Variation Characteristics And Influencing Factors Of Soil Aggregate-Organic Carbon Fractions In Agricultural Soils From A Saline-Alkali Soil Region

Soil organic carbon（SOC）pool is regarded as one of the largest and most active carbon pools in the terrestrial ecosystem.The active organic carbon（AOC）pool is supposed to be the most active fraction in SOC pools with high activity,rapid turnover,and easily oxidized and mineralized capacity.It works as an indicator to capture SOC changes.As the most crucial and essential unit in soil,soil aggregates store a large proportion of SOC in the surface soils.A tiny variation in aggregate-organic carbon pool or AOC fractions would have a great impact on global climate change.Western Jilin,which belongs to the northeast sample zone for global climate change research,is located in one of the three major concentrated distribution areas of saline-alkali soils in the world.In order to meet the food demands,largely scaled saline-alkali fields in the region have been reclaimed into agricultural fields.However,the influences of extensive maize planting on soil aggregates,SOC,soil physical-chemical properties,and soil microorganisms in the saline-alkali fields are still poorly understood.The mechanisms of SOC sequestration in soil aggregates need to be further studied.Based on the field investigation,maize fields with different saline-alkali status following the same tillage practice were sampled from 2017 to 2018,when the surface（0～20 cm）and sub-surface（20～40 cm）soils were collected separately.Soils were separated into large macro-aggregate（>2 mm）,small macro-aggregate（0.25～2 mm）,and micro-aggregate（<0.25 mm）using dry-sieve method.We tested SOC fractions,soil physical-chemical properties,and enzyme activities to assess the variation characteristics and influencing factors of aggregate-organic carbon fractions during maize growth.Combined with straw amendment simulation experiment,we clarified SOC cycling mechanisms from the perspective of aggregates,which would provide a theoretical basis for the further development and utilization of saline-alkali soils.The main results are shown as follows:（1）During maize growth,the concentrations of total organic carbon（TOC）,readily oxidizable organic carbon（ROC）,microbial biomass carbon（MBC）,and potentially mineralizable carbon（PMC）in aggregate fractions were characterized by heading>mature>seedling.Dissolved organic carbon（DOC）in small macro-aggregate of sub-surface soils were characterized by mature>seedling,while in other aggregates DOC were in order of mature<seedling.The results inferred that maize growth was beneficial to the accumulation of MBC,ROC,and TOC,promoting SOC mineralization,but was not conducive to DOC maintenance.（2）The textures of soils were silty loam and sandy loam for the content of clay was 3.35～5.55%in the surface soils,lower than that in the sub-surface layer（4.14～7.69%）,although surface soils absorbed more soil particles.During the maize growth periods,the content of large and small macro-aggregate,as well as the mean weight diameter（MWD）was in the order of seedling>mature>heading,contrary to the changes of micro-aggregates.The results indicated that maize growth accelerated the fragmentation of macro-aggregates,promoting the formation of micro-aggregates,but reduced soil stability.Soil cation exchange capacity（CEC）showed in order of heading>mature>seedling,which indicated that maize growth would increase the content of exchangeable cations in soils.Soil exchangeable sodium percentage（ESP）and electrical content（EC）were in order of mature<seedling,implying that maize growth would decline soil saline-alkali status.During maize growth periods,soil water content（WC）,bulk density（BD）,and soil porosity（SP）of the surface layer decreased and then increased.Higher WC and BD but smaller SP values were observed in the mature stage than those in the seedling stage indicated maize growth regulated soil moisture,increased soil pore structure,reduced soil compactness,and improved soil environment of surface soils.Different dynamics of soil p H were detected in our study,for instance,in the surface layer soil p H values in mild saline-alkali soils gradually increased,while the values in the moderate and severe saline-alkali soils decreased gradually.The results demonstrated that maize growth was conducive to reducing soil p H in moderate and severe saline-alkali soils.（3）Soil aggregate-organic C fractions were affected by soil physical-chemical properties.The redundancy analysis（RDA）showed that,in the surface soils,soil ESP and CEC could explain 41.90%and 20.60%of the variances of organic carbon fractions in large macro-aggregates,while in small macro-aggregates ESP and BD could explain 20.97%and 20.00%of the variances.Soil p H and CEC explained27.30%and 20.90%of the variances of micro-aggregate organic carbon fractions,respectively.In the sub-surface soils,soil p H and MWD could explain 24.30～41.90%and 16.30～25.00%of organic carbon variances in aggregates.Soil p H was the major factor affecting soil organic fractions in macro-aggregates（>0.25 mm）,while MWD held the largest explanation in micro-aggregates.（4）Soil invertase（INV）,amylase（AMY）,and urease（URE）with the activities of 113.13～1785.63μg h^-1,12.40～72.33μg g^-1 h^-1,and 170.44～478.78μg g^-1 h^-1,were mainly settled in small macro-aggregates.Catalase（CAT）with the activity of1.26～2.10 m L g^-1h^-1 was mainly distributed in large macro-aggregates,while the highest activity ofβ-glucosidase（GLU）ranging from 27.37～183.28μg^-1 h^-1 was observed in micro-aggregates.The highest activity ofβ-N-acetylglucosaminidase（NAG）was detected in small macro-aggregates in the surface soils,while it was observed in large macro-aggregates in the sub-surface layer.RDA analysis showed that,in the surface soils,INV could explain 61.00%,50.90%and 60.00%of the variations of organic carbon fractions in large,small macro-aggregate,and micro aggregate,respectively.In the sub-surface soils,CAT,NAG and AMY explained57.60%,60.10%and 62.00%of the variations of organic carbon fractions in large macro-aggregate,small macro-aggregate,and micro-aggregate,respectively.（5）Straw amendment simulation experiment showed that the decomposition of maize straw altered soil particle surface structures.Dynamics of TOC,ROC,DOC,MBC,as well as the mineralization of SOC in aggregates,were caused by the straw amendment,which resulted in a rapid increase of DOC,MBC,and mineralization rate in aggregates.After 120-d incubation,the increments of cumulative mineralization of SOC in large macro-aggregate,small macro-aggregate,and micro-aggregate were39542.11 mg kg^-1,40142.25 mg kg^-1,and 44340.13 mg kg^-1,respectively.The contents of TOC,ROC,DOC,and MBC in straw amendment treatment were significantly larger than those in control,which indicated that maize straw amendment and decomposition were conducive to aggregate-organic carbon fractions.