Effect Of Reduced Chemical Fertilization With Green Manuring On Soil Organic Carbon Of Aggregates And Clay Minerals In A Paddy Soil

Evaluation of soil organic carbon(SOC)sequestration at aggregate scale and its stabilization processes under various long-term fertilization regimes are critical to accomplish sustainable agricultural cropping systems.Long-term field experiment(10 years)under reduced amount of NPK(F;100%,80% and 60%)with Chinese milk vetch(MV,Astragalus sinicus L.;15t,22.5t,30 t and 37.5t)as green manure was conducted in a fluvo-aquic paddy soil of Southern China.This study was aimed to assess soil water stable aggregates(WSA)and their stability index,aggregate associated SOC,physical protection mechanisms of SOC within aggregate associated density fractions(f LOC,ci POC,f-i POC and m SOC)at 0-20 cm soil depth.Moreover,changes in soil chemical properties and clay mineralogy were also evaluated.Results revealed that soil macroaggregation(WSA0.25-2 mm)was improved significantly(P<0.05)under 37.5t + F80(76.7% and 10.1%)and 30 t + F60(80.6% and 12.5%)plots,when compared with CK and F fertilized plots respectively.Mean weight diameter(MWD)and geometric mean diameter(GMD)were increased under 37.5t + F80(51.4% and 22.7%)and 30 t + F60(54.3% and 22.7%)treatments than CK,accordingly.In comparison to CK,the bulk soil OC was significantly increased by 31.0% for 22.5t + F80,38.8% for 37.5t + F80 and 18.0% for 30 t + F60 fertilized plots.After F and MV amendments,SOC concentrations of 0.25-2,0.053-0.25 and <0.053 mm WSA fractions exhibited cumulative increments of 18.0%,11.2% and 6.8% than CK,irrespective of the treatments.The highest proportion of additional OC through MV residues was located in m SOC isolated from 0.25-2 mm and 0.053-0.25 mm WSA fractions.However,c-i POC0.25-2 mm and f-i POC0.053-0.25 mm fractions were the most sensitive indicators to increased SOC,suggesting shift of SOC towards microaggregates and thereby long-term SOC sequestration within microaggregates through physical protection as revealed by agglomerative cluster analysis which authenticated the transfer of SOC contents from m SOC fraction to f-i POC fraction within the microaggregates(0.053-0.25 mm WSA).The 10 years of substituted fertilization of F and MV significantly(P < 0.05)decreased soil p H across the treated plots.The increments in primary soil nutrient contents such as total nitrogen(TN)and available nitrogen(AN),available phosphorous(AP)and available potassium(AK)were independent of the treatments,except AK.The increase in TN(66.5% and 58.2%)and AN(78.4% and 59.7%)content was much higher under 22.5t and 30 t of MV combined with F80 than F60,compared to control.Likewise,the AP contents were significantly increased under the 30 t + F80(256.3%),37.5t + F60(259.5%),while of AK by 19.5% under 22.5t + F80 treated plot,compared to CK,respectively.However,the AK contents were dramatically slightly reduced under F60 fertilization along with all MV rates.A highly significant negative relationship of soil p H was detected with soil nutrient contents.The X-ray diffractograms(XRD)have not exhibited any clue of mineralogical phases transformations across the treatments(no obvious mineralogical changes were detected),however,the differences in the peak intensities(structural transformations within the existing phases)were found.The clay mineral assemblage of studied fluvo-aquic paddy soil was complex,comprising of vermiculite,illite,kaolinite,hydroxy-interlayered vermiculite(HIV),pedogenic chlorite,and quartz,across the treatments.Based on semi-quantitative analysis,it was evident that alone chemical fertilization(F)for 10 years has weathered and slaked the 1:1 minerals(kaolinite)and soil potassium-bearing minerals such as illite,which ultimately significantly reduced(65.3%)the vermiculite contents,compared to CK.While in comparison to F plot,long-term combined application of F with MV prominently increased vermiculite mineral contents with highest positive effect under 22.5t + F80,30 t + F80,37.5t + F80 and 22.5t + F60 treated plots.The correlation analysis indicated a nonsignificant positive relationship of bulk soil organic carbon(SOC)contents with vermiculite;however,a negative linkage was observed with soil p H,kaolinite and HIV minerals,respectively.Conclusively,reduced NPK fertilization with MV(especially,22.5t + F80,37.5t + F80 and 30 t + F60 amended plots)could be an efficient strategy to improve soil structure and long-term SOC sequestration within microaggregates through physical protection mechanisms.Moreover,the obtained results demonstrated that long-term application of reduced chemical fertilization in combination with green manuring(MV)has improved fertility status of the fluvo aquic paddy soil as well as promoted the vermiculitization process at the expense of illite(K-bearing mineral)and chlorite.