Effects Of Long-term Fertilization On Soil Organic Carbon Mineralization And Its Microbial Mechanism In Different Fractions In Yellow Paddy Soil

Soil organic carbon mineralization is the basic biochemical process of terrestrial ecosystems,which is closely related to the release of soil nutrients and the maintenance of soil quality,while the organic carbon of different size fractions shows different fertility effects and bio-efficiencies because of the differences in their stability.Soil microorganism as an important part of soil fertility,which changes of community structure and biodiversity can affected by different fertilization measures,thus affecting soil fertility.The study based on the long-term located monitoring base of yellow soil fertility and fertilizer efficiency,and the field experiment was designed to have four fertilization treatments:CK?no fertilizer?,Treatment NPK?application of chemical fertilizer?,Treatment M?application of cow dung?and Treatment MNPK?application of NPK,normal in rate,plus organic manure?.Three fractions of coarse particle component?250-2000?m?,microaggregate component?53-250?m?and silt-clay particle component?<53?m?were obtained by wet sieving.Under indoor simulated culture conditions,the mineralization characteristics of soil fractions organic carbon and microbial biomass carbon and nitrogen were studied by alkali solution absorption and chloroform fumigation-K₂SO₄ extraction.The bacterial community structure and its diversity were analyzed by Miseq high-throughput sequencing.This research can reveal the microbial mechanism that affects organic carbon mineralization in yellow paddy soils and provide data reference for predicting the balance of soil organic carbon pool and define the optimal fertilization mode.The main results and conclusions of this study are as follows:?1?After 22 years of long-term fertilization,soil total organic carbon content in NPK treatment did not change significantly compared with CK treatment,but M and MNPK treatment significantly increased?p<0.05?.Under simulated culture conditions,the CO₂ production rate showed a rapid decline trend in the D3 until D12and a slow decline trend until the D30.The dynamic changes of soil total organic carbon mineralization rate with time in each treatment accord with logarithmic function?p<0.01?.At the end of cultivation?30d?,the cumulative mineralization and cumulative mineralization rate of the soil total organic carbon were better than that of the organic manure treatment?M and MNPK?compared with the CK treatment.The cumulative mineralization of soil total organic carbon with the number of days of dynamic changes can be used first-order kinetic equation fitting?p<0.01?.The simulation results showed that organic manure treatments?M and MNPK?could increase the amount of potentially mineralizable carbon?70.7%-113.0%?in the soil to varying degrees.Among them,M treatment can significantly increase the total organic carbon turnover rate and reduce turnover time.The above results showed that long-term application of organic manure on yellow paddy soil could increase the total organic carbon mineralization rate,promote the accumulation of soil total organic carbon,reduce the cumulative mineralization rate and enhance the soil carbon sequestration.?2?Under different fertilization methods,the content of organic carbon and potential mineralizable organic carbon in the coarse particle component were the highest,and the lowest in the silt-clay component.Under simulated culture conditions,the organic carbon mineralization rate and cumulative mineralization of all the components showed as organic manure treatment?M and MNPK?>chemical fertilizer treatment?NPK?>no fertilizer treatment?CK?,and showing the consistent trend with the soil total organic.At the end of cultivation,organic manure treatments?M and MNPK?resulted in a significant decrease in the cumulative mineralization rate of organic carbon in each component,ranging from 0.47 to 0.15 percentage points,and the cumulative mineralization rate decreased with the decrease of grain size.In all treatments,the turnover rate of soil fractions organic carbon was the best by application of organic manure,followed by application of chemical fertilizer,the lowest in no fertilization treatment and the opposite in half-cycle effect.The above results indicated that organic manure could reduce the cumulative mineralization rate while promoting the accumulation of organic carbon in soil components,and the cumulative mineralization rate decrease with the decrease of grain size,so the carbon sequestration ability enhance with the decrease of grain size.?3?Long-term application of organic manure increased the content of SMBC and SMBN in whole soil and each component better than that of chemical fertilizer treatment,and the content of SMBC and SMBN decreased with the decrease of grain size.Compared with the CK treatment,the whole soil qMB and SMBN/TN were significantly different under the MNPK treatment and M treatment respectively,and SMBC/SMBN varied greatly in each fertilization treatment?NPK<M<CK<MNPK?.The effect of component qMB on organic manure was better than that on chemical fertilizer and qMB decreased with the decrease of particle size,while SMBN/TN was better under chemical fertilizer treatment than organic manure.The content of SMBC and SMBN in coarse component and microaggregate component were significantly correlated with the content of whole soil SMBC and SMBN.The accumulative mineralization of organic carbon in each component was related to its SMBC and SMBN,and the correlation decreased with the decrease of grain size,which indicated that SMBC and SMBN of coarse component and microaggregate component had a great effect on whole soil SMBC and SMBN and cumulative mineralization.?4?The results of the classification on the genus level of Venn figure showed that NPK>M>CK>MNPK,consistently with the trend of OTU number and Chao index.Bacterial?diversity and?diversity showed that there were significantly different in bacterial community structure under different fertilization measures.The diversity of bacterial community in the long-term application of organic fertilizer was close,but the bacterial community without fertilizer treatment and chemical fertilizer treatment was clustered alone.In the long-term application of organic manure,the diversity of bacterial species and genus were higher and the distribution ratio was more uniform,but the abundance of bacterial species and genus were less than that of NPK.The?diversity showed that the three predominant bacteria in the bacterial community was Proteobacteria,Chloroflexi and Acidobacteria,respectively.Among them,the Proteobacteria had the highest correlation with the TN content of silt-clay component,the correlation between Chloroflexi and T_1/2/2 of microaggregate component was the highest,and the Acidobacteria was closely related to SMBN/TN of coarse particle component.