不同轮作中油菜季土壤氮素矿化对水分和有机质组分的响应机制

Both soil water and soil organic matter fraction are important factors influencing soil N mineralization. More studies on a single factor’s effect on soil N mineralization have been done. Actually the effects of soil water on soil N mineralization are closely related to the quality and quantity of soil organic matter. Moreover, under field conditions soil water and soil organic matter are interwined joint affected soil N mineralization, especially under paddy-upland rotation. The influences and its mechanisms of soil N mineralization effected by the interaction of soil water content and soil organic matter fraction are still unclear. Considering the differences of soil N supply in oilseed rape field between rice-oilseed rape and cotton-oilseed rape rotations which are closely related to soil water content and soil organic matter fraction in these two rotation, the characteristics of soil N transformation rate are estimated from rice or cotton harvest to oilseed rape harvest with 15N pool dilution technique under field condition; besides the abundance of ammonia oxidizing bacteria and ammonia-oxidizing archaea are analyzed with next generation sequencing technique; and the relationships among soil N transformation, soil microbial commnuity, soil water content and soil labile organic matter fraction are analysized. Meanwhile, according to the differences of soil water contents and soil organic matter fractions between these two different rotations, the analysis the effects of soil labile organic matter fractions on soil ammonia oxidizing bacteria and ammonia-oxidizing archaea abundance and soil N mineralization with different soil water content are paid. Overall it will be benefical to undersand the relationships among soil microbes, soil water and soil organic matter and the mechanisms of soil N mineralization. Further it will provide the theoretical basis for N management of high yield and high fertilizer use efficiency in different winter oilseed rape rotations.

水分和有机质组分是影响土壤氮素矿化的重要因素，关于它们单个因素对土壤氮素矿化的影响积累了大量工作，然而水分对土壤氮素矿化的影响往往和土壤有机质含量和性质密切相关，并且田间条件下它们通常也是交织在一起的，尤其是水旱轮作中，但它们相互作用对土壤氮素矿化的影响及机制并不明晰。本项目以稻-油和棉-油轮作中油菜季土壤水分和有机质组分差异为切入点，采用15N稀释法和高通量测序技术，通过田间原位监测水稻/棉花收获开始到油菜收获这一阶段土壤氮素矿化特点、土壤氨氧化菌群相对丰度的变化特征，并分析其与土壤水分和有机质组分的关系；同时结合两种轮作模式下土壤水分和有机质组分的变化特点，采用室内模拟，研究不同水分条件下活跃有机质组分在数量和性质上的差异对土壤氮素转化、氨氧化菌群活性和结构的影响，探讨水分-有机质-微生物间的相互作用及其对土壤氮素矿化的影响机制，进而为不同轮作模式下油菜高产高效的氮素调控提供理论依据。

水稻-油菜和棉花-油菜是我国非常重要的耕作制度，前期调研发现稻油轮作中油菜产量低于棉油轮作，其土壤氮素供应低于棉油轮作。因此本研究利用稻-油和棉-油轮作同田对比定位试验，研究了两种轮作油菜季土壤氮素矿化特征，从前季作物氮素残留和当季土壤氮素转化剖析了影响土壤氮素供应的主要因素，并从土壤有机质组分和水分相互作用、有机氮形态和功能微生物三个角度剖析了两种轮作下油菜季土壤氮素供应差异的机制。主要结果表明，两种轮作下油菜季各生育阶段土壤氮素净矿化特征差异明显，油菜生长前期棉油轮作土壤氮素净矿化量（21.4 kg N/hm2）显著高于稻油轮作（15.7 kg N/hm2），而花期后则表现相反趋势。15N稀释结果进一步验证了该结果，苗期稻油轮作土壤总矿化速率和总硝化速率明显低于棉油轮作。苗期稻油轮作中氨氧化古菌的拷贝数明显低于旱地轮作，氮肥施用显著提高了氨氧化细菌的拷贝数，但对氨氧化古菌无明显影响。此外，棉油轮作中棉花季较高的氮素残留（43.4%）提高了油菜季土壤氮素供应能力。经过三年种植后，稻油轮作土壤活性有机质组分含量明显高于棉油轮作，去除活性有机质组分后，两种轮作土壤氮素矿化潜势分别降低了29.2%和36.6%，土壤微生物氮呈现相同的变化趋势。土壤活性有机质组分含量和土壤水分存在明显交互作用，模拟落干处理土壤的总固定速率明显高于持续淹水和干旱处理，高活性有机质含量土壤表现出高的总矿化速率和总固定速率。从土壤有机氮形态来看，两种轮作土壤酸解态氮含量无明显差异，但酸解态氮各组分的变化却不相同，稻油轮作中酸解铵态氮增加的比例 （33.8%）低于棉油轮作（53.9%），但其酸解未知态氮含量增加的比例（36.0%）高于棉油轮作（16.6%）。综上所述，水旱轮作有利于土壤有机质积累，但由于其活性有机质组分生物有效性低，导致油菜生长前期土壤氮素供应能力较低，不利于油菜前期生长，因此稻油轮作油菜生产中应适当增加前期氮肥投入，“前促后稳”的氮肥管理策略对于实现油菜高产高效具有重要生产意义。