Grain Fractionation Of Chemical Characteristics Of Soil Organic Matter In A Subtropical Paddy Field And Its Response To Fertilization

Soil organic matter（SOM）is the basic element of soil fertility,crop production and ecological environment functions,and it is also an important carbon pool on the terrestrial surface.At present,the changes in chemical composition and oxidation state of organic matter with different particle sizes in subtropical paddy soils are still unclear.To this end,this study relies on the red soil long-term fertilization experimental platform in Jinxian County,Jiangxi Province,and takes the typical subtropical paddy field soil in China as the research object.The five fertilisation regimes were:unfertilised（control）,nitrogen（N）,nitrogen+phosphorus+potassium（NPK）,NPK+straw（NPKS）,and NPK+Astragalus sinicus L.+pig manure（NPKM）.Based on the particle size fractionation and pyrolysis-gas chromatography/mass spectrometry technology（Py-GC/MS）,the carbon and nitrogen content and organic matter chemical composition of different components under different fertilization treatments were measured to reveal the change law of carbon and nitrogen content and chemical characteristics of soil organic matter with different particle sizes under long-term fertilization in paddy fields for37 years.The main results are as follows:（1）After 37-years,the SOM content of the control,N,NPK,NPKS and NPKM treatments had increased by 16.3%,12.3%,31.6%,41.7%and 59.8%compared with the initial level,respectively.The 37-year cumulative C inputs were minimal for the control(12.2 t C ha^-1)and maximal for the NPKM amendment(177.4 t C ha^-1).The soil organic carbon（SOC）sequestration（current minus initial level）was linear with cumulative total C input,with a conversion efficiency from input-C to SOC of 6.6%（2）The mass of mineral-associated organic matter（MOM）accounted for more than 90%of the bulk SOM,and the proportions of coarse particulate organic matter（cPOM）and fine particulate organic matter（f POM）were approximately 2%–6%for all fertilisation treatments.The NPKM increased the mass proportions of POM and decreased the massproportions of MOM compared with the control.（3）37-years of NPKS and NPKM increased the bulk SOC and TN pools.The NPKM treatment significantly increased the mass proportion of cPOM and f POM and organic matter content,which was mainly due to the contribution of MOM,while NPK and NPKS had little effect.The N treatment decreased soil organic matter content.Particle size also limits the carbon and nitrogen content of soil organic matter.Partical size had a significant effect on soil C/N,and the ratio in the particle size is:cPOM>f POM>MOM.（4）The particle size classes controlled the chemical composition and oxidation state of SOM.The cPOM and f POM selectively preserved the aromatic compounds and lignin due to their chemical recalcitrance,while the MOM was enriched with N-containing compounds,especially amino N,because of their microbial origin and high chemical affinities to mineral surfaces.The origin（plant-and microbial-derived）and oxidation state of SOM were also regulated by particle size（especially the coarse particles）but not fertilisation treatment.（5）Fertilization decreased the ratio of Alkanes/Alkenes and Phenolics/Lignin derivatives in cPOM,but increased the ratio of Polycyclic aromatics/Monocyclic aromatics.Compared with other four fertilization treatments,the phenolic/lignin value of NPKS is the smallest in cPOM,but it is the largest in f POM.The ratio of Alkanes/Alkenes and Polycyclic aromatics/Monocyclic aromatics reduces with the decrease of particle size.