Soil N₂O Uptake And Emission Under Different Land-use Types And Its Enlightenment For Land Management

To address the challenges of global climate change,reducing Carbon emission and adding Carbon sinks have been considered as a consensus of the international community around the goal of a Carbon peak and the vision of Carbon neutrality.Improving land resource management is a good way to achieve Carbon neutrality.For a green environment,“low Carbon”land-use types could provide a new visual angle for climate governance.Nitrous oxide（N₂O）and carbon oxide（CO₂）,etc.were uniformly called“Carbon”.Carbon dioxide equivalent（CO₂e）was proposed to uniformly measure the impact of greenhouse gases warming potential on the environment.N₂O was an important powerful greenhouse gas for warming potential,and 1 unit of N₂O was equivalent to 298 units of CO₂.Soil is not only the source,but also the sink of N₂O.However,the process of N₂O uptake in soil was easily covered up by its higher emissions.So,soil N₂O uptake was ignored by many studies.The processes of N₂O production,uptake,and net emission were depended on soil properties and management measures under land-use types.For decrease N₂O emission,the intrinsic relationship between land-use types,soil properties,and the processes of soil N₂O production and uptake should be studied.The net N₂O net emission process and its concentration in the atmosphere could be effectively mitigated,which was of practical significance for achieving The Paris Agreement’s goal of limiting the increase in global temperature to 1.5°C.The key factors and regulation mechanisms（soil moisture and pH）of N₂O emission was investigated in different land-use types to cope with climate change and promote the sustainable use of land resources.The results were shown as followed:（1）Different land-use soils[Farmland（Upland,Vegetable）,Plantation（Orchard）,Grassland,and Forest]were collected to explore the main driving factors of soil N₂O uptake and emission.The ¹⁵N₂O pool dilution method was used to measure the rates of soil N₂O production,uptake,net emission,and total accumulation during incubation.The results were shown as followed:Based on the application of nitrogen fertilizer,the N₂O production,uptake,and net emission rates were of the same order of magnitude.The highest rates of soil N₂O production and uptake were 0.57 and 0.23μg N kg^-1h^-1,respectively,indicating that N₂O uptake played a regulatory role in its net emission process.The total accumulation of N₂O production under different land-use types were Forest>Grassland>Vegetable>Orchard>Upland;The total accumulation of N₂O uptake under different land-use types were Forest>Grassland>Vegetable>Orchard>Upland;The total accumulation of N₂O net emission under different land-use types were Forest>Grassland>Orchard>Vegetable>Upland.（2）Soil physicochemical and biological traits differed significantly among land-use types:Forest was significantly higher than other land-use types in soil organic carbon（SOC）,total nitrogen（TN）,alkaline nitrogen（AN）,nitrate-nitrogen（NO₃^--N）,carbon to nitrogen ratio（C/N）,species richness and bacterial diversity;Upland were significantly lower than those of other land-use types in soil pH,loam,maximum field water holding capacity（WHC）,SOC,TN,and C/N ratio,but Upland soil clay were significantly higher than other soils;The complexity of soil bacterial networks was high in Upland,Vegetable and Orchard,while the bacterial networks of Forest and Grassland were relatively simple.The greater the differences in soil TN,SOC,ammonium nitrogen（NH₄⁺-N）,soil pH,and C/N ratio,the more significant the differences in microbial community structure,indicating that the differences in land-use types were important factors for changes in soil physicochemical and biological traits.Among them,the significant increase of TN content in Forest caused the differences in microbial communities.（3）The differences in soil physicochemical properties and biological characteristics were important factors in the process of N₂O uptake and emission.The results of correlation analysis showed that:The main factors affecting soil N₂O net emission included pH,nitrification rate,C/N,NO₃^--N content,nirK,nirS gene abundance,etc.Soil moisture,texture,and soil N₂O reductase（nosZ clade Ⅱ genotype）were included in the main factors of N₂O uptake.Among them,Upland had the lowest total accumulation of N₂O uptake and emission due to four factors,namely,microbial activity,soil permeability,nitrogen supply,and carbon supply,which were all lower than the other types.Grassland,Vegetable,and Orchard had relatively no significant differences in the above four conditions.Therefore,the total accumulation of N₂O uptake and emission were also no difference.The four conditions were significantly highest in Forest than in the other soils,so the total accumulation of N₂O uptake and emission were the highest（adding N fertilizer）.Among them,Forest with less clay,more sand,better soil aeration,higher WHC,and soil moisture had important effects on the abundance of soil N₂O reductase（nosZ clade Ⅱ）,which eventually could effectively increase the N₂O uptake in Forest.Based on the above conclusions,Forest（without N fertilizer as a stimulation source）may have the potential of N₂O sink.（4）To further clarify the effects of soil moisture and N₂O reductase on soil N₂O uptake and emission processes.N₂O uptake under different soil moisture was conducted by ¹⁵N₂OPD.The results showed that N₂O uptake and emission in the wetter soil（80-100%WHC）were significantly greater than in the drier soil（20%-60%WHC）.Soil nosZ gene abundance was sensitive to soil moisture,and nosZ gene copy number,soil moisture,and N₂O uptake had significant positive correlations with each other,which was the dominant factor for the greater N₂O uptake in the wet soil（80%-100%WHC）.Soil N₂O uptake（58.58%）,soil moisture（12.46%）,nosZ clade Ⅱ（12.13%）,and nosZ clade I（8.95%）had a 92.13%cumulative explanation for soil N₂O uptake PCA model.Soil denitrification potential had the same tendency as N₂O uptake,which also was the main factor of N₂O emission in the wet soil（80%-100%WHC）.Therefore,soil moisture content determined the N₂O emission and uptake process,and soil nosZ gene abundance can be used as an important indicator to reflect soil N₂O uptake capacity.（5）The abundance of nosZ gene in soil N₂O reductase was a key factor affecting the process of N₂O uptake,and soil pH was the most important influencing factor of N₂O reductase.Clarifying the effects of N₂O uptake and emission processes under different soil pH conditions（3.80-7.32）by ¹⁵N₂OPD,the results were shown that when soil pH was in the weak acid and neutral range（pH=6.08-7.32）,the peak value of soil N₂O uptake occurred at pH 6.69,and the peak value of soil nosZ gene copy number occurred at pH=7.15.Both the nosZ gene copy number and soil N₂O uptake were sensitive to soil pH,which may be the main reason N₂O uptake followed pH value.In acidic soil（pH≤4.98）,soil nosZ gene abundance was significantly inhibited by low pH.However,N₂O uptake was not significantly affected by low soil pH,suggesting that nosZ gene abundance was not a limiting factor for N₂O uptake in acidic soils.Soil pH has important effects on N₂O emission.In acidic soils（pH≤4.98）,N₂O emission increased and then decreased with the increase of soil pH,and the peak of N₂O emission was found at pH=4.47.In weak acid and neutral soils（pH=6.08-7.15）,the increase of N₂O emission was promoted by soil pH,indicating that there were different responses of N₂O uptake and emission to soil pH ranges.In conclusion,soil N₂O uptake could occur in different land-use types.Sometimes,it had the same order of magnitude as the net emission rate,indicating that N₂O uptake could significantly affect the net emission rates of N₂O.Soil moisture,texture,pH value,nitrification rate,C/N,NO₃^--N content,and the abundance of soil functional genes（nirK,nirS,nosZ）were the main factors of soil N₂O uptake and emission.Soil acidity regulation and soil moisture management play important roles in soil N₂O uptake and emission.