西北旱农区长期保护性耕作影响土壤碳排放的功能微生物响应机制

Agriculture is the major source of greenhouse gas emission, the carbon pool and its dynamic change in farming system could influence CO2 emission and carbon cycle process directly in terrestrial ecosystem. Research showed that conservation tillage practices could improve the soil structure, reduce water and soil erosion and increase soil organic carbon (SOC) content. Microbial communities play important role in material cycle and energy transfer of soil; they participate in soil carbon sequestration, methane metabolism and carbon decomposition which contribute to the main part of soil carbon cycle process. However, the microbial responding mechanisms for long-term conservation tillage practices affecting soil carbon emission on loess plateau are not much clear. Therefore, based on the long-term conservation tillage practices experiment commenced in 2001, the quantitative PCR and high-throughput sequencing techniques are employed in this study, combined with results of soil carbon emission measurement and data accumulation of the experiment, this project is designed to 1) Quantify the effect of long-term conservation tillage practice on soil carbon emission; 2) Characterize the effect of long-term conservation tillage practices on soil microbial diversity (composition and abundance) involving carbon cycle; 3) Explore the coupling relation among soil functional gene abundance involving carbon cycle, soil carbon emission and carbon transformation. Aim to characterize the microbial responding mechanisms for long-term conservation tillage practices affecting soil carbon emission on loess plateau, thereby to provide scientific base for understanding the role of microorganisms in soil carbon cycle process and adoption of conservation tillage practice on loess plateau.

农田土壤是重要的温室气体排放源，其碳库储量与动态变化直接影响陆地生态系统中CO2的排放及碳循环过程。保护性耕作能有效改善土壤结构、减少水土流失和提高土壤有机碳含量。微生物是土壤中物质循环和能量传递的主要驱动者，直接参与碳固定、甲烷代谢和碳降解等碳循环过程。但在我国西北旱作农业区，针对长期保护性耕作影响土壤碳排放的功能微生物响应机制的研究还较为缺乏。为此，本研究拟依托2001年建立在该区的保护性耕作长期定位试验，运用高通量测序等技术手段，结合土壤碳排放测算及该定位试验的长期数据积累，研究:1)长期保护性耕作对土壤碳排放的影响； 2）碳循环功能微生物类群对长期保护性耕作的响应与反馈；3）碳循环功能基因丰度与土壤碳排放及碳素转化动态的耦合关系。通过对旱农区长期保护性耕作影响土壤碳排放的功能微生物响应机制的研究，为深入了解微生物在土壤碳循环过程的作用和该区保护性耕作的推广应用提供科学依据。

为了探索不同耕作与秸秆还田措施对黄土高原半干旱区土壤碳固存与碳排放的微生物响应机制，本研究依托始于2001年的保护性耕作长期定位试验，研究了不同耕作与秸秆还田措施对土壤碳固存和碳排放、微生物碳源代谢特征、土壤固碳功能细菌丰度、群落结构及共发生网络的影响，并分析了土壤碳循环功能微生物群落与土壤碳固存及碳排放的相互关系。试验共设4个处理，分别为免耕（NT）、免耕秸秆覆盖（NTS）传统耕作（T）及传统耕作秸秆还田（TS）。本研究主要结果如下：（1）保护性耕作措施（NT、NTS与TS）有效提高了土壤碳固存，并降低了土壤碳排放量。相比于T处理， NT、NTS与TS处理下的土壤有机碳含量分别提高3.2%，13.4%和6.1%。NT、NTS与TS处理下的土壤碳排放量较T处理降低13.1%、18.4%和3.5%。（2）秸秆还田（NTS与TS）处理能够有效提高土壤微生物碳源代谢多样性；NT与TS处理降低了土壤微生物胺类碳源代谢水平，同时提高了微生物羧酸类碳源代谢水平；胺类、酚酸类和多聚化合物类物质是引起不同处理下土壤微生物代谢功能特征变化的主要碳源类型。（3）NTS处理中的土壤cbbL基因丰度较T处理显著提高52%，NTS与TS处理下的土壤RubisCO酶活性较T处理分别提高95.2%和63.4%。保护性耕作（NT、NTS与TS）处理提高了放线菌门相对丰度但降低了变形菌门的相对丰度，土壤固碳功能细菌群落结构与土壤有机碳及土壤微生物量碳含量具有显著相关性。（4）土壤碳排放量与土壤微生物量碳、RubisCO酶活性及固碳微生物多样性呈显著负相关，而土壤碳排放效率与土壤有机碳含量，全氮含量，土壤微生物量碳，cbbL功能基因丰度、RubisCO酶活性及土壤固碳微生物多样性均呈显著正相关。综上所述，在黄土高原半干旱区，免耕、免耕秸秆覆盖及传统耕作秸秆还田等保护性耕作措施能有效提高土壤微生物碳源代谢活性，增加土壤固碳细菌丰度，并提高土壤碳固存功能细菌群落多样性，从而增加土壤碳氮固存，减少农田土壤碳排放量。