The Responses Of Ecosystem Carbon Exchanges To Experimental Warming And Grazing Exclusion In A Meadow Grassland On The Northern Shore Of Qinghai Lake, China

The carbon cycle is one of the major biogeochemical cycles describing the flow of essential elements from the environment to living organisms and back to the environment again. The terrestrial ecosystems carbon cycles both drive and respond to environmental changes and human activities ranging from local to global scales. This process is essential for the building of all organic compounds and involves the participation of many of the earth’s key forces. The carbon cycles not only helped facilitate the evolution of life, but also contributed to the major climate changes that we currently observed. Therefore, studies about how ecosystem carbon cycles respond to the observed global climate change factors and human activities can contribute substantially to our understandings of carbon-climate feedbacks and modeling of future climate change. However, there are still large uncertainties in how ecosystem carbon exchanges respond to climate change and human activities.We conducted a field experiment to simulate the experimental warming by open top chambers(OTCs), and another experiment by grazing exclusion in a meadow grassland on the Tibetan Plateau. Our aims were to evaluate how ecosystem carbon exchanges respond to experimental warming and grazing exclusion.Field experiments by OTCs significantly increased soil temperature, while decreased soil moisture. Based on our three years field observations, our results showed that warming by OTC significant increased soil temperature by 1.03 oC, while OTC decreased soil moisture by 3.7%. Meanwhile, compared with the control blocks, warming by OTC significantly increased above ground biomass by 17.4%, and below ground biomass by 24.3%, respectively. In addition, warming by OTC also significantly altered the importance value index of plant community. Compared with the the control blocks, warming by OTC significantly increased the importance value index of graminoids, legumes, and sedges, while warming decreased the importance value index of forbs. Warming by OTC also significantly alter the biomass accumulation for different plant functional groups. In detail, warming significantly increased the biomass of graminoids, legumes, and sedges by 12.9%, 27.6%, and 21.5%, respectively, but warming had non-significant effects on forbs biomass.Warming by OTC had non-significant impacts on soil respiration and ecosystem respiration, suggesting that soil respiration and ecosystem respiration were insensitive to experimental warming. Importantly, the sources component of soil respiration and ecosystem respiration responds differentially to experimental warming. In detail, compared with the control blocks, experimental warming significantly increased aboveground plant respiration and plant respiration by 28.7% and 19.9%, respectively, while warming by OTC significantly decrease soil heterotrophic respiration by 10.4%, and experimental warming had non-significant impacts belowground plant respiration. Experimental warming also profoundly altered the contribution of the components of ecosystem respiration to the ecosystem respiration. Compared with the control blocks, experimental warming significantly increased the contribution of aboveground plant respiration and plant respiration to ecosystem respiration by 17.3% and 8.4%, respectively, while experimental warming decreased the proportion of soil respiration and soil heterotrophic respiration to ecosystem respiration by 10.8% and 19.0%, respectively. Multiple regression analysis showed that warming-induced reductions in soil moisture played critical roles in controlling the responses of the components of ecosystem respiration to experimental warming. In addition, there were significant positive relationships between warming-induced increases in aboveground biomass and warming-induced increases in plant respiration and aboveground plant respiration, as well as warming-induced reductions in microbial biomass carbon and warming-induced decrease in soil heterotrophic respiration.Experimental warming significantly increase net ecosystem exchange and gross primary productivity by 31.0% and 18.7%, respectively, compared with the control blocks. Warming-induced changes in soil temperature and soil moisture are mostly failed in the explanation of warming-induced changes in ecosystem carbon exchanges. Interestingly, warming-induced changes in ecosystem carbon exchanges are significantly correlated with warming-induced changes in plant functional groups biomass. Our results provide clear evidence that experimental warming-induced changes in plant functional groups had significant impacts on ecosystem carbon exchanges.Compared with the free grazing site, grazing exclusion significantly increased soil moisture but decreased soil temperature. Grazing exclusion also significantly increased aboveground biomass by 40.5%, but had no impacts on belowground biomass. Compared with free grazing sites, grazing exclusion also significantly decreased soil microbial biomass carbon.Grazing exclusion decreased soil respiration by 23.35%, while it also significantly increased net ecosystem exchange, ecosystem respiration, and gross primary production by 47.37%, 36.55%, and 33.14%, respectively. Multiple regression analysis showed that grazing exclusion-induced changes in soil respiration are mainly accounted by grazing exclusion-induced changes in soil temperature, soil moisture, and microbial biomass carbon. Meanwhile, grazing exclusion-induced changes in net ecosystem carbon exchanges, ecosystem respiration, and gross primary production are all significantly correlated with grazing exclusion-induced changes in plant productivity. Our results suggest that grazing exclusion is an effective method to store more carbon.Experimental warming significantly decreased the temperature sensitivity of both soil respiration and ecosystem respiration. Our results suggest that the carbon-climate feedbacks may not as strong as we previously hypothesized. On the other hand, grazing exclusion significant increase the temperature sensitivity of both soil respiration and ecosystem respiration, although grazing exclusion significantly decreased soil respiration. Our results suggest that grazing exclusion sites are more likely than grazed sites to release more carbon under future climate warming scenarios, although grazing exclusion in an effective way to store more carbon. The amount of carbon stored by grazing exclusion may be highly sensitive to the future warming conditions, especially on the Tibetan Plateau. Our results suggest that both experimental warming and grazing exclusion can strongly affect ecosystem carbon exchanges, therefore, these issues should be taken into consideration when we are estimating or modeling the global or regional carbon budgets. Also, there might be some interactive effects between experimental warming and grazing exclusion, which should be concerned in our future studies.