摘要
为准确评估亚热带稻田土壤的固碳特性及其有机碳的储量动态变化情况，进一步提升稻田土壤肥力，从亚热带稻田土壤碳库储量的时空分布与演变规律、稻田土壤有机碳的输入、微生物固碳的功能、有机碳矿化特性及其作用机制等方面进行了探讨。我国亚热带区稻田土壤有机碳储量与固碳效应明显高于旱地，其突出的碳固持能力主要是由于水稻光合碳的输入、土壤自养微生物固碳及淹水限制了微生物活性、抑制植物残体微生物分解过程，促进以植物残体直接积累。稻田土壤有机碳的矿化速率受碳氮磷计量、水肥管理措施以及温度等因素调控，从而影响着稻田土壤的固碳减排效率与潜力。本文提出“碳中和”背景下稻田土壤固碳减排的研究展望：基于亚热带稻田土壤对我国农田土壤碳固碳减排方面的重要性，提出了系统研究稻田生态系统中土壤有机碳累积和转化的作用机制，构建高精度的区域土壤有机碳库模拟模型及储量估算方法，以助力提升农田土壤肥力，加快我国农业双碳目标和农业绿色发展。
土壤碳库是陆地生态系统碳库中最大的储库，全球约1.5×1
我国水稻种植历史悠久，是世界上稻田面积最大的国家，总面积在3 000万h
近30年来，我国亚热带水稻土壤有机碳含量持续增加，显著高于旱地和林地等其他类型土壤，表明亚热带稻田生态系统具有重要固碳作
我国亚热带区稻田土壤有机碳储量与固碳效应明显高于旱地土壤，具备较强的固碳潜
图 1 水田和旱地土壤碳储量差异的影响因素概念框架
Fig.1 Mechanisms leading to greater C and N stocks in paddy soils compared with those in upland soil
土壤碳库中的有机碳库集中分布于土层1 m左右深度，而长期水稻种植条件下，1 m深土壤剖面中约有50%以上的有机碳储存在20 cm以下的底土层，极易受到人为干扰和环境变化的影
土壤有机碳储量受气候、水文和地理环境、土壤条件、植被与土地利用等要素的影
稻田土壤突出的碳固持能力主要是由于水稻光合碳的输入、土壤自养微生物固碳及淹水限制了微生物活性、抑制植物残体微生物分解过程，促进以植物残体直接积
图2 水稻秸秆、根系、根际沉积碳和微生物同化碳在不同碳库中的分
Fig.2 Distribution of different C substrates （shoot-C，root-C，rhizosphere-C，microbial-C） in different C pool
水稻光合碳是作物光合作用固定的大气二氧化碳经过凋落物、根系和根系分泌物等根际沉积形式进入土壤的碳，在农田碳循环中起着重要的作用，是水稻土壤高碳库的重要碳汇来源之
农田土壤中自养细菌分布广泛，能够通过多种生物固碳途径固定大气中的CO2，其具有巨大的固碳潜
土壤固碳微生物和固碳酶的活性受长期施肥的影响，从而显著影响稻田土的固碳速
土壤碳矿化是有机碳分解转化成CO2的过程，与土壤活性有机碳库关系密切，因此，土壤有机碳矿化释放CO2的数量与强度是评价环境因素对土壤有机碳分解转化的重要依
稻田土壤微生物生物量通常数倍于旱
图3 微生物生活策略对激发效应和土壤碳平衡的化学计量调节机制的概念
Fig.3 Adjustment of microbial life strategies to the amount and stoichiometry of labile resources，and outcomes in terms of soil C processing and net C balanc
厌氧条件下有机碳分解不彻底，会伴随有机酸、乙醇等中间产物的产生，有机碳矿化的最终产物包括CH4和CO2。厌氧条件下，水稻土中有机碳矿化和甲烷排放在低温下（<15 ℃）对温度变化更敏感。CO2排放的Q10值（温度敏感性指标）在5~15 ℃时比在25~35 ℃时高1.1~3.4倍；CH4排放的Q10值在5~15 ℃时比在25~35 ℃时高2.8~13.5
尽管过去几十年间针对稻田土壤碳循环的关键生物地球化学过程已进行了广泛研究，但相关过程的内在作用机制及生态环境效应认识仍存在许多不足。面对全球气候变暖、温度逐渐升高的巨大挑战以及国家粮食安全和生态环境建设的双重要求，作为世界第二大CO2排放国，进一步增强稻田土壤的碳汇功能，是推进水稻绿色低碳高质量发展的重要内涵，也是助力国家碳达峰碳中和的“双碳”战略的重要手段。现提出如下展望：
系统揭示稻田生态系统中土壤机碳累积和转化的作用机制，助力提升农田土壤肥力。针对亚热带稻作系统生产力提升、减投增效和农产品安全的国家需求，研究亚热带稻作系统水稻土有机碳积累、持续生产力和微生物过程的内在机制，突破水稻土肥力提升的科技瓶颈；借助生态化学计量学新工具，深入研究“水稻-土壤-微生物”系统的根际沉积碳周转与稳定过程，揭示驱动稻田生态系统中土壤有机碳持续累积的内在调控机制，为稻田土壤培肥和地力提升提供科学依据。
构建高精度的区域土壤有机碳库模拟模型及储量估算方法。由于时空不平衡、监测方法不一致等因素的制约，我国稻田碳汇功能的评估还存在较大不确定性。依据已有研究数据和成果支撑，结合野外调查与遥感同步反演，获取系统全面的土壤监测数据，通过分析土壤有机碳密度随深度变化的普遍性规律，剖析土壤剖面有机碳密度与表土碳密度和土壤深度的函数关系，构建基于表土碳密度估算土壤剖面有机碳库的模拟模型，优化土壤剖面碳库估算方法，从而提高全球或区域土壤有机碳储量的估算精度。同时，随着人工智能模拟技术越来越成熟，神经网络模拟方法需要逐步引入到生态学和表层地球科学等领域，将在区域土壤有机碳模拟与储量估算方面展现很好的前景。
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