Effects Of Biochar And Straw Returning On Soil Aggregates And Enzyme Activities In Purple Soil

The problems of low fertilizer utilization rate,deterioration of soil quality and decline of crop yield and quality caused by unreasonable application of biochar fertilizer are the limiting factors for the green and sustainable development of agriculture in China.The above problems are particularly prominent in the Three Gorges Reservoir area with rich rainfall,shallow soil layer and high reclamation coefficient.In order to formulate effective measures to improve soil quality,economically feasible technologies such as reducing the amount of chemical fertilizers and replacing biochar or straw,has attracted more and more attention.Both of them play an important role in soil structure,soil microbial activity and crop yield while maintaining soil quality.Purple soil dry slope land is the most important agricultural land in the Three Gorges Reservoir area.Exploring the rational fertilization method of purple soil dry slope land has important theoretical value and practical significance for the green and sustainable development of agriculture in the Three Gorges Reservoir area.Therefore,through the long-term experiment from 2017 to2020,five treatments were set up:no fertilization（CK）,conventional fertilization（NPK）,optimized fertilization（GNPK）,reduced chemical fertilizer combined with corn straw（RSD）and reduced chemical fertilizer combined with biochar（BC）.The effects of different fertilization conditions on the stability and composition of soil aggregates were studied Effects of organic carbon,nitrogen,phosphorus and potassium contents in aggregates and soil enzyme activities.The main results are as follows:1.Fertilization can significantly increase the content of 5～0.5 mm particle size aggregates and increase the average weight diameter（MWD）,geometric average diameter（GMD）and R0 25（>0.25 mm particle size aggregate content）and reduce the fractal dimension（D）and the damage rate of soil structure（PAD0.25）（P<0.05）.Fertilization could significantly increase the content of soil organic carbon,and the effects of BC(6.73 g kg^-1)and RSD(5.45 g kg^-1)were significantly better than NPK(5.05 g kg^-1)and gnpk(3.63 g kg^-1).The contribution rate of organic carbon of<0.25 mm aggregates was the highest（34.92%～59.49%）,and that of>5 mm aggregates was the lowest（1.55%～6.01%）.BC treatment significantly increased the organic carbon contribution rate of 5～2 mm and 2～1 mm aggregates（P<0.05）,while NPK,RSD and gnpk treatment increased most significantly on 0.5～0.25 mm aggregates（P<0.05）.All fertilization treatments can improve the yield of rape and maize,with great interannual difference,but the difference between treatments is not significant.2.Fertilization could significantly improve soil total nitrogen,total phosphorus,total potassium,alkali hydrolyzable nitrogen,available phosphorus and available potassium.The effect of BC treatment on soil total nitrogen,alkali hydrolyzable nitrogen and available phosphorus was significantly higher than that of other fertilization treatments（P<0.05）.BC treatment increased total nitrogen,alkali hydrolyzable nitrogen and available phosphorus by 11.84%～230.25%,compared with other fertilization treatments;BC and RSD treatments increased soil total potassium and available potassium less,while NPK and gnpk treatments increased significantly,with an increase of 22.81%～154.88%.With the decrease of soil aggregate particle size,the contents of total nitrogen,total phosphorus,alkali hydrolyzable nitrogen and available phosphorus in purple soil showed an increasing trend,while the contents of total potassium and available potassium were evenly distributed in each particle size.Fertilization can improve the contents of nitrogen,phosphorus and potassium nutrients in aggregates of each particle size.On the whole,the smaller the particle size is,the higher the effect is.Among them,the effect of BC treatment on the content of available phosphorus in each particle size is the most obvious<The contribution rate of<0.25 mm aggregates to soil nitrogen,phosphorus and potassium was the highest（34.92%～59.49%）,and the contribution rate of>5 mm aggregates was the lowest（1.55%～6.01%）;BC treatment significantly increased the contribution rates of organic carbon,nitrogen,phosphorus and potassium in 5～2 mm and 2～1 mm aggregates（P<0.05）,while NPK,RSD and gnpk treatment increased the most significantly in0.5～0.25 mm aggregates（P<0.05）.3.Compared with CK treatment,the contents of Microbial Biomass Carbon and Microbial Biomass Nitrogen in BC treatment increased the most significantly（P<0.05）,42.46%and 75.64%respectively;Gnpk and RSD treatment significantly increased the content of MBN（P<0.05）,but did not significantly increase the content of MBC;The content of MBC in NPK treatment was significantly lower than that in CK treatment,with a decrease of 24.87%,but there was no significant difference in MBN.Compared with CK,BC,NPK,gnpk and RSD treatments increased the activities of soil catalase,urease,acid phosphatase and alkaline phosphatase,and BC treatment increased the most significantly（P<0.05）,with an increase of 40.05%～201.20%.In conclusion,biochar and straw returning can promote the formation of soil large and medium aggregates,effectively improve the stability of soil aggregates and increase the content of soil organic carbon;Biochar instead of chemical fertilizer had better effect on the increase of total nitrogen,alkali hydrolyzable nitrogen and available phosphorus of soil small particle aggregates and the improvement of soil enzyme activity;Both biochar and straw returning are helpful to the stable yield of crops.This study is of great significance to formulate management measures such as scientific fertilization,soil fertility and soil improvement.