| 英文摘要： |
Carbon fluxes of grasslands could be deeply affected by nitrogen (N) enrichment, however, it remains unclear how other factors modulate the N-induced changes in carbon fluxes and the underlying mechanisms. In this study, we quantified the effects of N addition on net ecosystem CO2 exchange (NEE) and its two components (gross primary productivity (GPP) and ecosystem respiration (R-e)), and how water availability modulates these effects in a temperate grassland, China. We found that N enrichment significantly increased foliar N content of grasses (related to a higher photosynthetic rate), leading to higher aboveground biomass (AGB) and therefore vegetation cover, implying higher photosynthetic leaf area. AGB was further enhanced as root: shoot ratio tended to be reduced by N-addition. Consequently, GPP was significantly stimulated by N-addition. R-e insignificantly increased with N addition. With N-induced larger increase of GPP and little change of R-e, NEE, which is negative when an ecosystem sequestrates more carbon than it releases to the atmosphere, became more negative. Due to a significant sharper versus a relatively gentler N-induced increase with increasing water availability for GPP and R-e, respectively, the N-induced reduction of NEE distinctly increased linearly with increasing water availability. The enlarged N-induced stimulation of GPP and thus NEE at ample water conditions is mainly associated with luxuriant growth of grasses who got extra increase of foliar N content, AGB, and decrease of root: shoot ratio. Our findings facilitate an advanced understanding of carbon fluxes responding to climate change and provide knowledge for management in grassland ecosystems. |
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