Effects Of Long-term Fertilization On Dissolved Organic Carbon Biodegradation And Mineral Content

Dissolved organic carbon(DOC),as an active component of soil organic carbon(SOC),is sensitive to agricultural management.It is also important for global carbon cycle and a source of greenhouse gases emission.Fertilization and soil mineral morphology affect the DOC content,structure,and degradation.Soil minerals combined with SOC as organic-inorganic complex can improve SOC accumulation.Therefore,the study on the effect of fertilization on DOC content,degradation and soil minerals morphology will help to enrich the theory of soil organic carbon cycle.Two long-term fertilization sites at Yuanyang,Henan with Fluvo-aquic soil and Jixian,Jiangxi,with Red soil were chosen.Four typical treatments at the two sites were selected as:(1)no fertilizer(CK);(2)nitrogen,phosphorus,potassium only(NPK);(3)NPK + straw(Yuanyang)/ NPK + green manure + straw(Jinxian)(NPKS);(4)NPK+ manure / NPK + green manure + pig manure(Jinxian)(NPKM).Indoor incubation experiment was carried out aimed to explore the effect of long-term fertilization on: 1)soil properties and mineral forms;2)DOC biodegradation and GHGs emission;3)DOC component structure and microbial community structure.The main results were as follow:(1)In Fluvo-aquic soil at Yuanyang,the Ca CO3 and inorganic carbon content did not differ from treatments.Carbonate combined state calcium was the main form,and with the highest proportion of 77.06% under NPK.In the Red soil at Jinxian,the Ca CO3 and inorganic carbon content was higher under NPKS than that under NPK.The water-soluble and exchange calcium was the highest,accounted for 54.13% under NPKM.Generally,the different forms of Fe-Al oxide at Fluvo-aquic soil and Red soil were ordered as: free Fe-Al oxide(Fed,Ald)> amorphous Fe-Al oxide(Feo,Alo)>complex Fe-Al oxide(Fep,Alp).In Fluvo-aquic soil,both NPKS and NPKM increased the content of Fed,Feo and Alo compared to CK.In Red soil,NPKM significantly increased the content of Feo,Alo and Fep,Alp compared with CK.At the two sites,the HA/FA ratio of Fe-Al bond humus was the highest under NPKS as 2.34 and 4.46,respectively.This was indicated that mineral fertilizer combined with straw was help to improve the degree of organic matter humus.(2)In Fluvo-aquic soil at Yuanyang,the DOC content and the DOC degradation rate were higher under NPKS and NPKM than that under other treatments.The constant of mineralization rate of the labile DOC(k1)and mean residence time of stable DOC(MPT2)were higher under NPKS than that under NPKM.The emission flux and cumulative emission of CO2 and N2 O were higher under NPKS and NPKM than that under CK and NPK.However,NPKS significantly reduced the emission flux and cumulative emission of CH4.In Red soil at Jinxian,comparing with CK and NPK,the DOC content was increased,while DOC degradation rate was decreased under NPKS and NPKM.Compared with CK,the mean residence time of labile DOC(MPT1)(9.09 ~ 10.90 days)was increased,while mean residence time of stable DOC(MPT2)(103.31 ~ 303.03 days)was decreased under NPKS and NPKM.Compared with CK,the CO2 and N2 O emission flux was increased while the CH4 emission flux was decreased under NPKS.(3)The carbohydrates,aromatic and aliphatic compounds were the main components of DOC at both Yuanyang and Jinxian.The macromolecules such as aromatic ring materials and hydrophobic components were relatively enriched with the incubation.There was higher molecular polymerization under NPKS and NPKM,which was indicated that manure and straw returned was help to organic matter accumulated.The Fourier infrared scanning results showed that the DOC components were similar at Yuanyang and Jinxian.The the intensity of simple carbohydrate and alcohol-soluble material became weaker in the late stage of incubation,indicated that the labile DOC components were mainly degraded.UV spectrum results showed that the AI,SUVA260 and CDOM at both sites were increased,indicated that the ratio of aromatic and hydrophobic substances increased.The result of three-dimensional fluorescence spectrum showed that,compared with the early incubation stage,the substances moved toward the humoid fluorescent region(region III and V),indicated that most of the small molecules of protein-like substances(region I and II)had been used by microorganisms,leaded to the relative enrichment of similar humic substances with complex structures.(4)The high-throughput sequencing of microorganism showed that,during the biodegradation,the TOP-3 species of microorganism levels at the two sites included Proteobacteria,Actinobacteriota,Bacteroidota;the TOP-3 species on categories included Legionella,Nocardioides,and Microbacterium.The microbial abundance was increased,while the microbial diversity was litte affected under NPKS.The abundance of Proteobacteria was higher at both sites,it contained variety of carbon-degrading bacteria,which can degrade complex organic matter.The Actinobacteriota abundance was higher under NPKM at both sites,which can catalyze the production of glucoside hydrolase and store carbohydrates.In conclusion,in Fluvo-aquic soil,the Fe-Al oxide was increased under NPKS and NPKM;the constant(k1)of mineralization rate of labile DOC,mean residence time of stable DOC(MPT2),CO2 and N2 O emission flux were increased under NPKS.In Red soil,the content of amorphous Fe-Al oxide(Feo,Alo)and complex Fe-Al oxide(Fep,Alp)was increased under NPKM.The mean residence time of labile DOC(MPT1)was increased and the mean residence time of stable DOC(MPT2)was decreased under NPKS and NPKM.The carbohydrates,aromatic and aliphatic compounds were the main components of DOC at both Yuanyang and Jinxian.During the biodegradation,the bacteria for carbon degradation at both sites increased.In addition,the NPKM in Fluvo-aquic soil and the NPKS in Red soil increased the Actinobacteriota abundance.