Do warming-induced changes in quantity and stoichiometry of root exudation promote soil N transformations via stimulation of soil nitrifiers, denitrifiers and ammonifiers?

Abstract

Despite the recognized importance of root exudation to soil biogeochemical processes and soil function in forest ecosystems, few studies have investigated the impact of quantity and stoichiometry of root exudation on soil nitrogen (N) transformations. In addition, the potential mechanism of how climate warming influences soil N cycling via root-soil interactions remains unclear. We performed an experiment in which in situ root exudates were collected from Picea asperata seedling to examine the response of root carbon (C) & N exudation rates and the C: N ratio stoichiometry along with the soil N transformation rates to warming using infrared heaters in the Eastern Tibetan Plateau. The results showed that warming enhanced root C exudation rates (ug C g−1 root biomass h−1), with higher C: N ratio stoichiometry simultaneously. Experimental warming also significantly increased the soil net mineralization rates, net nitrification rates, and denitrification rates. Then, we conducted an in vitro experiment in which three bacterial strains (Bacillus subtilis, Nitrobacter hamburgensis, and Brachymonas denitrificans) representing ammonifiers, nitrifiers, and denitrifiers, respectively, were cultured with exudates as the culture media. The results indicated that root exudates collected in warmed plots significantly stimulated the growth of the three soil N transformation bacteria, compared with the treatments in control plots. Collectively, our results, to some degree, evidence a linkage that climate warming-induced changes in C fluxes and C: N ratio stoichiometry of root exudation stimulated the growth and reproduction of soil N transformation bacteria, consequently accelerating the soil organic matter decomposition and soil N transformations.