Effects Of Cotton Straw And Its Biochar On Soil C And N Contents And Ammonia Volatilization From A Drip Irrigated Cotton Field

[Objective] Biochar can improve soil fertility, enhance the soil’s ability to retain nutrients, reduce nutrient loss, and improve fertilizer use efficiency. The objectives of this study were to compare the effects of cotton straw and its biochar on（i） soil total organic C and organic C fractions,（ii） inorganic N and ammonia volatilization, and（iii） cotton yield and N fertilizer use efficiency. The results of this study provide a theoretical basis for improving N fertilizer use efficiency and for using cotton straw in a more reasonable and efficient way.[Method] A field experiment was conducted at an ecological demonstration garden in Shihezi, Xinjiang Province during 2014 and 2015. Two factors（soil treatment and N application rate） were used in this study.The biochar was produced by pyrolyzing cotton straw at 450 oC in a low oxygen environment for 6 h. The three soil treatments were（i） unamended（control, CK）,（ii） cotton straw amendment（ST, 6 t/ ha） and（iii）cotton straw biochar（BC, 3.7 t/ ha）. Equivalent amounts of C were applied to the soil in ST and BC. The three N application rates were 0, 300, and 450 kg N/ha（N0, N300, and N450, respectively）. A closed chamber technique was used to measure CO₂ emission and ammonia volatilization. The CO₂ was trapped with Na OH. The ammonia volatilization was trapped with dilute sulfuric acid.[Result]（1） Cotton straw and its biochar both significantly increased soil organic C. The ST treatment increased soil total organic C concentrations by 7% in 2014 and by 20% in 2015 compared with CK. The BC treatment increased soil total organic C concentrations by 35% in 2014 and by 58% in 2015 compared with CK. Cotton straw and N fertilizer application both significantly increased CO₂ emission. In comparison, biochar application（BC） significantly increased CO₂ emission in the N450 plots but not in the N0 and N300 plots.（2） Cotton straw application significantly increased water soluble organic C and microbial biomass C,but there was no significant difference in non-labile organic C between ST and CK. Water soluble organic C was 44% greater in BC than in CK. The BC treatment also increased microbial biomass C compared with CK（3） There was no significant difference in soil inorganic N among CK, ST and BC in the N0 plots. In the N300 and N450 plots, NO3--N and NH4+-N concentrations were both significantly greater in the topsoil of BC than in CK. Cotton straw application significantly increased topsoil NO3--N but had no significant effect on NH4+-N compared with CK.（4） Ammonia volatilization rates were greatest during the first two days after fertigation and then declined rapidly. Cumulative ammonia volatilization was 38.2% less in ST and 52.6% less in BC than in CK. There was no significant difference in ammonia volatilization between ST and BC in the N0 plots. In the N300 and N450 plots, ammonia volatilization was significantly lower in BC than in ST.（5） Cotton biomass and N uptake were significantly greater in ST and BC than that in CK. In the N0 plots, there was no significant difference between BC and ST in either cotton biomass or N uptake.However, in the N300 and N450 plots, cotton biomass and N uptake were significantly greater in BC than in ST.（6） In the N0 plots, cotton straw（ST） significantly increased yield by 16.4% in 2014 and by 10.2% in2015 compared with CK. Biochar application significantly increased cotton yield in 2014, but had no significant effect in 2015. In the N300 and N450 plots, cotton yields were 16.3% greater in ST and 24.8%greater in BC than in CK（averaged across both years）.（7） Nitrogen fertilizer use efficiency decreased as the N fertilizer rate increased. The N fertilizer use efficiency was 18.3% greater in ST and 48.6% greater in BC than in CK. In addition, N fertilizer use efficiency was 25.6% higher in BC than in CK.[Conclusion] Cotton straw and its biochar significantly increased soil organic C. Biochar application increased organic C more than did straw application. Soil CO₂ emission increased after cotton straw application. Biochar had no significant effect on soil CO₂ emission when the optimum amount of N fertilizer（i.e., N300） was applied; however, biochar significantly increased soil CO₂ emission when N rates were excessive（i.e., N450）. Cotton straw and its biochar increased inorganic N concentrations in topsoil and reduced ammonia volatilization. The effect of biochar was greater than that of straw. The straw and biochar both increased cotton biomass, N uptake, yield, and N fertilizer use efficiency. However, biochar application needs to be coordinated with rational fertilization practices. In N-fertilized plots, the positive effect of biochar was greater than that of cotton straw.