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Background and aims: Dissolved organic nitrogen (DON) has been increasingly recognized as a crucial component of the terrestrial nitrogen (N) cycle that regulates the ecosystem feedback to climate change. Yet, little information is available about the factors that control soil DON in the alpine ecosystems of the Tibetan Plateau, a region that is extremely sensitive to climate change. Methods: Here, we examined the relationship between DON and climate, plant, and soil attributes along a precipitation gradient in five alpine ecosystems (alpine wetland, alpine meadow, alpine shrub, alpine steppe, and alpine desert) across 20 sites that were up to 4000 km apart on the Tibetan Plateau. Results: Results showed that soil DON concentration varied significantly across these alpine ecosystems, and had a positive relationship with mean annual precipitation. Belowground biomass, soil moisture, micro biomass carbon (MBC), and soil total phosphorus (TP) explained 84% of the variation in DON concentrations. The percentage of DON in soil total dissolved N (TDN) varied with vegetation type, with the lowest percentage observed in the alpine meadow (52%) and the highest in the alpine desert (77%), indicating the slow turnover of DON into inorganic N in systems with low litter input and microbial activity. Conclusions: Precipitation and plant biomass input determine the concentration and turnover of soil DON on the Tibetan Plateau. A warmer and wetter climate that has been predicted for the Tibetan grasslands may lead to a larger and more active DON pool. [ABSTRACT FROM AUTHOR]

Background and aims: Nitrogen (N) limitation leads to intense competition between plants and soil microorganisms for available N. However, it is unclear how grazing affects the acquisition of N by plants and microorganisms. Methods: We conducted short-term N tracer experiments during the growing season (June, early growing season; July, mid-growing season; and September, late growing season) in an alpine grassland on the Tibetan Plateau to investigate the effects of grazing on the acquisition of NO -N, NH -N, and glycine-N by plants and soil microorganisms. Three dominant plant species (one graminoid, Kobresia pygmaea, and two forbs, Potentilla bifurca and Potentilla multifida) were selected for the study. As these species represented >90% of the vegetation, the plant recovery of N reflected competition at the plant community. Results: Grazing decreased the recovery of N by soil microorganisms and plants by 46 and 69%, respectively, indicating that grazing strongly reduced the uptake of N by plants and microorganisms and altered the partitioning of N between them. Significant interactions were found between grazing, season and the different forms of N. In the absence of grazing, plants acquired relatively more N than soil microorganisms for the three forms of N in July and September, whereas the microorganisms obtained relatively more N glycine in July and all three forms of N in September under grazing conditions. Under grazing, the plant root biomass played an important role in controlling plant-microbial N acquisition. Conclusions: Grazing alters the partitioning of inorganic and organic N between plants and soil microorganisms by reducing microbial N recovery to a lesser extent than plant N recovery. This indicates that heterotrophic microorganisms play an important part in N cycling in N-limited ecosystems. [ABSTRACT FROM AUTHOR]

Highlights: [•] Timing of phenological events is not necessarily delayed as elevation increases. [•] Changes in timing of phenological events are inconsistent as elevation increases. [•] Individual reproductive components have independent responses to climate change. [•] Duration of reproduction stage is dependent on timing of phenological events. [•] Response of phenological events to climate change is species-specific. [Copyright &y& Elsevier]