稻田与旱地土壤固碳效应差异形成机理：微生物周转及残留视角

As the two critical and contrasting roles of microorganisms, their catabolism and anabolism control the soil organic carbon turnover. Based on our the previous researches and the phenomenon that organic carbon content in paddy soil is generally higher than that in adjacent upland soil, we hypothesize that the microbial biomass turnover rate in paddy is slower than in upland, whereas microbial carbon use efficiency (CUE) and microbial residue accumulation have an opposite trend. This could be one of the key reasons for the higher carbon sequestration efficiency in paddy than in upland in subtropical hilly region of red soil. In the present project, 30 paired typical paddy and upland soils will be used to test the above hypotheses based on a combination of 13C, 18O isotope tracing incubation experiment and microbial biomarker identification technology. We will first establish the H218O-based method to determine microbial turnover time in paddy and upland soils, and thus improve the 13/14C-substrate-based method of microbial gross turnover time. Then, we aim to quantify the turnover time of microbial biomass and its cellular components (amino sugars and phospholipid fatty acids) in paddy and upland soils, and to clarify the distribution of carbon sources by microorganisms and the accumulation of stable cellular components residues (amino sugars). This project will reveal the microbial mechanisms of organic carbon sequestration discrepancy between paddy and upland soils from the perspectives on microbial turnover, microbial CUE and microbial residue accumulation. The results will enrich and improve the theoretical understanding of the carbon cycle in agricultural soil, and provide a scientific basis of sustainable management for agricultural ecosystems.

微生物的分解代谢与合成代谢功能是土壤有机碳周转的重要控制阀。基于前期研究基础及稻田比旱地土壤有机碳含量高的实际，假设稻田比旱地土壤微生物周转速率慢、碳源利用效率及微生物残留量高是亚热带红壤丘陵区稻田较旱地土壤有长期固碳效应的重要原因之一。项目拟以30对典型稻田和旱地土壤为研究对象，采用室内培养试验结合13C、18O双标记和微生物标识物鉴别技术，首先建立基于18O-H2O的稻田和旱地土壤微生物周转时间测定方法，修正依赖13/14C底物的微生物表观周转时间测定的不确定性，量化稻田与旱地土壤微生物及其细胞组分（氨基糖和磷脂脂肪酸）的周转速率与差异，揭示土壤微生物碳源分配特征及其稳定细胞组分（氨基糖等）的残留规律，从微生物周转、碳源利用效率和微生物残留效应角度，系统阐明红壤丘陵区稻田和旱地土壤碳固持差异的微生物机理。研究结果将丰富和完善农田土壤碳循环理论，为农田生态系统的可持续性管理提供科学依据。

本项目以我国东部中温带、暖温带、亚热带和热带气候区40对典型稻田和旱地土壤为研究对象，采用18O和13C示踪、微生物标识物分析技术，从微生物的碳源利用效率、生物量碳周转、活性微生物种群结构及其残留物贡献角度，系统阐明了稻田与旱地土壤有机碳积累差异的微生物机制。主要结果如下：（1）四个气候区农田土壤微生物碳源利用效率范围为0.10-0.54，且在稻田土壤中显著高于旱地土壤，表明稻田土壤有机碳通过微生物矿化作用的损失量低于旱地土壤。（2）四个气候区农田土壤微生物生物量碳周转时间为27-280天，在北方地区（中温带和暖温带）稻田与旱地土壤之间无显著差异，而在南方地区（亚热带和热带）稻田土壤中的周转时间显著长于旱地土壤，暗示了南方地区稻田土壤有机碳周转速率低于旱地土壤。（3）除暖温带外，稻田土壤总PLFAs和细菌PLFAs含量均显著高于旱地土壤，表明淹水的稻田土壤抑制了微生物对外源碳的利用，降低对其利用速率，尤其是真菌。（4）四个气候区农田土壤微生物残留物对有机碳积累的贡献为21%~73%，且稻田土壤显著低于旱地土壤同时，稻田与旱地土壤细菌残留物及微生物总残留物对有机碳的贡献在南方地区均显著高于北方地区。稻田土壤13C-氨基葡萄糖含量及其周转率在四个气候区中均显著低于旱地土壤，而稻田土壤13C-胞壁酸含量在南方地区显著高于旱地土壤。南方地区农田土壤有机碳积累过程中具有相对较高的微生物参与度；而稻田土壤淹水环境降低了微生物在有机碳积累过程中的参与度，抑制了真菌利用外源碳合成细胞壁残体。总体上，项目明确了稻田比旱地土壤有机碳积累强的机制：稻田土壤微生物的碳源利用效率高，呼吸速率低，降低了稻田土壤有机碳矿化量；南方地区稻田土壤微生物生物量碳周转慢，使其有机碳周转速率低，促进了有机碳积累；稻田土壤有机碳积累过程中微生物较低的参与度，降低了其残留物对有机碳积累的贡献，尤其是真菌残留物。