The Combined Characteristics Of Soil Organic Carbon And Minerals With Aggregate Stability Under Green Manure Returning

Green manure returning can improve the structure of paddy soils and increase soil organic carbon（SOC）content,which is an important field management measure for fertilizer reduction and carbon sequestration.Chinese milk vetch（Astragalus sinicus L.）,ryegrass（Lolium multiblorum Lam.）and rape（Brassica chinensis L.）are widely used as winter green manure in southern China.In the present study,the paddy soils derived from river and lake sediments in Nanxian County and Quaternary red clay in Qiyang City,Hunan Province are taken as the research objects.The 10-year long-term experimental site in Nanxian County was set up with control（CK）,100%chemical fertilizer（F100）,22.5 t/hm²Chinese milk vetch combined with different proportions of chemical fertilizer（MV+F100,MV+F80,MV+F60 and MV+F40）.The 36-year long-term experimental site in Qiyang City was set up with winter fallow（WF）,rape returning（RP）,Chinese milk vetch returning（MV）and ryegrass returning（RG）.Different organic carbon fractions were obtained by density grouping of macroaggregates and microaggregates.The composition characteristics of mineral associated organic carbon（m SOC）and mineral lattice structure changes were analyzed by solid ¹³C nuclear magnetic resonance（NMR）,X-ray diffraction（XRD）and high resolution transmission electron microscopy（HRTEM）.The distributions of free and amorphous iron,aluminum and manganese oxides in organic carbon fractions were analyzed.The retention mechanism of organic carbon in paddy soils and the effect of organic carbon and mineral combination on aggregate stability under green manure returning were explored.The main results are as follows:（1）Chinese milk vetch returning with chemical fertilizer reduction and ryegrass returning significantly improved aggregate stability.After returning green manure to the field,the aggregate was mainly concentrated in the>5 mm fraction.The content of>5 mm aggregate was significantly positively correlated with mean weight diameter（MWD）and geometric mean diameter（GMD）of soil aggregates.In the paddy soils derived from river and lake sediments,the MWD of aggregates increased with the decrease of chemical fertilizer application.The aggregate stability under MV+F80 was the strongest,compared to winter fallow,the values of MWD,GMD and>0.25 mm water-stable aggregate content(WSA_>0.25)were significantly increased by 63.0%,58.2%and 28.8%,respectively.In paddy soils derived from Quaternary red clay,RG treatment significantly increased the content of>5 mm aggregates compared to RP and MV treatments.Compared to winter fallow,ryegrass returning increased aggregate stability,and WSA_>0.25 had an increasing trend after three green manures returning.（2）The effects of Chinese milk vetch and ryegrass on enhancing soil organic carbon（SOC）were better than those of rape,and Chinese milk vetch returning was beneficial to the formation of m SOC.In the paddy soils derived from river and lake sediments,Chinese milk vetch returning with chemical fertilizer reduction significantly increased the SOC content of>5 and 2-5 mm aggregates,meanwhile,promoted the conversion of fine particulate organic carbon（f POC）to m SOC in microaggregates.Compared with F100,the relative content of polysaccharide C in c POC of macroaggregates under MV+F80 and MV+F60 increased by 21.6%and 17.7%,respectively.In paddy soils derived from Quaternary red clay,green manure returning increased SOC content in>5,0.053-0.25 and<0.053 mm aggregates,and MV treatment was beneficial to the accumulation of m SOC.Compared to RP,MV and RG treatments significantly increased SOC content in 2-5 and<0.053 mm aggregates.Compared with WF,the relative proportion of aromatic C within m SOC of macroaggregates increased by 60.2%（MV）,25.9%（RP）and 18.2%（RG）under green manure application,respectively.（3）Chinese milk vetch returning increased anti-decomposition organic carbon components within m SOC,and the effect of Chinese milk vetch on m SOC chemical structure in macroaggregates was stronger than that of microaggregates.The main organic carbon functional groups in the m SOC of paddy soil aggregates derived from river and lake sediments were O-alkyl C and alkyl C,accounting for 26.0%-32.3%and 26.4%-29.7%,respectively.In the m SOC of macroaggregates,MV+F80 increased the alkyl C content.Chinese milk vetch returning with chemical fertilizer reduction promoted the accumulation of carbonyl C within m SOC of macroaggregates and microaggregates,which may be caused by the Chinese milk vetch returning to enhance microbial activity and products.Chinese milk vetch returning increased the alkyl C/O-alkyl C ratio within m SOC,and the degree of organic carbon humification was enhanced.The single application of chemical fertilizer may reduce the aromatic structure in the m SOC of macroaggregates.Chinese milk vetch returning with chemical fertilizer reduction promoted the complexity of SOC structure within m SOC and effectively enhanced the stability of SOC.（4）Green manure returning promoted the transformation of illite to kaolinite within m SOC.In paddy soils derived from Quaternary red clay,the highest content of mineral within m SOC of macroaggregates and microaggregates was illite,accounting for 51.0%-61.9%;followed by kaolinite,accounting for 30.0%-36.8%;the content of hydroxyl-interlayered vermiculite and vermiculite was less than 10%.The lattice fringe spacing in the m SOC of microaggregates under WF gradually increased from 1.0 nm to 1.4 nm,and there was a solid phase transition from illite to 1.4 nm minerals.The crystallinity of illite in the m SOC of macroaggregates under MV treatment was poor,and the lattice fringes of illite in the m SOC of microaggregates were bent.The returning of Chinese milk vetch may strengthen the weathering of illite.The lattice fringe spacing in the m SOC of macroaggregates under MV shrinked from 1.0 nm to 0.7 nm,and illite may be converted to kaolinite.In general,the illite within m SOC of macroaggregates and microaggregates under MV changed significantly,and the content of kaolinite increased.（5）Green manure returning was beneficial to the accumulation of free iron oxide（Fed）and aluminum oxide（Ald）in organic carbon fractions,meanwhile,the effects of ryegrass and Chinese milk vetch were better than those of rape.In paddy soils derived from river and lake sediments,Chinese milk vetch returning with chemical fertilizer reduction significantly increased the contents of Fed,Ald and amorphous aluminum oxide（Alo）within c POC and f POC of macroaggregates,and promoted the accumulation of amorphous iron oxide（Feo）in organic carbon fractions.In paddy soils derived from Quaternary red clay,RG and MV treatments significantly increased the Fed content within f POC of macroaggregates by 61.1%and 46.0%,respectively,compared with RP treatment.Goethite and hematite were found within m SOC of macroaggregates and microaggregates in the two parent paddy soils,and the incorporation of Chinese milk vetch was beneficial to the formation of goethite in m SOC of microaggregates.The effects of green manure returning on oxides within organic carbon fractions of macroaggregates was stronger than those of microaggregates.（6）Illite had a significant positive effect on the SOC content of m SOC,while kaolinite had a negative effect.After the green manure returning promoted the conversion of illite to kaolinite,the SOC content of m SOC decreased accordingly.Illite and amorphous manganese oxide in m SOC of microaggregate were the most powerful factors affecting aggregate stability.Green manure returning altered the organic carbon chemical structure in m SOC of aggregates and promoted the evolution of clay minerals from 2:1 to 1:1,which was conducive to the interaction between organic carbon and minerals to form organic-inorganic complexes,thus promoting the stability of aggregates in paddy soils.Under the condition of 22.5 t/hm² Chinese milk vetch returning,60%-80%of conventional chemical fertilizer was recommended for southern paddy soils.The effects of Chinese milk vetch and ryegrass on carbon fixation and improvement of aggregate structure were better than those of rape.