水稻土氧化铁生物转化及其固碳机制研究

Paddy soils are the largest anthropogenic wetlands on earth, and it is urgent for China to increase crop production of the middle and low-yield paddy fields. However, the mechanisms underlying the accumulation of soil organic carbon in paddy soils in terms of microbial-mediated transformation of iron (hydro)oxides and management-induced turnover of soil aggregates are still unclear. Our preliminary results showed that the formation of short-range-ordered iron (hydro)oxides (SRO-Fe) enhanced the carbon sequestration potential of our studied paddy soil. This project aims to further explore the effects of transformation of iron (hydro)oxides mediated by the iron redox functional microorganisms on the preservation of soil organic carbon in paddy soils. Soil samples to 0-20 cm depth from 4 different rice growth stages will be collected from a typical calcareous paddy soil in Jinci city, Shanxi Province. Water-dispersible soil colloids and water-stable soil aggregates will be separated, respectively. Shifts in iron redox bacterial communities and microbial-mediated SRO-Fe transformation, as well as wetting and drying-induced soil aggregate turnover will be explored by using selective mineral extraction methods and laboratory incubation experiments in combination with advanced techniques including X-ray photoelectron spectroscopy (XPS)，synchrotron-based X-ray absorption fine structure (XAFS) spectroscopy, high-resolution transmission electron microscopy (HRTEM) and high-throughput sequencing. The results not only can promote our understanding of the mechanisms underlying the carbon cycling driven by coupling of SRO-Fe biotransformation and soil aggregate turnover, but will also provide theoretical basis for rational fertilization in paddy soils.

水稻土是地球最大的人为湿地。然而，微生物介导的氧化铁转化和水耕管理措施导致的团聚体周转对水稻土有机碳累积的影响及潜在机制还不清楚。本项目基于我国中低产稻田亟待增产的现实，针对前期研究发现的短程有序氧化铁(SRO-Fe)提升水稻土固碳潜力的现象进行深入剖析，以铁氧化还原功能微生物驱动的氧化铁转化为切入点，探索水稻土有机碳的累积机制。本项目拟在山西晋祠开展田间试验，采集水稻4个生育期表层(0-20 cm)土壤样品，筛分水分散胶体和团聚体，并利用矿物选择性提取法和培养试验，结合X射线光电子能谱(XPS)、同步辐射X射线吸收精细结构谱(XAFS)、高分辨率透射电镜(HRTEM)及高通量测序等技术，研究水稻不同生育期铁氧化还原功能微生物驱动的SRO-Fe转化和干湿交替导致的团聚体周转，旨在阐明微生物介导的SRO-Fe转化耦合团聚体周转在水稻土碳循环中的调控机制，为水稻土合理施肥提供理论依据。

水稻土是地球最大的水耕人为土，水耕管理措施导致的土壤结构和矿物活性的变化对稻田土壤有机质转化累积的影响尚不清楚。本项目以典型稻麦轮作水稻土长期定位试验为依托，以团聚体周转与氧化铁转化的耦合过程为切入点，利用裂区试验方差分析探究施肥处理和作物季节对无定形氧化铁、有机碳含量与稳定性及微生物代谢熵的影响；利用高清透射电镜、Al27 Si29核磁共振、X-射线光电子能谱技术，探究水稻土胶体中纳米矿物的形态，结构和配位状态；利用选择性提取法量化铁结合态有机碳的含量，利用改进Fe(Ⅱ)-二甲酚橙试验和对苯二甲酸补获试验量化过氧化氢和羟基自由基含量，基于作物产量量化外源碳氮投入量，利用荧光法测定碳转化酶的活性，利用培养试验分析有机碳的比呼吸速率，探究不同施肥处理对有机碳固存的影响及潜在驱动机制。研究表明：施肥处理和作物季节均对无定形氧化铁，有机碳含量与稳定性及微生物代谢熵有显著的影响，水稻季的无定形铁与有机碳含量显著高于小麦季；水稻土的复硅作用，铝硅酸盐的风化作用和氧化铁的还原溶解和氧化再生作用有利于植硅体、水铁矿和水铝英石等纳米矿物的形成，提升了稻田土壤固碳潜力；稻麦轮作系统本身有利于氧化铁的返老还童和有机碳的净累积，猪粪堆肥能以较高的固碳效率提高土壤的固碳速率，加速水稻土有机碳的固存，外源碳投入，微生物合成代谢和铁介导的自由基反应共同调控着水稻土外源有机碳的固存效率；水稻季的酚氧化酶活性和铁结合态有机碳的含量显著低于小麦季，无定形铁与铁结合态有机碳呈正相关，而与酚氧化酶活性呈负相关。水稻季的“酶栓”机制和小麦季的“铁门”机制耦合团聚体的重团聚过程赋予稻麦轮作系统周年有机质分解受阻；猪粪堆肥不但提高了氧化铁的返老还童，而且降低了酚氧化酶的比活性，进而提升了外源有机碳的固存效率，可用来定向培育中低产水稻土。