The Relationship Between Characteristics Of Soil Organic Carbon Pool And Enzyme Activities Under Combined Application Of Different Carbon Sources

Soil organic carbon is an important indicator of soil fertility.Its carbon pool quality affects the sustainable development of farmland ecosystem.Crop straw returning to the field,as a commonly used fertilization measure,can improve the content of soil organic carbon.In the agroforestry complex system,most of the organic matter in farmland and natural ecosystem enters the soil as a"mixture".The impact of this special mixed material returning to the field on the transformation of farmland ecosystem carbon pool is not clear.In view of this,this study took the sloping farmland in the Loess Plateau as the research object,and mixed the farmland corn straw with the litter leaves of Robinia pseudoacacia planted by the local tree in the mass proportions of 4:0,3:1,2:2,1:3 and 0:4.As an exploratory experiment,this study studied the effects of different proportions of litter leaves and corn straw on the characteristics of soil carbon pool,combined with the characteristics of soil carbon emission and the activity of soil carbon degrading enzymes,To explore the transformation mechanism of combined application treatment for farmland soil carbon pool in the fragility entironment and comprehensively evaluate the crop soil environment benefits of different proportion of mixed application and return treatment by factor analysis,in order to provide a theoretical basis for the improvement of agroforestry system and carbon sequestration and emission reduction in ecologically fragile area of the Loess Plateau.The main findings are as follows:（1）The combined application of Robinia pseudoacacia litter leaves and corn straw increased soil organic carbon（SOC）and fractions content,improved soil carbon pool activity（CAI）,carbon pool quality（CPI and CPMI）and soil CO₂ emission,but reduced soil carbon pool stability compared with straw returning treatment,which was more obvious in 0-20cm soil layer.Among them,the content of soil organic carbon in 0-20cm under the combined application of different carbon sources was 5.23%-17.08%higher than that of the control,and Y3C1 increased the most.Compared with Y4C0 treatment,the combined application of Robinia pseudoacacia litter leaves（Y1C3,Y2C2 and Y1C3）was conducive to the increase of soil active carbon pool fractions（ROOC and DOC）,while the content of MBC was on the contrary.Compared with the control,the quality of soil carbon pool（CPI and CPMI）in 0-20cm soil layer increased by 5.88%～18.01%and 30.08%～94.48%respectively.Soil carbon pool activities（CA and CAI）in straw returning treatment（Y4C0）and a small amount of Robinia pseudoacacia litter leaves combined application treatment（Y3C1）were lower than those in Y2C2,Y1C3 and Y0C4 treatments,which were similar in 20-40cm.Under different carbon source combined application treatments,the soil CO₂ flux showed that the growth and development stage of maize was greater than the mature stage,and each treatment increased by 21.00%～48.26%compared with the control.Corn grain yield,soil CO₂ emission（CE）and carbon emission efficiency（CEE）were similar,and each treatment was significantly higher than the control treatment.While Y3C1treatment increased corn grain yield,CE and CEE were lower than Y4C0 treatment.（2）Compared with the control,the combined application of Robinia pseudoacacia litter leaves and corn straw increased soil carbon degradation enzyme activities and corn grain yield.Among them,compared with the control,the activities of soilβ-1,4-glucosidase（BG）,phenol oxidase（PPO）and peroxidase（PX）increased by 3.31%-13.40%,2.99%-18.94%and 0.19%-6.08%,but cellobiose hydrolase（CBH）had no significant effect;The activity of PPO oxidase in straw returning treatment（Y4C0）and a small amount of Robinia pseudoacacia litter leaves combined application treatment（Y3C1）was lower than that in Y2C2 and Y1C3 treatment,while the activity of PX oxidase was the opposite;the activity of carbon degrading enzyme was mainly affected by corn filling stage.（3）The combined application of different carbon sources can directly or indirectly affect soil carbon emission by affecting the activity of soil carbon degrading enzymes,organic carbon pool and carbon pool stability,and soil water content and oxidase have a significant impact on carbon pool transformation.Among them,the content and proportion of ROOC have the highest explanation ratio of soil carbon pool stability index in each soil layer,which are 35.2%and 32.0%respectively.Soil carbon emission has a significant negative correlation with DOC,ROC and its proportion,carbon pool activity（CAI）and quality（CPMI）（P<0.05）.Soil carbon emission has a very significant positive correlation with soil carbon reducing enzyme activities,and soil carbon emission and carbon degrading enzymes are affected by soil water content in each soil layer.The results of partial least squares path model（PLS-PM）showed that the path coefficient of soil oxidase（β=0.460）max.Based on the comprehensive benefit evaluation results of factor analysis,Y3C1,that is the mixed application of a small amount of Robinia pseudoacacia litter leaves and corn straw,is more suitable for the farming measures of agroforestry system in the fragile habitat area of the Loess Plateau.