生物质炭与秸秆对农田深层土壤碳氮动态的影响：比较研究

The impacts of biochar amendment on carbon and nitrogen processes of deep soils are still unclear. Given that biochar is produced at the cost of consuming large amounts of the bioavailable crop straws, it is important to compare between the different roles of biochar and its feedstocks in processes of the whole soil profiles. It is hypothesized in this project that when compared to the direct straw incorporation to soil, pyrolyzing straws to biochar as a soil amendment will greatly reduce the inputs of labile carbon to the topsoil and by altering the topsoil carbon decomposition processes, carbon allocation to deep soil layers may be decreased. In addition, straw pyrolysis may decrease nitrogen immobilization by microorganisms, thereby enhancing nitrogen leaching and denitrification in deep soils. Consequently, deep soil carbon storage may be relatively lower when cropland straws are pyrolyzed, which will partly counteract the CO2 capturing effects of straw pyrolysis for the whole soil profile. Alternatively, we expect that keeping part of cropland straws and pyrolyzing the remaining part, i.e. to combine the amendments of straw and straw-derived biochar to soil, is likely a balanced measure to utilize the functions of both biochar and crop straws in soil. To test these assumptions, three treatments are set, including the soil incorporation of all straws produced in a cropland, incorporation of biochar derived by pyrolyzing all the straws, and the incorporation of part straws plus biochar derived from the remaining straws. In the field, soil solution and gas collection systems will be buried at different depths of a cropland, and dynamics of dissolvable carbon/nitrogen and greenhouse gases production in soil profiles will be monitored. Furthermore, materials labeled with enriched carbon and nitrogen isotopes will be added to soil columns, to investigate the contributions of various sources of carbon (straws/biochar, root exudates, and soil organic matter) and nitrogen (chemical fertilizers, straws/biochar, and soil organic matter) to carbon/nitrogen flows at different soil depths, which can provide insights into the mechanisms of how different forms of soil carbon inputs on soil surfaces influence deep soil carbon/nitrogen processes. Implement of this project can progress our scientific understanding of the agro-ecological functions of biochar.

生物质炭施加对土壤深层碳氮过程的影响尚不清楚。因生物质炭的生产以大量消耗生物可利用的秸秆为代价，应重视比较生物质炭与其来源秸秆在土壤全剖面过程中的不同角色。本项目假设：相比秸秆还田，秸秆炭化将大幅削减表土活性有机碳输入，通过影响表土有机碳分解而减少向深层的碳分配，降低微生物氮固持、增加氮素淋溶及深层反硝化，最终相对减少深层碳储存，弱化炭化对全剖面的净固碳效应；秸秆部分还田、部分炭化，或可兼顾秸秆与生物质炭的不同功能。为验证假设，设置秸秆全还田、全炭化、部分还田+部分炭化等处理，在农田不同深度埋设土壤溶液/气体采集系统，监测剖面可溶性碳氮及温室气体动态；此外，构建土柱并添加碳氮同位素标记，分析不同来源碳（秸秆/生物质炭、根系分泌、土壤有机质）、氮（化肥、秸秆/生物质炭、土壤有机质）对剖面碳氮过程的贡献，探讨土壤碳输入形式影响深层碳氮过程的机制。本项目将推进对生物质炭农业生态功能的科学认知。

生物质炭以其高度的难分解性，对生态系统固碳减排和“碳中和”的作用正日益受到重视。在农业生态系统中，生物质炭的生产以大量消耗生物可利用的秸秆为代价，应重视比较生物质炭与其来源秸秆在土壤剖面（尤其是土壤深层）碳氮过程中的不同角色。为此，本项目设置秸秆全还田、全炭化、部分还田+部分炭化等处理，监测了剖面可溶性碳氮及温室气体动态，并通过构建土柱及室内培养实验，研究了秸秆、生物质炭等对土壤剖面有机/无机碳的激发效应及其机理，比较了秸秆、生物质炭添加影响下土壤不同碳组分分解、微生物活性，并探讨了不同有机物影响下表层碳氮淋溶对深层碳动态的影响。研究结果表明：相比秸秆施加，生物质炭可明显降低土壤剖面温室气体(CO2、N2O、CH4)排放；秸秆添加向土壤中引入大量活性有机物，大幅提升了土壤微生物量及酶活性，为N2O的产生提供了充足的底物，并能促进土壤碳酸盐（无机碳）向CO2的转化，但其对深层土壤原有机碳分解的激发效应不明显；秸秆等外源活性碳添加量与激发效应密切相关，当添加量低于50%~100%微生物量时，主要引发正激发效应，更高的添加量则抑制激发效应或造成负激发效应，这可能是由于微生物类群间对微生物残体的二次利用降低了土壤有机质分解；无机来源的CO2占剖面总CO2排放的20%~50%，其释放与温度、氮肥施加正相关，且与微生物活性正相关，秸秆等活性碳可由此激发无机源CO2的产生；将秸秆炭化为生物质炭，可明显减少表层有机质向可溶态的解离及其向深层的运移，降低对深层碳的激发效应，但却因微生物氮固持作用的减弱而增加氮肥的淋溶，进一步可能促进深层碳酸盐向CO2的转化。综上，秸秆施加有利于降低表层向深层的N淋溶，减少深层无机碳的损失，但对剖面碳封存的净效应不明显；生物质炭对剖面上碳氮相关的生物过程几无直接影响，但却可大幅提升净碳封存量，有利于农业固碳。未来，将农业秸秆部分炭化、部分直接施入土壤，可能是兼顾秸秆、生物质炭二者生态功能的秸秆管理最佳方式。