Effect of N addition on root exudation and associated microbial N transformation under Sibiraea angustata in an alpine shrubland

Root exudates are generally known to play an important role in ecosystem carbon (C) and nitrogen (N) cycling. We aimed to quantify the responses of root exudates to increased N availability in an alpine shrub-dominated ecosystem, and unravel the associated ecological consequences to N cycling. After three consecutive years of N addition at three rates (N0, control; N5, 5 g N m−2 year−1; N10, 10 g N m−2 year−1), we measured in situ rate of root exudation for mature Sibiraea angustata shrubs, as well as soil microbial biomass, the activities of three extracellular enzymes, and net N mineralization rates (Nmin) in both rhizosphere and bulk soil. Exudation rates were 13% and 45% lower in the N5 and N10 plots compared to the control plots over the primary growing season. This, together with decreasing fine root biomass and length, lead to an annual decline of 44% and 66% in root exudate C inputs to soil, respectively. These decreased exudation rates were both directly and indirectly, through its effect on the rhizosphere effect (RE) on polyphenol oxidase activity, linked to decreased the RE on Nmin. As a result of N-induced decreases in both the RE on Nmin and rhizosphere volume, we estimated that the ecosystem-level rhizosphere effect for Nmin from 13% in the control plots to 8% in the N5 plots, and to 2% in the N10 plots. Overall, our results demonstrate that N addition may decelerate the C efflux from root exudates, resulting in a slower decomposition of soil organic matter and also gradually decelerating soil N cycling in alpine shrub ecosystems.