旱作类型及秸秆还田对水旱轮作土壤碳氮耦合循环及氮素利用的影响机制

Carbon and nitrogen cycle coupling in farmland is considered to be the important theoretical basis and possible solution to increase soil C sequestration and N immobilization, mitigate greenhouse gases emissions and improve nutrient use efficiency, and is becoming more and more attention. Upland crop-rice rotation is the important cropping system in Southern rice production region in China. There are various types of upland crops to form different rotation patterns with rice, however, the influential mechanism of upland-crop types on soil carbon and nitrogen cycle is still unclear. Therefore, three annual upland-paddy rotation patterns are to be studied in this project through continuous field experiment, isolated micro-zone in situ experiment, soil incubation, and by using 13C and 15N labeling technique, high-throughput sequencing technologies, etc. The aims are to determine the differences and dynamics in quantity, quality, their transformation of root-derived and straw-derived C and N inputs from different upland crops, and to trace the contribution of crop-derived N to the N utilization by the late season crops. Changes in soil C and N components, soil microorganism functional structure and activity under different upland crop-rice rotation after several years field experiment will be made clear, and the linkages of these changes to potentials in soil nitrogen immobilization mineralization will be further studied. Combining the above two aspects of research, influential mechanisms of soil carbon and nitrogen coupling cycle of nitrogen will be clarified on the stability of soil organic matter and N utilization by crops. Preliminary work on soil carbon and nitrogen coupling degree indexes also will be proposed and verified. The results we obtained can acquire more understanding on biological driving mechanism in soil C-N cycle coupling, and provide theoretical and technical supports for soil fertility promotion, reducing nitrogen fertilizer application and the cropping system optimization in rice production regions.

农田碳氮耦合循环被认为是土壤增碳保氮、温室气体减排、提高养分利用效率的重要基础与可能的解决途径，而日益受到关注。水旱轮作是我国南方稻区重要的种植制度，旱作类型多样，然而旱作类型的变化对土壤碳氮循环的影响机制尚不清楚。为此，本项目拟选择油-稻、麦-稻、玉-稻等年内水旱轮作模式为研究对象，通过大田定位试验、田间微区试验、土壤矿化培育试验，利用13C、15N双标记技术、高通量测序技术等，明确不同旱作类型根源和秸秆碳氮输入的差异、转化动态及对后季作物氮素吸收的贡献；探明多年定位试验后不同水旱轮作模式土壤碳氮库组分、土壤微生物结构与活性的变化特征，及其与土壤保氮、供氮潜力的关系；整合以上两方面的研究，弄清土壤碳氮耦合循环的变化对氮素利用及土壤有机质稳定性的影响机制；初步提出土壤碳氮耦合度指标并进行筛选验证，为深化认识土壤碳氮耦合循环的生物驱动机制、土壤肥力培育、减氮减排、及种植制度调整等提供依据。

我国南方稻区水旱轮作模式多样，然而旱作类型的变化及其秸秆还田对土壤碳氮循环的影响机制尚不清楚。为此，本项目设置了油-稻（RR）、麦-稻(WR)、玉-稻(MR)等3种水旱轮作模式及秸秆还田与不还田处理，通过4年大田定位试验，取得了以下结果：（1）不同旱作物根际C、N与秸秆C、N输入量差异明显，玉米秸秆碳与根际沉积C均高于油菜、玉米；油菜根际沉积N高于小麦与玉米，但玉米秸秆N输入远高于油菜与小麦。（2）土壤C库组成受种植模式与秸秆还田影响显著，秸秆还田显著提升了各模式活性C库比例; 秸秆还田显著提高了MR模式缓效C库比例，而对RR与WR模式影响不显著；秸秆还田显著降低了MR模式慢性C库的比例，却显著提高了RR与WR模式慢性C库的比例。秸秆还田显著改变了土壤TOC的官能团组成特征，芳香度显著降低，但烷基C/烷氧C比值显著提高。（3）土壤N库组成受种植模式与秸秆还田影响显著，土壤DON及DON/DN比例表现为MR>RR>WR，而土壤MBN及MBN/TN比例表现为RR>WR>MR; 秸秆还田显著提高了各N库指标。（4）种植模式对土壤微生物群落组成与多样性影响不显著，但秸秆还田显著改变了土壤微生物群落组成，提高了多样性指数。（5）不同种植模式C、N释放差异明显，MR模式C排放显著低于RR模式与WR模式，但其N2O排放量却显著高于RR模式，RR模式又显著高于WR模式；秸秆还田显著提高C释放量，但显著降低了N排放量。（6）水旱轮作C、N循环具典型的耦合特点，土壤TOC与TN的提升具有协同性，土壤微生物熵与氮肥偏生产力（PFPN）呈显著正相关，土壤DON/DN比值与土壤碳库稳定性指数（CSI）显著相关，当DON/DN约为60%时CSI较高；土壤MBC/MBN与TOC及PFPN均呈显著的二次曲线关系，当MBC/MBN为7-8时，TOC与PFPN均较高，该指标可作为水旱轮作区土壤管理的重要参考指标。（6）综合上述研究结果，建议MR模式可进行周年及连续多年秸秆还田，可减少N肥施用量20%；WR模式可进行秸秆还田，N肥施用不减少或适宜略减；RR模式不建议多年连续秸秆还田，且不宜在旱作物季减少N肥施用量。