Effects Of Different Organic Amendments On Soil Aggregation And Soil Organic Carbon Sequestration And Its Mechanism In Dryland Farming Areas Of Loess Plateau

Soil organic carbon（SOC）content is a key indicator of soil fertility,and increasing its content and sequestration capacity is crucial for improving soil fertility and mitigating global climate change.The semi-humid area prone to drought on the Loss Plateau is important arid food production area,and long-term input of organic materials is the most important way to improve soil organic carbon.Traditionally,manure application,straw returning,and green manure incorporation in the summer period of wheat are the main ways of organic amendment.The chemical composition of organic manure application,straw returning,and green manure incorporation are different,and they can constitute different organic fertilization measures when they used alone or in combination.However,the effects and potential mechanisms of these fertilization practices on soil aggregate formation and stability,SOC sequestration in different aggregate-size fractions,soil microbial community structure and enzyme activity,and the sustainability of crop production,as well as the effects and potential mechanism of the combination of mineral nitrogen and organic materials with nitrogen on the quantity of stable organic nitrogen are still unclear.Here,we take the wheat monoculture system in the Loess Plateau region as the research object,and field location split design experiment was used,and the main factors were green manure planting and incorporation:without green manure planting in summer（G₀）,green manure（Huai bean,Glycine ussuriensis Regel et Maack）planting and incorporation in summer（G）;The sub treatments included:without organic materials and NKP（CK₁）;application NPK alone（CK₂）;straw returning（S）;manure application（M）;straw returning and manure application（SM）.Our incubation experiments using ¹³C natural abundance and ¹⁵N isotope labeling techniques with four treatments:control,added straw（St）,added N（N）,and added straw and N（St+N）.At the same time,we employ the constant temperature incubation experiment with four treatments:control,added straw（St）,added N（N）,and added straw and N（St+N）.We using fourier transform infrared spectroscopy（FTIR）,high throughput sequencing,¹³C natural abundance and ¹⁵N isotope labeling techniques,to determined soil aggregate-size distribution,SOC sequestration,labile OC components,microbial diversity and community structure,soil enzyme activity,humus and other indicators.Our main conclusions are as follows:（1）Soil aggregate plays a crucial role in soil structure.Under the condition of balanced fertilization,the main factor affecting the formation of soil aggregates may be the quality of organic materials（i.e.,their chemical composition）,rather than the quantity of input C of organic material.To verify this possibility,we did a four-year field split plot experiment.The main treatments were excluding green manure planting（G₀）and green manure planting and incorporation（G）;The four sub-treatments were:Neither straw nor manure incorporation（CK）,wheat straw return（S）,manure application（M）,and wheat straw return and manure application（SM）.The results showed that compared with G₀,G increased the content of large macroaggregate（＞2 mm）and microaggregate（2-0.25 mm）by 66.85%and80.34%,respectively（p<0.05）;The content of small macroaggregate（0.25-0.053 mm）and large macroaggregate（＞2 mm）in straw returned soil was increased by 7.85%and 35.12%,respectively（p<0.05）,compared with no straw returning;The content of microaggregate（<0.053 mm）in the soil with manure application was 54.84%higher than that without manure application（p<0.05）.In terms of the influence on aggregate formation:green manure>straw>manure.Consistent with this,green manure had the greatest effect on the stability of soil aggregate followed by straw,while manure had no significant effect.When the three organic materials were combined in pairs or in three groups,given the lower C/N and higher labile C content of green manure,the interaction effect was most affected by green manure incorporation.Whereas,straw had the greatest effect on the activity of cellobiase andβ-glucosidase,and straw and green manure had the greatest effect on xylosidase activity,when single organic material was incorporated into soil.Correlation analysis showed that there was a significant positive correlation between the soil hydrolase activity and the total amount of cellulose C,hemicellulose C from organic materials.The results of structural equation model（SEM）indicated that there were differences between the path of the influence of green manure incorporation and straw returning on the aggregates stability and the path of the effect of manure application.However,the correlation between the C input of three organic materials and particle size mass of soil aggregate was not significant,indicating that the chemical composition of the three organic materials in this study were quite different.Therefore,it is impossible to predict the change of particle size mass of aggregates only from the quantity of C input.In conclusion,the organic materials with higher labile C content and lower C/N accelerate the formation of soil aggregate due to their higher degree of decomposition.The proportion of macroaggregates was related to the chemical composition of organic materials,while the quantity of C input of organic materials has little to do with it.Compared with the amount of organic C input,the formation of soil aggregate was significantly affected by the chemical composition of organic materials.