Grazing exclusion enhanced net ecosystem carbon uptake but decreased plant nutrient content in an alpine steppe
2020
Liu, Yongwen | Tenzintarchen, | Geng, Xiaodong | Wei, Da | Dai, Dongxue | Xu-Ri,
Grazing exclusion is widely conducted to restore degraded grasslands. The recovery status can be indicated by both nutrient and carbon cycling. However, the effect of grazing exclusion on net ecosystem CO₂ exchange and mineral nutrients in pastures on the Tibetan Plateau remains unclear, due to a lack of in situ investigations. Here, we conducted an in situ investigation of the grazing exclusion effect on net ecosystem CO₂ exchange and foliar mineral elements in an alpine steppe on the central Tibetan Plateau. Our results show that seven years of grazing exclusion enhanced net CO₂ uptake by 17% during the growing season via increased gross ecosystem photosynthesis, rather than unchanged ecosystem respiration. Grazing exclusion reduced the sensitivity of ecosystem respiration to soil surface temperature by 16–17% at both diurnal and seasonal scales. Grazing exclusion, therefore, may be an effective way to enhance carbon sequestration under the background of a warming climate. We further found that grazing exclusion significantly decreased the soil available nitrate-nitrogen concentration by 50% and foliar mineral concentrations such as nitrogen, calcium, sulfur, iron, and manganese. Grazing exclusion increased the foliar carbon:nitrogen ratio by 16% and decreased the foliar nitrogen:phosphorus ratio by 15%. These results indicate that grazing exclusion enhanced net ecosystem CO₂ uptake but aggravated mineral nutrient limitation of plant growth. Our study highlights that grazing exclusion influences alpine grassland carbon cycling by directly preventing grazing and by indirectly altering the status of plant nutrients. A quantitative distinction of such direct and indirect effects based on long-term and continuous in situ field observations is critical for revealing the mechanism underlying carbon-nutrient cycling evolution in the restoration of degraded alpine grasslands.
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