Characteristics Of Soil Fertility And Organic Carbon Variation Under Main Land Use Types In Hehuang Valley

Soil organic carbon（SOC）plays a very important role on land.As the most active carbon pool,its stability is restricted by many factors.The study of SOC and carbon pools is helpful to further understand the mechanism of soil carbon sequestration.The change of land use pattern can greatly affect soil organic carbon and carbon components.In different land use types,soil physical,chemical and biological properties are often different due to different vegetation conditions and anthropogenic disturbances,which affect the mineralization process and decomposition rate of organic matter.Based on the research of different land use types in Hehuang Valley,the comprehensive soil fertility index was calculated by Fuzzy method,and the effect of human activities on soil fertility was clarified.In order to provide theoretical basis for rational land use and sustainable development in Hehuang Valley,the turnover time of three kinds of soil carbon pools were calculated by indoor mineralization experiment and hypothesis fitting.The results are as follows:（1）The state of SOC,TN and TK were good in the soil of Hehuang Valley.In 0-10cm layer,the maximum value of SOC was 16.28 g·kg^-1（cultivated land）.The maximum value of TN was 1.59 g·kg^-1（cultivated land）.TP content was 1.60 g·kg^-1 in the 0-10cm layer of facility farmland,while TP contents in other types of land were low.The contents of TK in the four types of soil were all over 20.28 g·kg^-1.In Hehuang Valley,the soil p H value of facility land and grassland was low and in facility land,the soil compaction was serious.Planting Pinus sylvestris consumes a lot of potassium fertilizer in the soil.The soil nutrient status of cultivated land in Huzhu County was good,and the quality grade of single index was in the top three.From 2007 to 2016,the p H of cultivated land decreased by 5.20%due to long-term use of acid compound fertilizer.Long-term cultivation resulted in the depletion of cultivated land fertility and the SOM content decreased by 9.69%,TK decreased by 32.02%,Soil C/N ratio decreased by 27.28%.The comprehensive fertility index of cultivated land in Hehuang Valley was the highest（0.425）,while that of facility land was the lowest（0.300）.With the soil layer downward,the comprehensive fertility index decreased.The comprehensive soil fertility index of Hehuang Valley was mainly determined by SOC and TN,WC and p H value had great influence on it.The fertility of cultivated soil in Huzhu County was lower in southwest and higher in northeast.The comprehensive soil fertility index increased by 5.77%from 2007 to2016,mainly caused by the increase of nitrogen and phosphorus elements.In the subsequent cultivation process,attention should be paid to the addition of organic fertilizer and potash fertilizer,straw returning,broad bean and other crops rotation,less or no phosphate fertilizer,to ensure a good farming environment,sustainable growth of crops.（2）Soil content of C_p was high.In 0-10cm layer,the C_a content of grassland was the highest(0.27 g·kg^-1),accounting for 1.69%of SOC.The content in facility agricultural land was the lowest(0.17 g·kg^-1)accounting for 1.41%of SOC.Grassland had the highest slow-available carbon content(4.81 g·kg^-1),accounting for 29.89%of SOC.The content in facility agricultural land was the lowest(3.08 g·kg^-1),accounting for 25.73%of SOC.The C_p content of cultivated land was the highest(12.72 g·kg^-1),accounting for 74.95%of SOC.The content in facility agricultural land was the lowest(8.71 g·kg^-1),accounting for 72.87%of SOC.Generally,the soil layer is downward,and the carbon pool value decreases.The proportion of C_a and C_s in grassland was higher than that in other land use types,while the proportion in nursery was the opposite.Affected by SOC,C_a content decreased with the deepening of soil depth.In 0-10cm to 10-20cm layer,the C_a of grassland decreased by 0.7%,and that of other types decreased by 5%-17.65%.The content of soil MAOC was higher.In 0-10cm layer,POC content in the nursery was the highest(5.51 g·kg^-1),accounting for 38.44%of SOC.The content in facility agricultural land was the lowest,3.49 g·kg^-1,accounting for31.67%of SOC.The content of MAOC in cultivated land was the highest(10.07 g·kg^-1),accounting for 67.10%of SOC.The content in facility agricultural land was the lowest(7.25g·kg^-1),accounting for 68.33%of SOC.Other soil types were in the range of these values.The content of SOC was an important factor affecting the content of each carbon component,and the C_a,slow efficiency carbon,inert carbon,POC and MAOC have a very significant positive correlation with SOC.There was a certain similarity between C_p and MAOC,and the correlation between them was as high as 0.908.（3）The influence of soil physical and chemical properties on soil organic carbon pool was mainly reflected in the influence of SOC and TN,but the influence on turnover time of soil organic carbon pool was not significant.Among different land use types,the correlation between grassland and other three types and soil physical and chemical indexes was very different.The MRT_a and MRT_s were more correlated with WC,SD,p H and other environmental indexes.POC/MAOC had no significant relationship with MRT_a and MRT_s.In grassland and cultivated land,the MRT_a was negatively correlated with that of C_s.The opposite was true in facility farms and nurseries.MRT_a was mainly affected by C_a,SOC,p H value and water content.MRTs were mainly affected by slow-efficiency carbon content,water content,p H value and total phosphorus.In grassland,facility agricultural land and nursery,POC/MAOC were mainly determined by POC content,which was affected by SD and other basic physical and chemical properties.POC/MAOC in cultivated land was mainly determined by MAOC content,which was affected by soil properties such as TP,C/N ratio and SD.Soil consolidation in facility agricultural land made more organic carbon exist in the form of mineral combination in soil,and organic carbon was difficult to decompose and be used by crops.TP and C/N in cultivated land were proportional to POC/MAOC,and the increase of TP and C/N by phosphate and organic fertilizer could increase POC/MAOC and make soil carbon pool more active,which was beneficial to crop growth.The activity degree of carbon pool in the nursery was limited by TK,and the growth of Pinus sylvestris requires more potassium fertilizer.Applying potassium fertilizer can enhance the activity of soil organic carbon in the nursery and produce more economic benefits.