Net ecosystem carbon budget of a grassland ecosystem in central Qinghai-Tibet Plateau: integrating terrestrial and aquatic carbon fluxes at catchment scale

Abstract The effects of aquatic carbon exports on grassland ecosystem carbon balance is unclear. Here we quantified the seasonal and annual net ecosystem carbon budget (NECB) of an alpine grassland ecosystem in the Qinghai-Tibet Plateau (QTP) permafrost region with integrative terrestrial and aquatic carbon flux measurements. The terrestrial carbon fluxes, including the net ecosystem production (NEP) and CH4 flux, were derived from eddy covariance and previous chamber-based measurements. The aquatic carbon fluxes, including dissolved organic carbon, biogenic dissolved inorganic carbon, particulate carbon, and riverine CO2 efflux of the catchment, were determined with stream monitoring. We found that NECB exhibited distinct seasonal features for the grassland ecosystem, which shifted from a carbon sink in growing season (68.8±8.7 g C m−2) to a carbon source in non-growing season (−41.1±2.4 g C m−2), while the NECB (27.7±6 g C m−2 yr−1) demonstrated a net carbon sink at an annual basis. The total aquatic carbon flux (TAC, 11.8±1.5 g C m−2 yr−1) offset 14% of the land carbon assimilation for growing season and 30% of the annual land carbon assimilation after subtracting ecosystem respiration. The higher TAC/NEP ratios in low NEP months indicated that aquatic carbon was more important on offsetting terrestrial carbon sink when NEP was low. Our results show a large contribution of aquatic carbon to NECB in a QTP grassland ecosystem, which suggest that disregarding the aquatic carbon flux can substantially overestimate the strength of terrestrial carbon sink. As aquatic carbon is coupled with hydrological processes, TAC is crucially important for NECB in the cryosphere ecosystem since the drastic hydrology and permafrost change can transport more soil carbon into the fluvial networks. More studies on NECB is needed, especially when the ecosystem have a low NEP and actively changing hydrological processes.