Effects Of Different Organic Fertilizers On Aggregates,Iron Oxides And Organic Carbon Fractions In Upland Red Soil

Due to the long-term excessive application of chemical fertilizer,serious problems such as the decrease of organic matter concentration and soil acidification have appeared in red soil in China,which hinder the sustainable development of local agriculture.Soil degradation can be remedied by improved tillage practices and improved fertilizer inputs,which can increase soil organic carbon content and restore soil structure.In this experiment,rapeseed-sweet potato rotation experiment in dry red soil of Jiangxi Province was taken as the research object.Control group(CK),conventional chemical fertilizer(CF),conventional chemical fertilizer reduced by 40%(RF),conventional chemical fertilizer combined with straw(CFS),reduced 40%chemical fertilizer combined with straw(RFS),conventional chemical fertilizer combined with biochar(CFB),reduced 40% chemical fertilizer combined with biochar(RFB),reduced 40% chemical fertilizer combined with pig manure(RFP),reduced 40%chemical fertilizer combined with vermicompost(RFV)9 different fertilization patterns.Through three consecutive seasons of field experiments,the effects of reduced chemical fertilizer combined with different organic fertilizers on the structure of red soil aggregates,iron and aluminum oxides and organic carbon components were analyzed.The specific results are as follows:Compared with CF treatment,straw and biochar did not significantly affect crop yield,but crop yield was significantly increased under RFP and RFV treatment,with an increase of 113.2%-203.9%.There was no significant difference in yield among different treatments in sweet potato season.Compared with CF treatment,RFP and RFV treatments significantly increased the contents of available P,nitrate N,total N,total P,EC value and p H value in red soil,while straw and biological black carbon treatments had no significant changes.Animal organic fertilizer(pig manure,vermicompostes)was significantly better than plant organic fertilizer(straw,biochar)in promoting the yield and nutrient content of red soil crops.Different organic fertilizer application promoted soil organic carbon(SOC)accumulation,in which RFB,RFP and RFV treatments significantly increased SOC and particulate organic carbon(POC)contents by 11.1%-63.1% and 101.2%-273.1%,respectively.RFP treatment also significantly increased the content of mineral associated organic carbon(MAOC)by 23.4%-48.1%,but straw treatment had no significant effect on the organic carbon of each component.Compared with CF treatment,the application of different organic fertilizers also significantly changed the contents of dissolved organic carbon(DOC)and microbial biomass carbon(MBC).The improvement effect of pig manure and vermicompost treatment reached a significant level,with an increase of 112%-386%.However,there was no significant difference in the content of DOC in 2019 sweet potato season.The application of plant-derived organic fertilizers(straw,biological black carbon)increased the relative content of alip Hatic organic carbon in soil DOC,but decreased the relative content of aromatic organic carbon,while the application of animal-derived organic fertilizers(pig manure,vermicompost)showed an opposite trend.The application of organic fertilizer generally promoted the formation of aggregates and weak-crystalline iron oxides in the soil as a whole,and vermicomposturs increased the absorbance of free hydroxyl groups,aromatic rings,aliphatic compounds,polysaccharides and other functional groups in the red soil.The results of scanning electron microscopy(SEM)showed that the surface of soil colloid treated by RFP and RFV was rougher than that of other treatments.Compared with CF treatment,RFB,RFP and RFV treatments promoted the content of weak crystalline iron oxide in red soil,and RFP and RFV treatments had the best improvement effect,with an increase rate of 24.4%-52.2%.Compared with CF treatment,application of organic fertilizer promoted soil agglomeration,among which RFP and RFV treatments had the most significant effect on improving mean weight diameter(MWD),with an increase of20.1%-57.1%.Correlation analysis of soil aggregates,Fe oxides and soil organic carbon composition showed that the biochar and vermicompost treatments increased soil organic carbon content mainly by increasing POC,and aggregation protection was the main protective mechanism.Pig manure treatment not only protects organic carbon through aggregates,but also chemically protects organic carbon by promoting the formation of MAOC.To sum up,the application of organic fertilizer can promote the improvement of soil fertility,increase crop yield and effectively reduce the amount of chemical fertilizer.At the same time,different types of organic fertilizers will affect different components of soil organic carbon,and change the biogeochemical cycle of soil carbon,which provides a part of the theoretical basis for the development of sustainable agriculture under the substitution of organic fertilizers for chemical fertilizers.