| Nitrous oxide(N2O)and nitric oxide(NO),two important reactive nitrogen gases in the atmosphere,play important roles in greenhouse effect,stratospheric ozone depletion and photochemical pollution.The use of nitrogen(N)fertilizers is known as the major reason for atmospheric N20 and NO increase.Empirical bottom-up models therefore estimate agricultural N2O and NO inventories using N loading as the sole predictor,disregarding the regional heterogeneities in soil inherent response to external N loading.Several environmental factors have been found to influence the response in soil N2O and NO emission to N fertilization.Overlooking the influence of environmental factors may lead to errors in the extrapolation of bottom-up models and discrepancies between bottom-up and top-down estimates.Therefore,incorporating key factors into bottom-up models has significant consequences for regional and global N2O emission inventories,based on the comprehensive evaluation of the relative importance of environmental factors.Soil pH is an important factor regulating N2O and NO emission,but the relative importance and influence mechanism have not been addressed properly.Here,we conduct global multivariate analysis using an updated balanced dataset,thus emphatically analyze the relative importance of soil pH in multiple explanatory environmental factors(i.e.soil and climate)that influence the regional variation in N2O and NO emission,and improve the bottom-up models.Moreover,we conducted a series of incubation experiments to investigate the influencing mechanism of soil pH on N2O,NO emission and NO/N2O molar ratio during soil denitrification.The main conclusions were obtained as follows:(1)Global meta-analysis of 1104 N2O field emission measurements shows that soil pH is the chief factor explaining reginonal disparities in N2O emission.The emission factor(EF)of N2O increases significantly(p<0.01)with soil pH decrease.EF in acidic soil is more than 3 times that of alkaline soil.The default EF value of 1.0%,according to IPCC(Intergovernmental Panel on Climate Change)for agricultural soils,occurs at soil pH6.76.Therefore,IPCC method underestimates N2O emission for acidic soil,while it overestimates for alkaline soil.Moreover,changes in EF with N fertilization(i.e.?EF)is also negatively correlated(p<0.01)with soil pH.This indicates that N2O emission in acidic soils is more sentivie to changing N fertilization than that in alkaling soils.Incorporating these findings into bottom-up models,a new model for global estimates of agricultural soil N2O emissions is deduced.(2)Global meta-analysis of NO field emission measurements shows that soil pH is the primary environmental factor affecting NO emissions from forest and agricultural soils.NO emission from forest soils increases exponentially with decreasing soil pH(p<0.01).NO emissions in acidic soils are 5 to 10 times that of calcareous forest soils.In agricultural soil,NO emission factor(EF)increases significantly(p<0.01)with decreasing soil pH.The estimation model for global agricultural soil NO emission is therefore obtained based on these meta-analysis results.(3)Global meta-analysis of NO/N2O molar ratios shows that soil pH is the chief environmental factor that affects NO/N2O molar ratios from forest and agricultural soils.NO/N2O molar ratios increase significantly(p<0.01)with soil pH decrease,indicating that NO emission is more sensitive to changing soil pH than N2O emission.Equivalent NO and N2O molar emissions occur at soil pH5.5.NO emissions are higher than N20 in strongly acidic soil,while the opposite was found in neutral and alkaline soils.(4)The results of anoxic sterile and live soil incubation show that both microbial and chemical denitrification are important pathways for nitrate loss and nitrogenous gas emission,with significant influence of soil pH.In Shangzhuang(SZ)and Hongshi(HS)soil,nitrate loss and total gas emission in sterilized treatments were similar to those in live treatments,except for pH8.3 of SZ soil.In sterilized SZ soils,nitrate losses at pH6.5 and 7.1 are 47.65 and 27.48 mg N kg-1,accounting for 89.91%and 79.49%of those in live soils,respectively.Total gas products are 0.35 and 7.30 mg N kg-1,accounting for 1.25%and 61.29%of those in live soils,respectively.In sterilized HS soils,nitrate losses at pH5.5 and 7.1 are 63.88 and 63.59 mg N kg-1,and total gas products are 28.89 and 28.54 mg N kg-1,respecitively.In pH8.3 treatments of SZ soil,nitrate loss and total gas product in sterilized soil are significantly higher than those in live soils,which are 1.82 and 2.38 times that of live soils.The percentages of NO are negligible,while the percentages of N20 decrease with soil pH and N2 increase with soil pH.Therefore,N2O is the main gas product under acidic and neutral conditions while N2 dominates under alkaline conditions.There is a clear conversion between NO2-,NO,N2O and N2 during the incubation,and the conversion rate increased significantly with soil pH.(5)Sterilized anoxic incubations using Changbaishan(CBS),Zushan(ZS)and Luchongguan(LCG)soils show that chemical denitrification is still a potentially important pathway for nitrate loss and nitrogenous gas emission in acidic or strongly acidic soils.The average ratios of nitrate loss to nitrate addition are 43.52%,44.03%and 26.65%in CBS,ZS and LCG soils,respectively.The average ratios of total gas product to nitrate loss are 29.96%,29.63%and 11.53%in CBS,ZS and LCG soils,respectively.The NO/N2O molar ratio decreases with soil pH,which is consistent with global meta-analysis result.The conversion rates between gas products are generally low due to the lower soil pH,resulting in no significant conversion of N2O to N2 observed throughout incubation.Compared with fresh soil,stirring in slurry incubations significantly increases gas production and conversion rate during chemical denitrification. |
PDF Full Text Request