Effects Of Different Organic Material And Their Incorporation Depths On Soil Organic Carbon And Soil Microbial Community Characteristics In A Chinese Mollisol

Black soil is one of the important cultivated land resources in China and plays an important role in ensuring national food security.However,black soil is facing the problems of declining soil organic carbon(SOC)contents and degradation of structural function,which limits the capacity of cultivated land to improve.Organic material incorporation is widely used as an effective measure to improve soil organic carbon(SOC)contents.To clarify the effects of different organic material and their incorporation depths on organic carbon structure of black soil.In this paper,a three-year experiment was conducted in Guangrong country,Heilongjiang Province,with five treatments:conventional tillage(CK,tillage depth to 15 cm with no organic material incorporation),straw incorporation with conventional tillage(T1,tillage depth to 15cm),straw incorporation with subsoil tillage(T2,tillage depth to 35 cm),straw and organic fertilizer incorporation with subsoil tillage(T3,tillage depth to 35 cm)and organic fertilizer incorporation with subsoil tillage(T4,tillage depth to 35 cm).The effects of different organic material and their incorporation depths on the soil organic carbon,carbon pool management index,microbial biomass carbon and nitrogen,density fraction of soil organic carbon,humus structure and microbial community structure were investigated by soil organic carbon density grouping,humus grouping,fourier infrared spectroscopy and three-dimensional fluorescence spectroscopy and phospholipid fatty acid analysis.The results are as follows:(1)Different organic material and their incorporation depths were important measures to affect soil organic carbon and nutrient.T1,T2,T3 and T4 treatments increased the contents of soil organic carbon(SOC),available nitrogen(AN),available phosphorus(AP),microbial biomass carbon(MBC),and carbon pool management index(CMI)by 6.16%-15.38%,3.17%-12.76%,2.31%-134.98%,50.06%-79.31%and13.50%-34.96%in the 0-15 cm soil layer compared to CK.And in the 15-35 cm soil layer,T2,T3 and T4 treatments increased the contents of SOC,AN,AP,MBC and CMI by 4.35%-20.11%,15.89%-33.91%,232.97%-323.84%,43.44%-97.46%and 11.06%-43.68%compared to CK.(2)Different organic material and their incorporation depths increased the contents of soil organic carbon density fraction.Among them,the contents of free light fraction(F-LF),ollculled light fraction(O-LF)and heavy fraction(HF)significantly increased by 28.4%-62.5%,33.13%-60.24%and 16.67%-31.72%,respectively,in the 0-15 cm soil layer under T1,T2,T3 and T4 treatments compared with CK(p<0.05).In the 15-35 cm soil layer,F-LF,O-LF and HF significantly increased by 46.85%-65.03%,32.26%-51.61%and 21.12%-32.30%,respectively,under T2,T3 and T4 treatments compared to CK(p<0.05).Organic material incorporation with subsoil tillage increased the contents of methyl,methylene and aliphatic functional groups but decreased the relative proportion of aromatic functional groups in the light fraction(F-LF and O-LF)and their relative proportions.Organic material incorporation with subsoil tillage increased the contents of reactive organic carbon in F-LF and O-LF and promoted the decomposition of inert materials in the soil.(3)Different organic material and their incorporation depths increased the carbon contents(FA-C and HA-C)of FA and HA.FA-C and HA-C were significantly increased by 9.36%-40.90%and 14.86%-43.79%,respectively,in the 0-15 cm soil layer under T1,T2,T3 and T4 treatments compared to CK.In the 15-35 cm soil layer,FA-C and HA-C significantly increased by 25.19%-29.7%and 7.46%-41.79%under T2,T3 and T4 treatments compared to CK.The fulvic acid-like,humic acid-like and protein-like components were mainly present in FA,where fulvic acid-like content>humic acid-like content>protein-like content.The fulvic acid and humic acid components were mainly present in HA,where humic acid>fulvic acid.Organic material incorporation with subsoil tillage effectively increased the contents of FA and HA and led to a decrease in the molecular weight and complexity of HA.(4)Different organic material and their incorporation depths improved the soil microbial community characteristics.Compared with CK,T1,T2,T3 and T4 treatments increased the abundance of bacteria,fungi,actinomycetes and total phospholipid fatty acids(T-PLFA)by 20.32%-56.86%,1.17%-99.30%,1.69%-31.22%and 11.97%-48.98%in the 0-15 cm soil layer,respectively.The abundance of bacteria,fungi,actinomycetes and T-PLFA were increased by 79.57%-159.26%,133.74%-687.00%,52.21%-96.42%and 81.77%-139.02%under T2,T3 and T4 treatments in the 15-35 cm soil layer.And compared to CK,the ratio of fungi to bacteria increased by 53.37%and 23.62%in the0-15 cm soil layer and 15-35 cm soil layer under T3 treatment,while the ratio of Gram-positive(G~+)and negative bacteria(G~-)decreased by 17.5%and 20.99%in the 0-15 cm soil layer and 15-35 cm soil layer,respectively(p<0.05).Random forest results showed that SOC and G~-contributed more to CMI than other indicators in the 0-15 cm and 15-35 cm soil layer with 40.66%and 33.14%under different organic material and their incorporation depths,respectively.To sum up,organic material incorporation with subsoil tillage can enhanced the contents of SOC,MBC,CMI and nutrient,impoved the soil structure,enhanced the soil organic carbon sequestration capacity,increased the contents of F-LF,O-LF and HF,improved the fluorescence intensity of FA and HA fluorescent fractions.Therefore,it is recommended to use the straw and organic fertilizer incorporation treatment in black soil areas.