| Global warming is a serious eco-environmental problem of the world,the increase in greenhouse gas(GHG)emissions is a major driver of it.Globally,CO2,CH4 and N2O are the main greenhouse gases,which contribute 60%,15%and 5%,respectively,to the anthropogenic greenhouse effect.Grasslands cover 37%of the earth's terrestrial area and are a major GHG sink of the global ecosystem.However,how grasslands might contribute to climate change mitigation in the future depends on many factors,a major one is atmospheric nitrogen(N)deposition caused by human activities.The increasing input of anthropogenic reactive nitrogen changes the GHG emissions of grassland ecosystem by affecting biological(plant and soil microorganisms)and abiotic factors(soil physics and chemistry,etc.).Previous studies only focused on the inorganic nitrogen and the rate of nitrogen deposition,ignored the organic nitrogen.However,nitrogen deposition includes both organic and inorganic nitrogen.Thus,using a single N source,inorganic or organic,to estimate N deposition induced GHG fluxes can be problematic.In China,organic N constitutes?28%but can range from 7%to 67%in different regions.We conducted a field experiment that included two nitrogen addition level(low level 5 g N m-2y-1 and high level 30 gN m-2 y-1),every nitrogen level includes five different nitrogen compositions(organic N(ON)proportion in N addition,N1:0%ON,N2:10%ON,N3:20%ON,N4:40%ON,and N5:100%ON)in Hulunbuir meadow steppe to evaluate how changes in N composition would affect CO2,N2O,and CH4 fluxes.Further,we examined how biotic and abiotic factors might contribute to the GHG fluxes.By simulating realistic N compositions and evaluating relevant functional genes,our study provides more accurate estimates of GHG fluxes and advances our understanding of the underlying mechanisms that regulate the responses of GHGs to N deposition.The main achievements of this paper are as follows:(1)Nitrogen addition had a significant impact on biotic and abiotic factors controlling greenhouse gas emissions,and nitrogen addition levels and components also significantly affected the influence of nitrogen addition.Nitrogen addition significantly decreased soil moisture content and pH,but increased soil available nitrogen and dissolved organic carbon.Soil physical and chemical properties were also significantly affected by N compositions and levels,more strongly by inorganic N than by organic N and under high N addition than under low N addition.For vegetation communities,nitrogen addition significantly increased the plant community aboveground biomass,but the response of different functional groups to nitrogen addition were different:promoting the growth of grass;inhibiting the growth of forb and legume.Meanwhile,nitrogen addition also significantly increased the plant community underground biomass and the belowground net productivity,and decreased plant species diversity.Nitrogen level and components can significantly affect the magnitude of nitrogen effect.High level nitrogen addition can aggravate the negative effects of nitrogen addition.Greater organic nitrogen proportion in nitrogen addition weaken the influences of N addition on biomass and plant diversity.For microorganisms,nitrogen addition significantly decreased soil microbial biomass carbon and nitrogen.In addition,nitrogen addition had a negative effect on bacterial diversity,but had a positive effect on fungal nitrogen diversity,and significantly affected the community composition of soil microbial bacteria and fungi.Nitrogen addition significantly decreased oligotrophic taxa(acidobacteria)and increased copiotrophic taxa(actinomycetes,Patescibacteri,Proteobacterium and Bacteroides).Nitrogen composition affect Bacteroidetes and Nitrospirae,but not significantly affected on fungi.The increasing organic nitrogen in N deposition increased the relative abundance of Nitrospirae,but decreased Bacteroide at high N addition.(2)Nitrogen addition significantly increased the cumulative emissions of CO2 and N2O in 2018 and 2019.However,nitrogen deposition significantly increased the absorption of CH4 in 2018,but inhibited it in 2019.The nitrogen addition levels and compositions also significantly affected the influence of nitrogen addition.Compared with low level nitrogen addition,high level nitrogen addition significantly increased the cumulative emissions of CO2 and N2O in grassland.The increase of the proportion of organic nitrogen in nitrogen addition significantly reduced the effect of nitrogen addition on the cumulative emissions of CO2 and N2O,especially at high nitrogen levels.For CH4 uptake,compared with mixed nitrogen addition,organic nitrogen addition significantly increased CH4 uptake.(3)Nitrogen addition affects greenhouse gases emissions by affecting the biological and abiotic factors which influence the greenhouse gases emissions.Nitrogen addition increased soil available nitrogen content and then increased plant aboveground and underground biomass,thus finally increasing the plant autotrophic respiration to increase grassland CO2 emission.Due to the higher ammonia volatilization rate of organic nitrogen,the increase of the proportion of organic nitrogen in nitrogen addition reduced the emission of CO2,through weakening the promotion effect of nitrogen addition on aboveground and underground biomass.For N2O cumulative emissions,nitrogen addition increased N2O emissions through increasing soil NH4+and the activities of related microorganisms.N components change N2O emission mainly by affecting soil NH4+(Due to the higher ammonia volatilization rate of organic nitrogen,the increase of the proportion of organic nitrogen in nitrogen addition decreased soil NH4+);however nitrogen level change N2O emission mainly by affecting soil NH4+and the activity of related functional microorganisms.Compared with the addition of mixed nitrogen,the influences of increasing organic N occurred primarily via increasing activity of methanotrophs and reducing soil NH4+-N and then decreasing underground biomass,leading eventually to greater CH4 uptake.In summary,our results show that the GHG fluxes were significantly affected by N compositions.Greater organic N in N deposition lessened its effects on CO2 and N2O emissions,especially with high N addition,primarily due to low rates of increase in soil N resulting from high ammonia volatilization rather than changes in microbial activity.Our findings indicate that further N deposition studies should take into account the composition in N deposition to improve estimates and predictability of the effects on GHG fluxes in terrestrial ecosystems... |
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