Oxidative Degradation Characteristics Of Different Kinds Of Biochar And Their Effects On Mineralization Of Soil Organic Carbon

Biochar has high carbon content and exhibits high thermal chemical stability.It can be applied to soil to slow down the carbon cycle in terrestrial ecosystem.Therefore,the study on the environmental effects of biochar,especially its soil carbon sequestration,has been paid more and more attention in recent years.The carbon stability,surface morphology and pore structure of biochar are key factors influencing its environmental function.The stability of different biochars are quite different due to the factors such as the type of raw materials and the pyrolysis process.However,the biotic and abiotic degradation characteristics of different biochar in the soil are not very clear.In addition,the impact of biochar on soil organic carbon mineralization is still controversial.Both positive and negative priming effects were reported in exist research.Whether biochar addition will ultimately increase soil carbon storage or promote soil carbon emissions needs to be further studied.It is of great significance to realize the scientific utilization of biochar on soil carbon sequestration and mitigation.In this study,three kinds of typical agroforestry wastes in Zhejiang province,namely,rice husk biochar(RH),pecan shell biochar(PS)and bamboo biochar(BB),were used as materials for producing biochar.Basic physical and chemical properties and stability of the biochars were compared and analyzed.The biotic and abiotic degradation characteristics of these biochars in soil under submerged and simulated dry land conditions were studied and their half-life were estimated to reveal the oxidative degradation process of biochar in soil.In addition,the indoor mineralization experiments were also carried out.These three kinds of biochar were added to the soil at different ratios to explore their priming effect on soil organic carbon mineralization through the analysis of soil respiration strength.The results will provide some scientific basis for biochar on farmland soil carbon sequestration and mitigation.The main conclusions are as following:(1)The stable carbon contents of rice husk biochar,pecan shell biochar,and bamboo biochar were 87.1%,75.8%and 74.2%,respectively.The order of biochar stability was as following:rice husk biochar>pecan shells biochar>bamboo biochar,which was consistent with the order of stable carbon content.The results of electron scanning electron microscopy(SEM)showed that the order of the surface roughness of three kinds of biochar was:rice husk biochar>bamboo biochar>pecan shells biochar,which was consistent with the order of surface area with the values of 56.0,12.7 and 1.03 m2/g,respectively.(2)Under the conditions of waterlogging and simulated dry soil moisture,the total carbon content of biochar in biochar and soil mixing treatment decreased within 180 days.Rapid degradation was observed in the early stage of culture(the first 15 days),and the degradation rate became very slow after 30 days.The order of total carbon degradation rate was as following:pecan shell biochar(14.4-18.7%)>bamboo biochar(13.8-13.9%)>rice husk biochar(7.2-7.3%).Under the two moisture conditions,the total carbon contents of the three kinds of biochar were not obviously different after 180 days,indicating that the abiotic oxidative degradation has a little effect on biochar.Long-term degradation study was needed.Under the condition of waterlogging and simulated dry soil moisture,the trend of stable carbon content after mixing with biochar and soil for 180 days was similar to that of total carbon content.After clay pipes filled with biochar placed in the soil environment,the stable carbon content also showed a slow decreasing trend in descending order:rice husk biochar(36.4-40.2%)>pecan shell biochar(29.5-38.0%)>bamboo biochar(5.2-11.8%),and the degradation under dry condition was stronger than that in waterlogging condition.The result of half-life of calculated from our experiment showed that the survival time of rice husk biochar in the soil was the longest and the retention time of pecan shell biochar was the shortest.(3)The cumulative mineralized amounts of soil organic carbon(SOC)were decreased by 8.0-10.7%with 5%rice husk biochar,pecan shells biochar,and bamboo biochar treatments after 120 d of incubation.Biochar had a suppressing influence on the native soil organic carbon decomposition,and the effect increased with increasing ratio of biochar.The negative priming effect of biochar on the mineralization of soil organic carbon was mainly observed after 15 d of incubation,and increased with the incubation time.The priming effect depended on the type and amount of biochar applied.The negative effects of 5%bamboo biochar(with priming rate of 88.8%)and 5%rice husk biochar(85.4%)were significantly higher than that of 5%pecan shells biochar(72.3%).Thus,bamboo biochar and rice husk biochar had a greater potential for long term carbon sequestration in soils than pecan shells biochar.