems. This study was conducted to determine the effect of N addition on soil microbial composition, Rs and the temperature sensitivity of Rs (Q10). The N addition experiment was carried out in a semiarid grassland in China, with N fertilizer application rates of 0, 2, 4, 8, 16, or 32 g N m–2 yr–1. Microbial phospholipid fatty acids (PLFAs), Rs and Q10 were measured, and their relationships with soil properties were determined for three growing seasons. The results showed that N addition significantly increased the content of soil dissolved organic carbon (DOC) and inorganic nitrogen (IN), and decreased soil pH. With respect to soil microbes, N addition reduced soil PLFAs, reduced the fungi to bacteria ratio (F:B) and increased the gram-positive bacteria to gram-negative bacteria ratio (G+:G–). Rs under the N2, N4, N8, N16 and N32 treatments decreased by 2.58%, 14.86%, 22.62%, 23.97% and 19.87%, respectively, compared to the N0 (control) treatment. The results of structural equation models showed that N addition reduced Rs by lowering soil PLFAs and altering the microbial composition. However, N addition had no significant effect on either Q10, soil total organic carbon (TOC) or total nitrogen (TN), indicating that N addition alleviated soil carbon loss and was unlikely to change the potential for a bigger loss under global warming.