Responses Of Ecosystem CO₂ Exchange To Nitrogen Addition,Water Addition And Grazing In Songnen Meadow Steppe

Climate change and anthropogenic activity intensification would have profound influences on ecosystem structure and functioning, such as carbon cycle, which may subsequently feedback to climate change and exaggerate its impacts. Terrestrial ecosystem carbon cycle is an important part of the global carbon cycle. With vast area and huge soil carbon pool, grasslands play an important role in the global carbon balance process, which is often constrained by the timing and availability of water and nitrogen.The present study was conducted in a meadow steppe in Songnen grassland. Using a long-term grazing experiment platform, we tested the combined effects of grazing, precipitation and nitrogen addition on grassland ecosystem C exchange, soil respiration, as well as net ecosystem CO2 exchange(NEE) drought sensitivity. We conducted a 3-year field experiment to assess the responses of vegetation composition, ecosystem productivity, ecosystem C exchange and soil respiration to sheep grazing, manipulative water and nitrogen addition, as well as extreme drought. The important results and conclusions are as follows:(1) Nitrogen addition significantly stimulated aboveground biomass accumulation and NEE, which suggests that nitrogen availability is a primary limiting factor for ecosystem C cycling in the studied meadow steppe. Water addition had no significant impacts on either ecosystem C exchange or plant biomass, but ecosystem C fluxes strongly correlated with early growing season precipitation, rather than whole growing season precipitation, across the 3 experimental years. After the incorporation of water addition into the calculation of precipitation regimes, we found that monthly average ecosystem C fluxes correlated more strongly with precipitation frequency than with precipitation amount. These results highlight the importance of precipitation distribution in regulating ecosystem C cycling. Overall, ecosystem C fluxes in the studied ecosystem are highly sensitive to nitrogen deposition, but less sensitive to increased precipitation.(2) The manipulated extreme drought events significantly reduced grassland ecosystem CO2 exchange, but had little effects on plant biomass. However, we also found that the recovery rate of NEE positively correlated with both plant biomass and water supply after the drought. These results suggest that grass photosynthetic processes were significantly inhibited by the lack of water supply; meanwhile, plants tend to maintain the normal autologous water stability and tissue activity rather than acquire more carbon and energy during the drought, which is a strategy led to higher resilience after disturbance. Additionally, long-term nitrogen and water addition significantly enhanced NEE drought sensitivity(SNEE). After pooled all the data together, we detected SNEE depends strongly on both gross biomass and root/shoot ratio. These results highlight the negative influence of global changes associated variation in plant biomass and biomass allocation in the ability of ecosystem water balance between demand and supply to tolerate water stress.(3) Grazing greatly enhanced plant diversity in Songnen grassland, and also caused inhibition in the growth of dominant species, which benefited more from nitrogen and water addition. This phenomenon is caused by nitrogen addition induced improvement in the palatability of dominant species and grazing of herbivores on dominant species. At the same time, grazing induced enhancement in NEE was detected in all treatment plots. Significant interactive effects on NEE were detected between grazing and either nitrogen or water addition. Hence, although grazing partially removed vegetation biomass, its net effect may promote Songnen grassland ecosystem carbon sequestration through enhancement in the efficiency of resource utilization and transformation.(4) Water and nitrogen addition stimulated heterotrophic soil respiration(SRH) in 2012 and 2013, but they only significantly increased total soil respiration(SRTOT) and autotrophic soil respiration(SRA) in a dry year(2013). These results illustrated that SRH was mainly limited by environmental factors, whereas SRA was constrained by plants demand. Grazing decreased the supply of soil carbon source by photosynthesis, leading to a negative response of SRTOT and SRH in both experimental years. Using the structural equation model method, we found the root-shoot ratio, belowground biomass and soil microbial biomass C were the main factors to regulate soil respiration. These findings highlight the capability of respiratory substrate supply and efficiency of carbon utilization may play an important role in the controlling of Songnen grassland ecosystem soil respiration.In conclusion, the ecosystem productivity in the studied meadow steppe will benefit more from the predicted increase in N deposition than from greater total precipitation. Grazing can improve the stability of the grassland ecosystem community structure, and strengthen the effect of N deposition and precipitation increasing on grassland ecosystem CO2 exchange. According to the IPCC report, Songnen grassland may experience more severe decoupling in the balance of water supply and water demand due to more extreme weather, longer precipitation interval and lower rainfall frequency in the future. This imbalance in water supply and demand may constrain grassland ecosystem carbon sequestration function. However, grazing grassland, which own high resistance to drought stress, is likely to have higher productivity and carbon accumulation. Based on the ecosystem CO2 exchange and soil respiration measurements, our study further confirmed the important role of global climate change, land use and their interactions on grassland ecosystem carbon cycle and carbon sequestration function. Elucidating the principles underlying these influences has important theoretical value. More importantly, this study provided adequate experimental data support and guidance to the protection and rational utilization of grassland.