Study On Soil Biological Characteristics Of Located Experiment Of Green Manure In Weibei Rainfed Highland

As an important agro-ecological region,the Weibei rainfed highland needs urgently measures to build up its soil fertility and improve its eco-environment because of its insufficient annual precipitation,infertile soil and severe soil erosion.High in stress tolerance,leguminous green manure crops grow normally in severe ecological environment.Cultivation and incorporation of such crops may help reduce soil erosion as well as improving soil structure and soil fertility.In order to explore rules of changes in farmland soil nutrients,ecological stoichiometry characteristics and soil enzyme activities as affected by cultivation and incorporation of green manure crops in summer fallow seasons,a field experiment was designed to have three different species of leguminous green manure crops [Mung bean(Phaseolus radiatus L.),soybean(Glycine max(L.)Merr.)and Huai bean(Glycine ussuriensis Regel et Maack)],four nitrogen fertilization rates(N0,0 kg N hm-2;N1,108 kg N hm-2;N2,135 kg N hm-2 and N3,162 kg N hm-2)and three replicates for each treatment,laid out in a split plot design in a tract of farmland in the Weibei Rainfed Highland,and conducted for six years in a row.Cultivation and incorporation of green manure crops in the summer fallow period significantly improves soil carbon and nitrogen status,and hence is an effective practice to build up soil fertility in the Weibei rainfed highland.The specific conclusions are as follows:1.Results show that the long-term cultivation and incorporation of leguminous green manure crops improved soil organic carbon,total nitrogen,alkaline nitrogen contents significantly or by 4.47%~15.35%,5.21%~6.25% and 11.00%~14.35%,respectively,as compared to CK(the fallow treatment),and the effects of Huai bean were the most obvious in all the treatments.Two weeks after the incorporation of green manure crops,soil total nitrogen,soil organic matter and alkaline nitrogen all increased in content,but the margin of the increase of the former was higher than that of the latter two.In the treatments with Huai bean,organic carbon,total nitrogen,alkaline nitrogen,total phosphorus and available phosphorus all increased significantly,either in a short term or a long term,indicating that the effect of the green manure crop of Huai bean on soil building is the most obvious.Huai bean,high in number of root nodules and in nitrogen content,is a crop widely cultivated in the local area.After the incorporation of a green manure crop into the soil,soil C:N declined for the first short period of time,but in the long run,soil C:N was increased,which is beneficial to the accumulation of soil organic matter and improvement of soil nutrient balance.The ratios of C:N,C:P and N:P in the experiment field are all quite low,because the soil is low in soil organic carbon and total nitrogen,but high in total phosphorus.Soil C:P and N:P is closely related to dynamics of soil carbon and nitrogen contents.2.Generally speaking,soil enzyme activities of green manure treatment were significantly higher than CK when green manure crops cultivated and incorporated for six years and the effects of Huai bean and soybean were better than Mung bean.After wheat harvest,the enzyme activities of applying nitrogen fertilizer treatment were significantly greater than that of no fertilizer treatment as far as urease and alkaline phosphatase are concerned.For the other enzyme activities,the treatment of N1 was higher than that of N0 and had little difference with N2 and N3.Compared to the after wheat harvest stage,activities of alkaline phosphatase and dehydrogenase showed a tendency of increase after the incorporation of green manure,while sucrase,urease and arylsulfatase having decreasing trend,and total enzyme activity index(TEI)changing little.The principal component analysis(PCA)showed that composite scores of soybean and Huai bean treatment were higher than that of CK and Mung bean,which represents better effect of improving soil fertility.3.The Michaelis constant(Km),maximum reaction rate(Vmax)and Vmax/Km of alkaline phosphatase of green manure treatment all significantly increased when compared to CK.The Vmax/Km value of N1 was significantly greater than other applying nitrogen fertilizer treatment,which shows that the reduction of nitrogen fertilizer application rate is beneficial for the reaction’s performance.Analyzed in the perspective of enzyme thermodynamics,the change of alkaline phosphatase activity is led by activation enthalpy,which belongs to enthalpy control reaction.Summarized from results above,cultivation and incorporation of green manure crops in the summer fallow period significantly improves soil carbon and nitrogen status,and hence is an effective practice to build up soil fertility in the Weibei rainfed highland,while Huai bean has the best effect of improving soil fertility.Less amount of nitrogen fertilizer can be applied and the reduction level could be 20% when compared to the local common nitrogen fertilizer rate.