The Formation Mechanism Of Organo-mineral Complex In Typical Shajiang Black Soil Under Long-term Straw Returning Model

The clay fraction content is relatively high in Shajiang black soil,of which 2:1 type clay minerals are dominated by montmorillonite.The obvious swelling and shrinkage characteristic of Shajiang black soil is determined by the own property of 2:1 type clay minerals directly,which seriously restrains the improvement of farmland tillage conditions and soil quality.The promotion of soil organic matter(SOM)content can enhance the capacity in holding water and nutrition and eliminate negative production effect caused by swelling and shrinkage of this type of soil.Shajiang black soil was studied through field trials and indoor simulation experiments,so as to analysis the restricting factors in the accumulation of SOM.Based on the straw returning long-term location experiment,the formation and transformation mechanism of soil organo-mineral complexes were discussed with depth.In this experiment,fourier transform infrared spectroscopy,scanning electron microscopy and 13C solid state nuclear magnetic resonance technologies were used to study the morphology characteristics of soil clay particles,the structure of soil organic carbon and the formation and transformation mechanism of organo-mineral complexes under different straw returning modes,in order to explore the key factors that restrain the accumulation and stabilization of SOM in Shajiang black soil.The research results will be helpful to provide theoretical foundation about the reduction of the black soil attribute obstacle factor and the improvement of basic soil fertility.The long-term field experiment,which has been carried on since 2007,is designed to have five treatments,i.e.WMS+F(application of mineral NPK plus returning of both wheat and maize straws),WS+F(application of mineral NPK plus returning of wheat straw only),MS+F(application of mineral NPK plus returning of maize stalks only),F(application of mineral NPK only)and CK(no-fertilization).The main research results are as follows:1.There were significantly negative correlations between the content of amorphous Al oxides and easily oxidized SOM(r2=-0.48,p<0.01),and there were extremely remarkable difference between the content of easily oxidized SOM and inert SOM(r2=-0.46,p<0.01),which indicates that the emergence of amorphous Al oxides has relatively large impacts on accumulation of SOM.The content of total SOM was significantly and positively correlated with the inert SOM(r2=0.36,p<0.05),while was remarkably and negatively correlated with the content of easily oxidized SOM(r2=-0.59,p<0.01).It demonstrates that SOM is more likely to accumulate in the inert SOM,which is the key to accumulate the organic matter in Shajiang black soil.2.Organic matter within the organo-mineral complexes existed mainly in the form of Fe/Al-bound organic matter(Fe/Al-OM)and tightly combined organic matter(TCOM),and both of them added up to 88.70%of the total organic matter(TOM),In terms of contents of the three forms of organic matters(Fe/Al-OM,TCOM and TOM),the five treatments followed an order of WS+F?WMS+F?MS+F>F>CK.Fe/Al-OM,TCOM and TOM was significantly and positively related to soil short-range ordered and organo-bound Fe/Al oxides,and negatively to amorphous Fe/Al oxides in content.Straw returning remarkably promoted the accumulation of SOM and was affected by Fe/Al-bound organic matter.However,the slow formation of amorphous Fe/Al oxides brings about deficiency in content,as a result,the further accumulation of SOM was restricted.3.The average content of organic matter(OM)in soil organo-mineral complex was about 27.69 g/kg,of which accumulative contribution rate was up to 75%.The accumulation of OM in soil organo-mineral complexes by wheat straw returning is higher than that of maize straw returning.Scanning electron microscopy(SEM)found that surface structure of clay mineral is layer structure and sheet structure.Fourier transform infrared spectroscopy(FTIR)showed that compared with maize straw,wheat straw was the key to enrich organic matter in Shajiang black soil.The chemical bonds formed by metal oxides and metal hydroxides in Shajiang black soil played an important role in the accumulation of SOM.4.13C solid state NMR showed that OM in clay particles was mainly in the form of Alkyl C and O-alkyl C,especially in the form of proton carbon.Long-term straw returning and fertilizer application can increase the content of aromatic compounds(aromatic C and aromatic C-O)in the organic matter of soil clay particles,and improve the aromaticity of soil clay particles,presenting a trend of WMS+F=WS+F>MS+F?F>CK.The ratio of Alkyl C and O-Alkyl C(A/OA)can reflect the decomposition degree of organic matter,and shows the trend of CK>WMS+F>WS+F>MS+F>F in clay particles.This indicated that single application of chemical fertilizer was not conducive to the transformation of O-Alkyl C to Alkyl C,while straw mulching combined with chemical fertilizer was beneficial to the decomposition of O-Alkyl C compared with single application of chemical fertilizer.Straw returning plays an important role in improving organic matter content of Shajiang black soil.Fe and A1 oxides and amorphous silicon are the key factors for the formation of organo-mineral complexes.And the clay mineral is the key to the accumulation of organic matter in Shajiang black soil.