Effects Of Organic Materials And Nitrogen Addition On Soil Aggregate And The Distribution Of Carbon And Nitrogen

Soil organic carbon（SOC）is one of the core indexes of soil fertility.Increasing SOC sequestration plays an important role in improving soil fertility and mitigating global climate change.The combination of organic material and inorganic fertilizer application is often used to increase SOC sequestration in agricultural production.Due to the different chemical compositions of various organic materials,there are still uncertainties effect on the formation,stability and organic carbon in soil aggregates.Moreover,the effect of the combination of mineral nitrogen and organic materials on the content of C and N in aggregates and the potential mechanism are still unclear.Therefore,the following two laboratory incubation experiments were conducted in this study.¹³C natural abundance method and ¹⁵N isotope labeling methods were used to reveal the effects of organic materials with different chemical properties were added into the soil alone or combined with N on SOC mineralization,the distribution and transformation of C and N in different soil aggregate fractions and the possible mechanism.The main results are as follows:（1）In order to reveal the effect of organic materials addition on the formation of soil aggregates,soil particle size（<0.25 mm;<2 mm）and organic material type（no addition,green manure,straw,kiwi branches,biochar）as research factors,to form a complete experiment design.The results showed that the content of SOC,MBC and MBN could be significantly increased by organic materials addition.,and the effects of green fertilizer,kiwi branches and straw on soil aggregate content were consistent,and the aggregation effects were as follows:Straw>Kiwi branches>Green manure>Biochar.Soil particle size is one of the important factors affecting the formation of aggregates.When organic materials are added,in<0.25 mm soil,only the content of silt+clay particles decreased significantly（2.87%～61.28%）,while the content of other three aggregates fractions increased significantly.The content of micro-aggregate and silt+clay particles in<2 mm soil decreased significantly,while the contents of large macro-aggregates and small macro-aggregates increased significantly,and this change could occur within 10 days after the addition of organic materials.Correlation analysis showed that the content of labile organic matter（soluble sugar,hemicellulose）of organic materials,MBC,MBN,DOC and the cumulative release of CO₂were significantly positively correlated with the stability of soil aggregates,and the correlation between MBC,MBN and the stability of soil aggregates was the strongest.Therefore,organic materials with higher content of labile organic matter may promote stability of soil aggregates by increasing soil microbial activity.（2）In order to reveal the effects of straw and nitrogen fertilizer combined addition on carbon and nitrogen components of soil with different fertility,straw（added or not added）,nitrogen fertilizer（added or not added）and soil with different fertility（high carbon and high nitrogen soil,low carbon and low nitrogen soil）were used as research factors to form a complete experiment design.The results showed that,compared with no straw addition,the cumulative CO₂release amount of high or low fertility soil with straw addition was significantly increased by 347.81%and 413.85%,respectively.Compared with the control,the cumulative CO₂release amount of high or low fertility soils with nitrogen addition only increased by 14.55%and 19.66%,respectively,at the beginning of the experiment,but decreased by 26.02%and 20.64%,respectively,at the later stage.The SOC,DOC,MBC,TN and MBN of the two soils were significantly increased by straw addition alone or combined with nitrogen,and the low fertility soil was more sensitive to exogenous carbon and nitrogen addition.（3）On the basis of research content（2）,the effects of straw combined with nitrogen addition on soil aggregate formation,and carbon and nitrogen distribution with different soil fertility were further analyzed.The results showed that straw addition was beneficial to soil aggregation,while nitrogen addition was not.At the initial stage of experiment,compared with the control treatment,the macro-aggregates increased by 322.92%and 65.64%,respectively,while the micro-aggregates and silt+clay particles decreased by 5.57%and44.19%,respectively.High fertility soil also showed the same trend,but the increase of soil macro-aggregates was significantly less than that of low fertility soil.Compared with the control treatment,the contents of large macro-aggregate,small macro-aggregate and micro-aggregate in low fertility soil supplemented with nitrogen were significantly reduced by 31.69%,16.46%and 24.57%,respectively,while the content of silt+clay particle was increased by 31.43%.The change of soil aggregate content in late period was consistent with that in early period.In terms of SOC concentration of aggregates,after straw added only,organic carbon concentration of large macro-aggregate and small macro-aggregate showed a decreasing trend,while that of micro-aggregates and silt+clay particles showed an increasing trend,but the contribution rate of organic carbon of each aggregate fractions increased significantly.Straw combined with nitrogen can significantly promote the accumulation of¹³C and ¹⁵N in the different aggregate fractions.When straw and nitrogen were added in high and low fertility soils,compared with straw added only,the ratio of straw source carbon in large macro-aggregate increased by 16.65%and 57.40%,respectively,Compared with nitrogen added only,the proportion of ammonium sulfate ¹⁵N in large macro-aggregates of high and low fertility soil increased significantly by 872.73%and 451.75%,respectively,and similar results were also shown in other aggregate fractions.In conclusion,soil aggregate formation is affected by soil fertility,chemical properties of organic materials and mineral nitrogen fertilizer.The higher the content of labile organic carbon,the more beneficial to the improvement of soil microbial activity,and thus promote soil aggregation.Straw and nitrogen combined addition can significantly increase the proportion of straw derived carbon and mineral nitrogen in each soil aggregates fractions,which is conducive to soil carbon and nitrogen accumulation,and this effect is better in low fertility soil.Therefore,organic material and inorganic fertilizer combined application should be considered in agricultural production to fertilize soil fertility.These results can provide theoretical basis for the establishment of rapid fertilizer cultivation technology.