（2）Under the condition of balanced fertilization,green manure incorporation,straw returning and manure application had different effects on the organic carbon content of soil aggregates:compared with no green manure incorporation,the SOC of large macroaggregates（＞2 mm）,small macroaggregates（2-0.25 mm）,microaggregates（0.25-0.053 mm）and silt and clay（<0.053 mm）in green manure incorporation soil increased by 6.34%,7.57%,5.64%and 10.03%respectively（p<0.05）;Compared with no straw returning,straw returning significantly increased the contents of soil large macroaggregates（＞2 mm）,small macroaggregates（2-0.25 mm）,microaggregates（0.25-0.053 mm）by 9.26%,6.74%and 4.63%respectively（p<0.05）;Compared with no application of manure,the application of manure significantly increased the SOC content of soil small macroaggregates（2-0.25 mm）,microaggregates（0.25-0.053 mm）,which were12.65%and 11%respectively（p<0.05）.The decomposable green manure and straw have a significant impact on the organic carbon contribution rate of soil larger macroaggregates（＞2 mm）and microaggregates（0.25-0.053 mm）,while the decomposed organic manure only has a greater contribution rate to microaggregates（0.25-0.053 mm）.In the soil with green manure and straw incorporation,the organic carbon content of macroaggregates（＞0.25 mm）,microaggregates（0.25-0.053 mm）and clay particles（<0.053 mm）have a significant impact on soil C sequestration,the organic carbon content of macroaggregate（＞0.25 mm）in the soil with manure had no significant effect on soil C sequestration.In addition,green manure incorporation,straw returning and manure application were also affected the composition of organic C in the soil.Compared with organic manure,wheat straw and green manure,the content of total C,SOC,SIC and LOC in the soil were increased more significantly.The combination of green manure incorporation and straw returning had the largest increase in CPMI,and the three organic materials mainly had a significant impact on the labile C（aliphatic C）.The green manure incorporation has a relatively young and faster decomposition,which significantly increases the content of aliphatic C in large macroaggregates and reduces in silt and clay,and followed by the soil with organic manure application and straw returning.In conclusion,organic materials with high C/N and easier decompose had a greater impact on the soil organic carbon components and C sequestration of each aggregate-size fractions.（3）Green manure incorporation,organic fertilizer application and straw returning have different effects on theαdiversity of bacteria and fungi in soil In terms ofαdiversity,compared with no organic manure application,the Shannon index of bacteria in the soil treated with manure increased by 0.8%（p<0.05）,while in terms of fungal diversity,the three organic materials reduced soil fungiαdiversity,in which for Chao1 index,the application of manure decreased by 10.9%compared with no application（p<0.05）;Straw returning reduced 2.9%compared with non-returning（p<0.05）,which showed that the sensitivity of fungal community diversity to organic materials with different chemical properties was higher than that of bacterial community.While,we found dominant bacteria and fungi with relatively high abundance.The addition of three organic materials significantly improved the relative abundance of soil bacterial Proteobacteria,which was31.7%-41.2%,and the relative abundance of fungal Ascomycota,which was 42.9%-65.1%.Green manure incorporation has significantly improved the community similarity of bacteria and fungi in soil.Among them,green manure incorporation has increased the keystone taxa and links between communities,making the relationship between bacteria and fungi more closely and living in a mutually beneficial and cooperative way.Although green manure incorporation has reduced the diversity of soil bacteria,but with increasing of the relative abundance of Actinobacteria,the green manure decomposed more rapidly,this may be the key factor for green manure to promote soil aggregation and labile organic C fraction.（4）We performed incubation experiments to explore the mechanisms of the conversion of mineral N to stable organic N（humus N）,our research suggested that the effect of N addition on the CO₂ emission rate and cumulative CO₂ emissions showed a prominent two-phase characteristic:both the CO₂ emission rate and cumulative production were stimulated by exogenous mineral N during the initial stage,while the CO₂ emission rate gradually decreased during the middle and later stages.This indicates that added N was rapidly utilized by microorganisms;however,residual C may remain in the soil for a longer time,leading to great differences in soil C/N ratios at different stages.At the end of the incubation period,the proportion of ¹⁵N derived from labeled N to total N in humus was0.063%（N）and 0.085%（St+N）,while the proportion of ¹⁵N in humus to total mineral N added was 0.41%and 0.64%,respectively.This indicated that even if only mineral N was added,the stable organic N in humus was converted by microbes.Compared with the addition of only mineral N,the combination of N and straw promoted the conversion of mineral N to stable organic N.In conclusion,with sufficient mineral elements,the chemical composition of organic materials promoted soil aggregate more than organic C input;Meanwhile,the easily decomposed organic materials increased the complexity of soil microbial communities and increased the keystone taxa and connections between communities,therefore,it can promote the organic materials rapid decomposition and the formation of soil aggregates;With the organic amendment,the quantity of SOC sequestration depended on both the quantity of C input and the chemical composition of organic materials,organic materials with different chemical composition had different effects on soil aggregate,which resulted in significant differences in the proportion of SOC sequestration in aggregate-size fractions;The combination of organic material addition and mineral N input can promoted the transformation of mineral N to organic N and the formation of stable organic matter.These results provided scientific basis for the construction of soil rapid fertilization technology in dryland farmland.