Effects Of Chinese Milk Vetch And Straw Returning On Nutrients,Active Organic Carbon And Iron Oxide In Paddy Soil

Chinese milk vetch and straw are high-quality organic fertilizer sources for paddy fields in southern China.The effect of Chinese milk vetch and straw returning on paddy soil fertilization and substitution of chemical fertilizer has been paid more and more attention.In this paper,paddy soils developed from river alluvium Q₄（JZ）and quaternary sediments Q₃（WH）were taken as objects respectively,and the field positioning experiment of Chinese milk vetch rotation and the laboratory culture experiment of adding Chinese milk vetch were used.To study the effects of Chinese milk vetch returning（MV）and straw returning（S）and different amount of fertilizer NPK（40%¹00%）on rice season（October 2017 and October 2018）and Chinese milk vetch season（April 2018）in the paddy soil surface（0²0 cm）nutrients（organic matter,alkali-hydro nitrogen,available phosphorus and available potassium）,active organic carbon（particles organic carbon,easy oxidation organic carbon）,and iron oxide（free iron,amorphous iron,complexing iron）.The relationships among soil nutrients,active organic carbon and iron oxide were discussed by correlation analysis,and the soil fertility of Chinese milk vetch and straw returning was evaluated by principal component analysis.The purpose is to provide a theoretical basis for the rational utilization of Chinese milk vetch and straw and the improvement of farmland soil fertility in the southern rice region.The results show that:（1）The application of 40%-100%chemical fertilizer under Chinese milk vetch and straw returning increased the contents of soil organic matter（1.40%⁴6.49%）and alkali-hydrolyzed nitrogen（0.08%³5.88%）,returning Chinese milk vetch with straw was better than Chinese milk vetch returning alone;The content of available phosphorus and available potassium in the soil treated with Chinese milk vetch single planting and returning to the field decreased,and the content of available phosphorus and available potassium increased significantly after the straw returning and the application of chemical fertilizer,and decreased with the decrease of the application amount of chemical fertilizer;Chinese milk vetch and straw returning increased the content of particles organic carbon,increasing by 1.01%³5.77%,which had little effect on the content of easily oxidized organic carbon;under the Chinese milk vetch and straw returning,reducing the amount of chemical fertilizer can still increase the content of soil nutrients and active organic carbon,and the highest content is at 80%chemical fertilizer.（2）The application of 40%¹00%chemical fertilizer under Chinese milk vetch and straw returning increased the content of soil amorphous iron（0.06%¹2.97%）,complex iron（1.10%⁴3.61%）and the activation degree of iron oxide（0.12%¹7.36%）,little effect on soil free iron content;compared with the treatment of Chinese milk vetch alone,the content of amorphous iron and complex iron and the activation degree increased after JZ applied with straw,but decreased in WH;compared with the treatment of Chinese milk vetch alone,the content of amorphous iron and complex iron in the soil increased with the addition of chemical fertilizer,and there was no significant difference with the decrease of chemical fertilizer application.（3）Compared with the treatment without adding the Chinese milk vetch,The contents of soil pH（0.67%⁵.91%）,organic matter（4.16%¹7.69%）,available potassium（30.46%⁸5.88%）,and particulate organic carbon（8.10%²9.81%）were significantly increased in the 180 days after the addition of 2%and 4%of Chinese milk vetch,the increase rate of 4%is greater than 2%;In addition,the contents of various forms of nutrients and active organic carbon decreased with the increase of incubation time.The addition of 2%and 4%Chinese milk vetch had little effect on the content of free iron in the soil in the first and middle stage of culture（0¹20 days）,On the 180th day of culture,the content of free iron in the treatment with Chinese milk vetch was significantly lower than that in CK treatment;in 180 days of culture,the addition of Chinese milk vetch increased the content of complex iron（4.13%²3.10%）,decreased the content of amorphous iron（0.71%⁶.03%）and the degree of activation（0.24%⁶.93%）,and the variation range increased with the addition of Chinese milk vetch and the increase of culture time.（4）Organic matter,alkali-hydrolyzed nitrogen,available phosphorus,and available potassium have extremely significant positive correlation with amorphous iron,complexing iron,and activation degree（except for JZ available potassium）(r=0.288^*0.730^**);there was a significant or extremely significant positive correlation between particulate organic carbon and various forms of nutrients and iron oxide(r=0.338^*0.765^**).It is indicated that iron oxide is closely related to soil nutrients;among the active organic carbons,particulate organic carbon is more closely related to nutrients and iron oxide.（5）Compared with the control of Chinese milk vetch and straw without returning and no fertilization,both Chinese milk vetch and straw can improve soil fertility level,and the effect of Chinese milk vetch returning on soil fertility level is better than straw returning;compared with Chinese milk vetch alone,increasing the return of straw to the field increased the JZ fertility level,but decreased at WH.With the decrease of the amount of fertilizer,the soil fertility level first decreases and then increases,which is the highest under 80%of the amount of fertilizer.