Root exudation is involved in regulation of nitrogen transformation under mowing in a temperate steppe

Nitrogen (N) transformation in soil is an important process involved in regulation of functioning of grassland ecosystems by affecting soil N availability and nitrous oxide (N2O) emission from soil. As a common grassland management practice worldwide, mowing has impacts on nutrient status in soil and nutrient cycling in grassland ecosystems. Root exudates including the organic carbon (C) and N substances are stimulated by mowing and involved in regulation of microbial community in the rhizosphere. Despite reports of mowing effects on soil nitrogen transformation and root exudation in the literature, few studies have specifically evaluated whether and how the mowing-induced root exudates are involved in mowing-induced soil N transformation in grassland ecosystems. Here, we investigated the effects of mowing on root exudation and the role of root exudates in soil N transformation by both field experiments in a temperate steppe and greenhouse experiments. We found that mowing enhanced soil net N mineralization rate (NNMR) by 3.2-fold and N2O flux by 75.0% in field experiment. We further demonstrated that mowing stimulated exudation of organic acids, amino acids, sugars, and phenolics in both field and greenhouse experiments. To explore the function of the root exudates, microcosm experiments were conducted to evaluate the roles of exuded amino acids, organic acids, sugars, and phenolics in regulating NNMR, N2O flux, enzyme activity, and functional genes related to soil N transformation by adding these chemicals to soil. We discovered that the addition of glycine enhanced NNMR mainly by enhancing the abundance of ureC, gdh, and amoA related to soil N mineralization. The addition of oxalic acid and glucose increased N2O flux mainly by increasing the abundance of nirS related to denitrification. Our findings reveal that root exuded organic acids, amino acids and sugars play an important role in the mowing-induced increases in soil N transformation. These novel findings would contribute to development of mowing-based grassland management.