Stability Of Aggregates And Distribution Characteristics Of Organic Carbon Under Chinese Milk Vetch And Straw Returning

Soil organic carbon is an important cementing material of aggregates.The formation and stability of aggregates are closely related to the content of organic carbon.Therefore,the study of the distribution characteristics of soil aggregates and organic carbon is helpful to clarify the accumulation rule of organic carbon in soil under Chinese milk vetch and straw returning.Based on the rice-Chinese milk vetch rotation experiment,six treatments control?CK?,chemical fertilizer?NPK?,Chinese milk vetch returning?MV?,Chinese milk vetch returning with chemical fertilizer?MV+NPK?,straw returning with chemical fertilizer?S+NPK?,Chinese milk vetch and straw returning with fertilizer?S+MV+NPK?were selected.Paddy soils developed from fluvial alluvium Q₄?JZ?and Quaternary sediment Q₃?WH?parent materials were studied,the effects of Chinese milk vetch and straw returning on the stability of soil aggregates and the organic carbon fractions were studied by the methods of wet sieve and density fractionation;to explor the effects of Chinese milk vetch and straw returning on the soil bulk density and aggregate distribution of plough layer?0-20cm?;the changes of soil aggregates and the storage characteristics of organic carbon in the aggregates were analyzed by laboratory culture experiment.It provides a theoretical basis for the rational return of green manure and straw to the field and soil fertility in agricultural production.The main research results are as follows:?1?Chinese milk vetch and straw returning reduced soil bulk density,increased soil total porosity and hairy tube holding water,and improved soil physical properties.However,the variation of the two soils was different in each treatment.The soil bulk density of JZ soil was 1.06-1.16g/cm³,and the soil bulk density of Chinese milk vetch and straw returning with chemical fertilizer treatment?S+MV+NPK?was significantly reduced by 8.62%.The soil bulk density of WH was 1.24-1.36g/cm³,and the soil bulk density of Chinese milk vetch returning with chemical fertilizer treatment?MV+NPK?decreased the most,which was reduced by 6.06%.?2?The distribution of water-stable aggregates with different particle sizes showed that the content of>5mm aggregates was the highest?43.33%-88.22%?,followed by 2-0.25mm,5-2,0.25-0.053 and<0.053 mm aggregates.Compared with the control treatment,the content of aggregates>5mm was increased and the content of aggregates was reduced by 2-0.25 and 0.25-0.053mm after the Chinese milk vetch and straw returned to the field.Moreover,both Chinese milk vetch and straw returning increased the mean weight diameter,geometric mean diameter and water-stable macro-aggregates,but the fractal dimension?D?was decreased.The stability of aggregates in Chinese milk vetch returning with chemical fertilizer?MV+NPK?was higher than that in Chinese milk vetch and straw returning with fertilizer?MV+S+NPK?.?3?Chinese milk vetch returning with chemical fertilizer?MV+NPK?significantly increased the organic carbon content in both soils?13.13%-23.87%?,but there was no significant change in the organic carbon content after added straw.Soil organic carbon in different treatments was mainly concentrated in macro-aggregates,and the contribution rate of organic carbon in>5mm aggregates was more than 50%,while the content of micro-aggregates and clay was lower.Compared with the control treatment,the application of Chinese milk vetch and straw increased the organic carbon content of the aggregates from 2-0.25 mm?18.40%-65.85%?,and the application of chemical fertilizers with Chinese milk vetch increased the contribution rate of organic carbon in macro-aggregates?3.79%-22.77%?.The mineral-associated organic carbon was mainly composed in the aggregates,followed by fine particulate organic carbon,and the relative content of coarse particulate and light fraction organic carbon is the lowest.Chinese milk vetch returning with chemical fertilizer increased the content of mineral-associated organic carbon and fine particulate organic carbon in micro-aggregates.?4?Addition of 2%and 4%Chinese milk vetch increased the aggregate content of>2mm for 120 days.Addition of 2%Chinese milk vetch at 60 days increased the most?78.08%?and 4%Chinese milk vetch at 120 days increased the maximum?77.31%?and significantly increased the average weight diameter of the aggregate.The content of>2mm aggregates was the highest on the 60 days,and decreased with the increase of culture time.The content of<0.053mm aggregates decreased first and then increased with the increase of culture time,and reached the highest on the 180days.?5?The addition of Chinese milk vetch increased the soil organic carbon content in different culture periods,and the content of soil organic carbon in the soil treated with 4%Chinese milk vetch was higher than 2%,the organic carbon content of each treatment increased first and then decreased with the increase of cultivation time.The content of organic carbon in aggregates decreased with the decrease of particle size.The content of organic carbon in aggregates increased after 180 days of adding Chinese milk vetch,and the increase of organic carbon content in aggregates>2mm was the largest?17.17%-43.67%?.The relative content of organic carbon fractions in macro-aggregates?>0.25mm?was mainly increased after the addition of Chinese milk vetch 120 days,and significantly increased the content of fine particulate organic carbon in micro-aggregates?0.25-0.053mm?after 180 days.