还田秸秆碳驱动的土壤氮素转化及其微生物机制

Controlling the availability of soil nitrogen is critical for agricultural production, which needs to understand soil nitrogen transformation mechanism. Straw incorporation is one major practice to maintain or increase the stock of soil carbon, which has been applied widely in China. The soil nitrogen transformation under the condition of straw incorporation is impacted by straw-derived carbon transformation, but its relative mechanism is not clear. This project is designed to study the long term/short term effect of straw incorporation on soil nitrogen transformation by using filed and simulation experiments to analyze the content of different types of soil organic matter and nitrogen, the abundances, diversities, communities and activities of ammonia-oxidizing and denitrifying microorganisms. In order to reveal the effect of straw derived carbon flow on soil nitrogen fixation and denitrification and its relative microbial mechanism, stable isotope probing and modern molecular biology technique will be used to analyze straw derived carbon flow, the abundance and composition of functioning microbial community of soil nitrogen transformation. The results of the project will help us to better understand the soil fertility change mechanism and will provide effective guidance for controlling the availability of soil nitrogen, under the condition of straw incorporation.

把握土壤氮素转化机制进而控制土壤氮素可利用水平是农业生产的关键问题。秸秆还田是维持或增加土地利用过程中碳储量的主要方式，已在我国广泛应用。秸秆还田条件下的土壤氮素转化受秸秆碳转化影响，但影响机制仍较模糊。本项目拟以秸秆还田和不还田土壤为研究对象，采用大田实验、模拟土柱实验和碳化硅管实验三种研究尺度，结合稳定同位素示踪技术和现代分子生物学技术，对土壤中秸秆碳的转化分配规律、不同形态土壤有机质及氮素含量、固氮和反硝化微生物数量、多样性、组成和活性及主要影响因子进行研究，探明秸秆碳的转化分配特性；阐明还田秸秆碳转化对土壤氮素转化的长期和短期影响及关键因子；解析秸秆碳驱动的土壤氮转化微生物数量与组成；揭示还田秸秆碳驱动的土壤氮素转化及其微生物机制。本项目的结果对准确、全面揭示秸秆还田下的土壤肥力变化机制具有重要的科学意义；可为控制和应对秸秆还田后土壤可利用氮水平的下降提供科学依据和数据支撑。

秸秆还田后，秸秆碳在前期迅速进入土壤各有机碳库，分配比例依次为MBC、POC、DOC；后期则为POC、MBC、DOC；5年秸秆还田仅对0~60 cm土壤碳氮组分及氮素转化微生物群落产生显著影响。秸秆碳转化过程中可显著增加土壤反硝化微生物和固氮细菌的活性与丰度，同时增加其种群的多样性，改变其种群结构。秸秆碳直接驱动的反硝化细菌和固氮细菌比例分别占59.4%~65.5%和15.8%~28.9%，而由土壤老有机碳驱动的反硝化细菌和固氮细菌比例约34.55~40.6%和71.1%和84.2%。直接参与秸秆碳同化的反硝化细菌和真菌包括Rhodococcus、Pseudomonas、Sphingomonas、Burkholderia-Paraburkholderia和Paraphaeosphaeria；固氮细菌包括Methylobacterium、Bradyrhizobium、Camelimonas、Alsobacter。秸秆还田下土壤反硝化细菌群落活性、数量和结构组成主要受土壤POC和MOC的显著影响；固氮细菌除了受土壤有机碳组分，主要是SOC和POC显著影响外，还与土壤总氮和硝态氮含量密切相关。综上结果表明，秸秆碳直接驱动的反硝化细菌和真菌对土壤反硝化贡献更大，是造成旱地土壤硝态氮损失的关键菌属；而土壤固氮细菌则更多受土壤老有机碳和秸秆还田引起的土壤氮素变化的驱动，在缓解秸秆还田引起的有效氮损失方面扮演重要角色。本研究结果为今后秸秆还田条件下旱季作物的氮素管理提供了关键菌属信息和数据理论支撑。