Effects Of Warming And Nitrogen Addition On Soil Respiration,Microbial Properties And Plant Production Distribution In Grassland Ecosystems

Climate warming and atmospheric nitrogen deposition are two important components of global changes,which have profound impacts on the carbon cycle of terrestrial ecosystems.In the context of global changes,microbes and plants produce carbon feedback to the atmosphere through respiration.Therefore,a comprehensive understanding of the responses of soil respiration,microbial community and plant growth to climate change is of great significance for the parameterization of earth process models and the accurate prediction of the future global carbon cycle.Grassland,one important biota in terrestrial ecosystems,accounts for 40%of the terrestrial vegetation cover.However,the study of key soil carbon processes in grassland ecosystems in areas sensitive to climate change(high latitude or high altitude)is still lacking.Thus,we conducted field manipulative warming and nitrogen addition experiments in a semi-arid alfalfa-pasture and a fenced semi-arid natural grassland in the Loess Plateau of China,and evaluated the responses of above-and belowground biomass distribution and production to long-term(10 years)geothermal warming in a sub-arctic grassland.Open-top chambers were used to elevate temperature passively and geothermal warming was achieved due to an earthquake including three soil temperature treatments(+0 ^oC,+3.6 ^oC and+7.6 ^oC)above soil ambient.N was enriched at a rate of 4.42 g m^-2 yr^-1 with NH₄NO₃.The main findings are as following:(1)During April 2014 to March 2016 in the semi-arid alfalfa-pasture,N addition increased Rs by 14%over the two-year period;and warming stimulated Rs by 15%in the non-growing season,and inhibited it by 5%in the growing season,which can be explained by decreased plant coverage and soil water.The main effect of N addition did not change with time,but that of warming changed with time,with the stronger inhibition observed in the dry year.When N addition and warming were combined,an antagonistic effect was observed in the growing season,whereas a synergism was observed in the non-growing season.Overall,warming and N addition did not affect the Q₁₀ value over the two-year period,but these treatments significantly increased the Q10 value in the growing season compared with the control treatment.In comparison,combined warming and nitrogen addition significantly reduced the Q₁₀ value compared with the single treatment.These results suggest that the negative indirect effect of warming-induced water stress overrides the positive direct effect of warming on Rs.Our results also imply the necessity of considering the different Rs responses in the growing and non-growing seasons to climate change to accurately evaluate the carbon cycle in the arid and semi-arid regions.(2)During April 2015 and December 2016 in the fenced natural grassland,Warming significantly decreased Rt and Rh by 7.4%and 9.5%,respectively,but had no significant effect on Ra.N addition significantly stimulated Ra by 34%,whereas it inhibited Rh by 11%and had no significant effect on Rt.N addition significantly increased the contribution of Ra to Rt by 10%.Warming decreased the Q₁₀ values of Rt and Rh but had no significant effect on Ra.N addition significantly increased the Q₁₀values of Rt and Rh,whereas it decreased the Q₁₀ values of Ra.The combination of warming and N addition had a synergistic effect on the cumulant of Rh,whereas it had an antagonistic effect on Ra.No interactive effect between warming and N addition was observed on Rt.Our results emphasized that the Ra and Rh responded differently to warming and N addition.Our findings suggested that Rt has the potential to resist climate warming and increasing N deposition by differentiating the responses of its inherent components.(3)During April 2015 and December 2017 in the fenced natural grassland,Warming did not affect microbial biomass or the composition of microbial functional groups.However,warming significantly decreased microbial respiration,directly resulting from soil p H decrease driven by the co-mediation of aboveground biomass increase,inorganic nitrogen increase and moisture decrease.These findings highlight that the soil microbial community structure of semi-arid grasslands resisted the short-term warming by 2 ^oC,although its metabolic rate declined.(4)In the sub-arctic grassland,ten years soil warming did not increase standing root biomass and even fine root production,as well as decreased aboveground biomass and production.Structure equation modeling(SEM),used to understand underlying mechanisms,suggest that the reduction in fine root production was mainly attributed to the decreased aboveground biomass and the reduced soil organic carbon(magnitude index of soil organic matter)and total nitrogen.Long-term soil warming did not significantly change root-shoot ratio attributed to the balance between favorable thermal and unfavorable substrate C and N environments for plant growth.These counterintuitive responses suggest an urgent need to improve our understanding of the comprehensive mechanisms underlying the responses of subarctic grasslands to long-term soil warming.These results could provide important data support and theoretical basis for the parameterization of earth process models and more accurate prediction of future global carbon cycle.