Abstract

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.

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