少耕轮作对绿洲农田土壤有机碳累积的触发机制

Long term planting of a single species along with adoption of conventional tillage usually resulted in a relatively low balance of soil organic carbon (SOC) pool in arid oasis irrigated regions. Therefore, it necessitates the studies on how SOC pool responses to diversified cropping systems on a tempo-spatial scale in combination with reduced tillage and straw mulching on a farmland scale. This has important significance in further understanding the processes of SOC dynamics and scientifically evaluating carbon sequestration capacity for oasis irrigated farmland. In the present project, typical irrigated area of inland river basin in Hexi Corridor, Northwest China, was selected as target region, and staple crops such as spring wheat and spring maize alternated in an annual rotation cropping sequence was taken as research object, in order to elucidate the effects of reduced tillage and rotation systems on processes of SOC sequestration and emission in oasis irrigated region through a previously designed long-term experiment and the related laboratory measurements. Trigger mechanism of different types of SOC to reduced tillage and mulch management was thereby revealed through analyzing responses of SOC dynamics to crops litter decomposition, soil thermo-hydraulic characteristics, and soil dominant micro-flora structures in a soil profile and on an aggregate scale, in order to provide theoretical basis for evaluating carbon sequestration benefits in carbon pool management.

干旱绿洲灌区长期种植单一作物，并采用传统耕作措施，使农田土壤有机碳维持较低水平的相对平衡。因此，开展作物时间和空间尺度上多样化配置，并采用少免耕结合秸秆覆盖条件下农田土壤有机碳累积特征及其触发机制的研究，对于深入理解绿洲区农田土壤有机碳变化过程和科学解析土壤有机碳汇能力有重要意义。本项目选择西北内陆河灌区典型区域，以其区域内主栽作物小麦、玉米组成的时间和空间上的轮作模式为研究对象，通过基于前期布设的定位试验和相关室内测试等，阐明绿洲少耕轮作对土壤有机碳累积及排放过程的影响。通过分析剖面和团聚体尺度上不同类型土壤有机碳对作物凋落物降解、土壤水热特征、土壤微生物区系结构的响应，揭示农田土壤有机碳受耕作措施和覆盖管理的触发机制，为绿洲农田土壤有机碳过程管理和碳汇效益评价提供参考依据。

干旱绿洲灌区长期种植单一作物，并采用传统耕作措施，使农田土壤有机碳维持较低水平的相对平衡。开展作物配置、少免耕结合秸秆覆盖条件下农田土壤有机碳累积特征及其触发机制的研究，对于深入理解绿洲区农田土壤有机碳变化过程和科学解析土壤有机碳汇能力有重要意义。项目选择西北内陆河灌区典型区域内主栽作物小麦-玉米组成的轮作模式为研究对象，研究了农田土壤有机碳累积过程对作物配置、耕作措施和覆盖管理的响应；作物配置、耕作措施和覆盖管理对土壤有机碳累积的触发效应及其机制；分析了干旱绿洲条件下，农田土壤有机碳固持强度及速度。研究结果表明：经过两个轮作周期后，小麦秸秆还田显著提高了小麦-玉米轮作体系0-20cm土壤有机碳（SOC）含量，提高幅度在5.6%-11.2%之间。秸秆还田处理显著促进了大团聚体（粒径＞0.25mm）中SOC的积累的同时，也显著促进了0-10cm土层大团聚体（粒径＞0.25mm）中轻组有机碳（LFOC）和中颗粒有机碳（POC）的积累，较背景值最高增幅分别达到59.4%和63.7%。免耕秸秆还田及秸秆翻压均显著提高了小麦-玉米轮作体系0-20cm土壤微生物量碳含量，提高幅度在16.7%-36.6%之间。小麦不同耕作及覆盖措施条件下轮作玉米农田耕层剖面SOC、FLOC和POC与该土层微团聚体（粒径＜0.025mm）中SOC、FLOC和POC含量显著正相关。小麦、玉米农田土壤呼吸速率很大程度上决定于5cm处土壤温度。免耕显著提高了小麦-玉米轮作体系0-20cm土壤SOC储量，平均增加了0.61t hm-2。秸秆翻压覆膜处理小麦根茬碳投入量达到2.9 tC hm-2，免耕秸秆覆盖处理玉米根茬碳投入量达到4.3 tC hm-2，0-10cm土层土壤SOC含量与小麦根茬碳投入量存在显著的正相关关系。小麦秸秆还田显著提高了0-20cm土壤范围内大团聚体SOC、FLOC和POC对该土层SOC的贡献率，最高增幅分别高于背景值7.2、2.5和9.3个百分点。小麦不同耕作及覆盖措施下小麦季和玉米季均表现为土壤碳的“汇”，小麦免耕及免耕秸秆还田相对于传统翻耕表现出较强的碳汇效应。本研究结果充实了干旱绿洲灌区小麦玉米轮作体系土壤有机碳平衡及其机制对耕作及覆盖措施的响应理论。