Carbon and Nitrogen Losses from Soil Depend on Degradation of Tibetan Kobresia Pastures

Degradation of Kobresia pygmaea pastures has strongly increased on the Tibetan Plateau over the last few decades and contributed to a high loss of soil organic carbon and nutrients. The pathways of carbon (C) and nitrogen (N) losses from degraded K. pygmaea pastures are still unclear, but this is a prerequisite to assess the recovery of Tibetan grasslands. We investigated the response of day- and nighttime CO2 efflux and leaching of dissolved organic C and N, NH4+ and NO3- from K. pygmaea root mats in three degradation stages: living root mat, dying root mat and dead root mat. Dying root mat had the highest C loss as CO2 and as leached dissolved organic carbon. This indicates K. pygmaea pastures shift from a C sink to a C source following plant death. In contrast, living root mat had the lowest daytime CO2 efflux (038 +/- 01 mu gCg(-1)h(-1)) because CO2 was assimilated via photosynthesis. Nighttime CO2 efflux positively correlated with soil moisture for living and dead root mats. It indicates that increasing precipitation might accelerate C losses due to enhanced soil organic carbon decomposition. Furthermore, dead root mat had the highest average NO3- loss (23 +/- 26mgNL(-1)) from leaching compared with other root mats. Consequently, leaching increases the negative impacts of pasture degradation on N availability in these often N limited ecosystems and thus impedes the recovery of K. pastures following degradation. Copyright (c) 2016 John Wiley & Sons, Ltd.