Effects of warming and grazing on dissolved organic nitrogen in a Tibetan alpine meadow ecosystem

The production and fate of inorganic N as regulated by environmental change are relatively well understood, but we know comparatively little about how these factors influence dissolved organic nitrogen (DON). We measured total N, DON and dissolved inorganic nitrogen (DIN) concentrations in the soil solution and plant N uptake in a factorial warming×grazing experiment in a Tibetan alpine meadow. Results showed that warming significantly decreased DON concentration by up to 36%. The effect of warming on DON concentration in the soil solution varied with sampling date and soil depth. However, our results showed that moderate grazing set off the effect of warming on DON concentration in the soil solution. Grazing increased soil DON, opposite to the effect of warming. Previous studies have found warming to increase a range of factors which contribute to the supply of DON in soil. Our results show that the observed decrease of DON under warming was correlated with plant N uptake, suggesting that warming stimulated processes making DON available to plants. Our study highlights the complex interaction of land management regime and climate warming in the regulation of DON cycling in N-limiting environments.