Effects Of Nitrogen Reduction Integrated With Using Biochar On Soil Organic Carbon And Nitrogen Mineralization And Rice Yields In Rice Paddies

Reducing applications and increasing fertilizer efficiencies of the nitrogen(N)fertilizer are one of the research hotspots at present.As a new material,biochar plays an important role in soil improvement and crop yield increase.In this study,field positioning experiments were carried out,and six treatments were set up according to the principle of equal N quantity:No fertilization(CK),conventional fertilization(150kg/hm~2,N100,T0),N reduction 10%(135 kg/hm~2,N90)combined with biochar(2.5t/hm~2,T1),N reduction 20%(120 kg/hm~2,N80)combined with biochar(5.0 t/hm~2,T2),N reduction 30%(105 kg/hm~2,N70)combined with biochar(7.5 t/hm~2,T3),N reduction 40%(90 kg/hm~2,N60)combined with biochar(10.0 t/hm~2,T4).In order to explore the effects of N reduction integrated with different biochar proportions on soil organic carbon(C)and N mineralization,C and N components and rice yields in rice paddies,and determine the optimal ratio of N reduction combined with biochar to return to rice paddies,results of this study provided theoretical basis for the rational fertilization of farmland soil in Guizhou region.The related results are as follows:(1)The soil organic C mineralization rates were divided into four stages during the whole culture period.After C mineralizing,the cumulative mineralization amounts and mineralization rates of organic C in each treatment were respectively among 2.25～2.67 g/kg and 10.1～14.7%,which were the lowest in T2 and respectively decreased significantly by 2.4%and 22.4%compared with of that in T0.The soil organic N mineralizing rates were divided into three stages during the whole culture period.After N mineralizing,the cumulative mineralization amounts and mineralization rates of organic N in each treatment were respectively among 182.70～271.92 mg/kg and 14.8～19.6%,which the highest values were in T2 and respectively increased significantly by 32.7%and 23.5%compared with of that in T0.(2)Compared with T0,the contents of soil organic C(SOC)and dissolved organic C(DOC)were increased under N reduction integrated with biochar treatments and were proportional to the ratio of used biochar treatments.The contents of the microbial biomass C(MBC)and readily oxidized organic C(ROC)in soil were the highest under T2 and respectively increased significantly by 35.5%and 10.9%compared with T0.MBC/SOC,DOC/SOC and ROC/SOC were decreased by N reduction integrated with biochar treatments.The contents of organic C and active organic C in all treatments after C mineralizing decreased compared with those before C mineralizing,and the decrease was most significant in T2.(3)Compared with T0,the contents of total N(TN),microbial biomass N(MBN),soluble organic N(SON),soluble inorganic N(DIN)and soluble total N(TSN)in soil were increased by N reduction integrated with biochar treatments,of which the highest values were found in T2 were significantly increased by 9.2%,15.5%and 25.5%,25.4%and 25.5%,respectively compared with T0.There were increased SOC/TN,MBN/TN,SON/TN,DIN/TN and TSN/TN under N reduction combined with biochar treatments.After N mineralizing,the contents of TN,MBN and SON in each treatment decreased compared with that before N mineralizing,while the contents of DIN and TSN increased compared with that before N mineralizing and was the most significant in T2.(4)Rice yields,partial productivity,agronomic efficiencies and apparent use efficiencies of N fertilizer were the highest under T2,which were significantly increased by 10.2%,39.5%,30.5%and 106.7%compared with of that under T0respectively but was decreased under T4.Correlation analysis showed that rice yields were negatively correlated with soil organic C cumulative mineralization rates,and positively correlated with soil organic N cumulative mineralization rates,partial productivity,agronomic efficiencies and apparent use efficiencies of N fertilizer.In this study,under nitrogen reduction by 20%combined with 5.0 t/hm2 biochar treatment,there were best efficiency to increase the content of soil active organic carbon,nitrogen of different forms and rice yield,inhibite the process of soil organic carbon mineralization,enhance the ability of soil carbon sequestration and fertilizer preservation,and promote the effect of soil organic nitrogen mineralization and soil inorganic nitrogen release.It can be concluded that nitrogen reduction combined with isonitrogen biochar fertilization is feasible in Guizhou yellow paddy soil.