Effects Of Biochar On Soil Ecological Function Under Dryland Farming

In the background of food security threats and climate warming,the demand of improving soil quality and carbon sequestration is increasing,it is urgent to take effective measures to improve soil fertility and carbon sequestration capacity of dryland soil.Biochar has become a research hotspot in agriculture,energy and environment because of its broad prospect of industrialization.However,whether biochar can significantly improve the quality of dryland soil and reduce soil carbon emission remain unclear.In this paper,we taken the typical dry farmland as research object,adopted the methods of soil column and incubation experiments,pot and true field experiments,to systematically reveal the environmental effects of apple branch biochar on the dryland farming ecosystem.Our specific objectives are(1)screening a biochar application method to improve the hydraulic characteristics of dryland soil and mitigate inorganic nitrogen leaching;(2)studying the effects of biochar on winter wheat growth under different water and nitrogen conditions;(3)exploring the mechanisms of interactions of biochar and nitrogen fertilizer on soil nitrogen migration and soil carbon emissions.The main results are as follows:(1)A higher amount(4%;w/w)of biochar was added to the plough layer soil(0-20 cm),or an appropriate amount(2%)of biochar was added to the subsoil(10-20 cm),could effectively improve the water infiltration capacity of the silty clay soil.Biochar application method and amount significantly influenced soil nitrogen leaching and water conductivity,and there is a dynamic equilibrium relationship between nitrogen leaching and soil water conductivity.High dose of biochar(4%)improved soil water conductivity but significantly increased nitrogen leaching.Only the method of adding 2%biochar to the subsoil(10-20 cm)could improve soil water conductivity and mitigate nitrogen leaching.Therefore,adding 2%of biochar to the subsoil is recommended to improve the soil hydraulic characteristics and reduce the risk of nitrogen leaching.(2)The effect of biochar on winter wheat yield was relied on the amount of biochar,nitrogen fertilizer level and water content.Compared with the application of nitrogen fertilizer alone,the combined application of 1%and 2%carbon and nitrogen fertilizer increased the wheat yield by 7.40%-12.00%,but drought stress inhibited the synergistic effect of biochar and nitrogen fertilizer.Additionally,combined high dose of biochar(4%-6%)with nitrogen fertilizer resulted in a significant reduction in yield(6.25%-21.83%).Under specific water and nitrogen conditions,1%and 2%biochar treatments had higher content of NO3-and available phosphorus,which is helpful to increase yield.However,a higher amount of biochar(>4%)may absorb a large amount of NO3-,form a higher C/N ratio,which led to the reduction of wheat yield directly.(3)In a vegetation-free soil system,soil organic carbon and microbial biomass carbon increased with the addition of biochar.However,biochar amendment at rates of 2%and 4%primed the C-mineralization rate,whereas 1%biochar amendment decreased the C-mineralization rate,regardless of the N level.In addition,biochar increased the activity of alkaline phosphatase and the microbial biomass P content,regardless of the soil N level.The enzymes associated with soil carbon and nitrogen cycling are depended on biochar addition rate and soil nitrogen level.(4)In the dryland wheat ecosystem,compared with the application of N fertilizer alone,the application of biochar at 20 t ha-1 combined with N fertilizer not only increased wheat biomass by 12.2%-13.8%but also significantly decreased residual NO3-in the subsoil by 13.2%-74.7%.Nevertheless,long-term N fertilization at 240 kg N ha-1 led to large amounts of residual NO3-without a significant increase in crop biomass,which inevitably increased the risk of leaching during the fallow period.Field-aged biochar absorbed a certain amount of NO3-,thereby sequestering N in the soil after two years of N fertilization.The application of biochar at 20 t ha-1 combined with N fertilization at 120 kg N ha-1 is a promising dual-win strategy for improving N availability while concurrently mitigating inorganic nitrogen leaching.(5)Biochar application alone or combined with N fertilizer persistently increased SOC and improved several soil parameters over three years.However,soil respiration significantly decreased in response to biochar combined with 120 kg ha-1 N fertilizer,whereas the opposite response was observed when biochar was combined with 240 kg ha-1 N fertilizer.Biochar(20 t ha-1)combined with 120 kg ha-1 N fertilizer enriched some species that contributed to higher microbial C use efficiency and lower SOC mineralization.Nevertheless,the application of biochar or N fertilizer at high rate strongly disturbed the soil microbial ecosystem,thus decreases the C sequestration potential of biochar.Ultimately,Biochar(20 t ha-1)combined with 120 kg ha-1 N fertilizer is recommended to simultaneously maintain long-term C sequestration and to improve soil properties of dryland soils.