Mechanisms Of The Responses Of N₂O Emission From Latosol Aggregates To Continuous Application Of Organic Materials In Tropical Vegetable Fields

The application of organic materials can significantly improve soil fertility,promote the formation of soil aggregates and thus improve soil structure.Therefore,the application of organic materials on farmland not only serves as soil fertilizer,but also effectively addresses the environmental risks associated with improper disposal of organic materials.Soil aggregates,as important soil building blocks and reactors of greenhouse gas N₂O,are profoundly affected by the application of organic materials.At present,the effects of organic material addition on N₂O emissions in soils are mostly at the whole-soil level or limited to short-term effects,and the dynamics of soil nitrogen turnover and N₂O emissions under continuous organic material application have not been deeply analyzed from the perspective of soil aggregates.Based on the above scientific questions,this study focused on Hainan tropical latosol pepper-bean rotational crop ecosystem,and the field experiment started in 2015with three fertilizer application methods,i.e.,chemical fertilizer alone（N）,chemical fertilizer with straw（N+straw）and chemical fertilizer with sheep manure（N+manure）.Indoor experiments were conducted to study the effects of two different organic materials additions on soil aggregate distribution,bulk soil and soil aggregate nutrients,bulk soil and aggregate N₂O emission fluxes,and to analyze the community diversity and structural changes of soil bacteria under different organic materials applications.The main findings obtained in this study are shown below.（1）Six consecutive years of application of both organic materials did not change the overall distribution pattern of aggregate:the smallest percentage of macro-aggregate and the largest percentage of micro-aggregate.Compared with fertilizer treatment,the percentage of soil macro-aggregate increased by 60.05%and 217.74%for fertilizer plus straw treatment and fertilizer plus sheep manure treatment,respectively,while micro-aggregate decreased by 9.76%and 14.39%,respectively.Fertilizer with organic material increased soil organic carbon（SOC）,total soil nitrogen（TN）,dissolved organic carbon（DOC）,DOC/SOC and cation exchange capacity（CEC）compared to fertilizer alone treatment with increases ranging from 7.43%to 40.28%,28.57%to 42.86%,83.31%～180.83%,60.08～97.87%,and 0.93～20.03%;reduced soil C/N values in the range of 1.44%～15.35%,where the effect of sheep manure application was significantly higher than that of straw application.Compared to chemical fertilizer application alone,straw and sheep manure addition significantly increased the mean weight diameter of soil aggregates by 11.43%and 31.42%,respectively.Therefore,fertilizer with organic material addition was beneficial to improve soil structure,mitigate soil acidification and improve soil fertility.（2）Effect of organic material addition on nutrient distribution of aggregates:straw or sheep manure addition significantly increased SOC,TN,DOC,p H,NO₃^--N,CEC,and TP of each particle size aggregates under the same fertilization treatment,the nutrient content（except p H and electric conductivity）of macro-aggregate increased more significantly compared with micro-aggregates.The increase in nutrient content in the whole soil was dominated by the micro-aggregate because of their large proportion.（3）Effect of organic material addition on bulk soil N₂O emissions:Compared with chemical fertilizer application,N₂O emissions from straw or sheep manure application soil was significantly increased by 3.95 and 21.44 times,respectively.Incubation experiments revealed that N₂O emissions in the sheep manure addition treatment were concentrated at 9 days after fertilizer application and were consistent with changes in NH₄⁺-N and NO₃^--N in the soil,while N₂O emissions were consistently lower in the fertilizer and fertilizer with straw treatments.By random forest analysis and least squares path analysis,increased soil organic carbon,dissolved organic carbon,and total nitrogen concentrations were the main controlling factors for the application of soil organic material to increase bulk soil N₂O emissions;nitrification and denitrification in soil were increased by increasing AOA,nir S,nar G gene abundance,and nar G/nos Z ratio,respectively.（4）After six years of fertilization,there were differences in N₂O emissions between fertilizer treatment aggregates.Fertilizer treatment and fertilizer plus straw treatments showed that the highest N₂O cumulative emissions were observed in the macro-aggregates of 109.49μg·kg^-1 and 45.79μg·kg^-1,respectively,and the lowest from micro-aggregates of 42.251μg·kg^-1 and 7.231μg·kg^-1,respectively.However,the fertilizer plus sheep manure treatment showed micro-aggregate emitting higher N₂O of211.01μg·kg^-1 and the lowest in macroaggregates of 145.05μg·kg^-1.（5）Influence of N₂O emissions from aggregates on bulk soil N₂O emissions:The contribution of different particle size aggregates to bulk soil N₂O emissions was calculated by combining different particle size aggregates and cumulative bulk soil N₂O emissions from the same fertilizer application treatment,and it was found that the largest contribution of macro-aggregates to cumulative bulk soil N₂O emissions was47.58%in the treatment of fertilizer with straw,and the largest contribution of micro-aggregates to cumulative bulk soil N₂O emissions was 42.71%and 59.22%in the treatment of fertilizer alone and fertilizer with It was found that the contribution of micro-aggregates to the cumulative soil N₂O emissions was the highest in the fertilizer alone and fertilizer with sheep manure treatments（42.71%and 59.22%,respectively）.（6）Soil bacterial community diversity and species composition of tropical latosol were altered after six years of fertilizer management.Straw and sheep manure applications altered both soil bacterial community richness（Chao1 index）and bacterial community diversity（shannon and simpson indices）.Sheep manure application increased soil-wide bacterial community richness and community diversity compared to chemical fertilizer treatment,while straw addition decreased bacterial community richness and community diversity.Straw and sheep manure application significantly increased bacterial abundance in the Proteobacteria phylum and decreased bacterial abundance in the Chloroflexi and WPS＿2 phylums,while bacterial abundance in the Firmicutes,Gemmatimonadetes,and Nitrospiare phylums responded differently to the two organic materials,with sheep manure addition significantly increasing bacterial abundance in the Gemmatimonadetes phylum compared to chemical fertilizer application alone.The bacterial abundance of Gemmatimonadetes and Nitrospiare phylum was significantly increased by sheep manure addition compared to chemical fertilizer alone,while straw addition did not have a significant effect,and for Firmicutes phylum bacterial abundance was significantly decreased by sheep manure addition,but significantly increased by straw addition.The results showed that fertilizer plus organic material treatment could improve the properties of Latosol,in which fertilizer plus sheep manure treatment was more effective than fertilizer plus straw treatment.However,fertilizer plus sheep manure treatment significantly increased soil N₂O emissions,mainly by increasing soil nutrient content and p H to promote nitrification and denitrification to promote N₂O emissions,and in fertilizer plus sheep manure treatment mainly by increasing the contribution of micro-aggregates to bulk soil N₂O to promote bulk soil N₂O emissions.