Effect Of Mixing Ratio On Soil Organic Carbon Pool Of Alfalfa/Smooth Bromegrass Mixture In Longdong Loess Plateau

Soil organic carbon（SOC）is an important indicator of soil fertility,and the quality of carbon pool affects the sustainable productivity of farmland ecosystem.Mixture of legume and grass,an important planting pattern in rain-fed farming area of the Loess Plateau,can improve soil organic carbon content and storage.However,the effects of mixing ratio on soil organic carbon and its components need to be further clarified.In this study,alfalfa（M）/smooth bromegrass（B）mixture in the Longdong Loess Plateau was used as the object and different mixing ratios（M7B3,M5B5,M3B7）were set.The content and storage of soil organic carbon（SOC）and its components,including easily oxidized organic carbon（EOC）,soluble organic carbon（DOC）,granular organic carbon（POC）,microbial biomass carbon（MBC）and mineral bound organic carbon（MOC）were measured.The changes in activity of soil carbon metabolism-related enzymes,includingβ-1,4 glucosidase（BG）,fibrinose hydrolase（BX）and xylosidase（CBH）,and soil respiration rate were investigated at various stand ages and cuts of grasslands.The relationships between soil organic carbon pool,soil enzyme activity and soil respiration were analyzed.The main results are as follows:1)Mixing ratio had significant effects on the content,storage and carbon pool management index（CPMI）of SOC and its active components in 0-10 cm soil layer of grasslands with different stand ages.With the increase of stand age,SOC and POC contents increased.M5B5 was the highest at 2 and 4 year-old,and M7B3 was the highest at 5 and 6 year-old.They were higher in the mixtures than sole cropping.With the increase of stand age,soil DOC content decreased significantly.M5B5 was the highest at 2,4 and 5 year-old,and M7B3 was the highest at 6 year-old.Soil EOC content increased with the increase of stand age,and M5B5 was the highest at 2,4and 6 year-old.M7B3 was the highest at 5 year-old.Soil MOC content increased first and then decreased with the increase of stand age,and peaked in the 5 year-old stand.With the increase of stand age,CPMI of surface（0-10 cm）soil under different mixtures increased first and then decreased,that is,it peaked in the 5 year-old stand,and M5B5 was the highest at all ages.2)Mixing ratio had significant effects on the content,storage and CPMI of SOC and its active component in 0-10 cm soil layer of different grasslands.The content of SOC and its active components increased first and then decreased with the increase of cuts.With the increase of cuts,CPMI under different mixtures increased first and then decreased,and M7B3 was higher at the first and third cut,and M5B5 was higher at the second cut.3)Mixing ratio had significant effect on soil carbon metabolism-related enzyme activity in 0-10 cm soil layer.Mixing had a significant effect on soil enzyme activities,and M7B3 and M5B5 were generally higher.The changes of soil enzyme activities in the 5 and 6 year-old mixtures were similar at different mixing ratios.Compared with sole cropping,mixing increased the activities of three carbon metabolism-related enzymes.The contents of SOC,MBC,DOC,EOC,POC and MOC were positively correlated with the activities of BG,BX and CBH.Carbon pool activity（L）was positively correlated with soil enzyme activity,and was positively correlated with carbon pool index（CPI）and CPMI.4)Mixing ratio had significant effect on soil respiration rate.There was a significant exponential correlation between soil respiration rate and soil temperature at different mixing ratios,that is,soil respiration increased significantly with the increase of soil temperature.There was a nonlinear correlation of soil respiration rate with soil humidity,showing that soil respiration rate gradually increased with the increase of soil humidity,and then gradually decreased when the temperature was higher than 20%.Soil respiration temperature sensitivity coefficient(Q₁₀)value was higher in mixtures than sole cropping,and M7B3 was higher.The two-factor model considering both soil temperature and humidity was better for simulating soil respiration.SOC and its components had a certain correlation with soil respiration,except for DOC,and for the others,there were not significant.Soil respiration rate was negatively correlated with soil carbon pool stability index（Carbon pool activity index LI and CPMI）,that is,the higher the soil respiration rate,the lower the activity and quality of soil carbon pool.Conclusively,higher proportion of legume in alfalfa/smooth bromegrass mixtures can increase SOC content and storage in 0-10 cm soil layer at different stand ages and cuts,improve soil enzyme activity and affect soil respiration rate.Considering the stability of SOC pool and its effector,it is suggested that mixing of legumes and grasses at 5:5 in Longdong Loess Plateau can be used in practice,which can improve the quality of soil carbon pool,improving soil fertility,and enhancing the sustainability of the mixture.