低分子量水溶性有机物对红壤菜地磷有效性的调控作用机制

Low phosphorus (P) availability in red soils of vegetable land is mainly resulted from strong fixation of P by iron and aluminum oxides, which has become a constraint for vegetable growth and development under long-term use of chemical fertilizers without organic manures and continuous cropping of vegetables with high intensity. Application of exogenous low-molecular-weight dissolved organic matter (LMW-DOM) including organic acids, simple sugars, and amino acids may play contrasting roles in regulating the potential release of P from red soils of vegetable land, but relevant mechanisms involved in the improvement of soil P availability have mainly been investigated in terms of abiotic processes such as ligand-enhanced dissolution and competitive adsorption between the rhizosphere and non-rhizosphere soils. In the present study, different types of LMW-DOM compounds are used to regulate P transformation and bioavailability in red soils of vegetable land. On the basis of systematic and in-depth study of relevant abiotic mechanism, we further focus on microbially-mediated mechanism (abundance of P-transforming functional microorganisms, turnover of microbial biomass P, and dissimilatory iron reductive dissolution) and biological-abiotic coupling mechanism. These three mechanisms may be involved in controlling the changes of soil physical, chemical and biological processes and thus affecting P availability in red soils of vegetable land. Two experiments are designed: 1) A laboratory incubation experiment on the gamma-ray-sterilized and non-sterilized red soils is conducted with different levels of P application and water content, in order to examine the effects of different LMW-DOM compounds on P transformation in red soils and futher to reveal the abiotic and microbially-mediated mechanisms; and 2) A pot rhizosphere bag experiment on the planted and non-planted red soils is conducted with fixed levels of P application and water content, in order to examine the effects of representative LMW-DOM compounds on P bioavalability in planted red soils and further to reveal the biological-abiotic coupling mechanism. The results will provide reference data for developing practical regulatory measures of improving P availability in red soils of vegetable land.

红壤菜地铁铝氧化物对磷的固定引起磷有效性降低并日益成为蔬菜生长发育的限制因子。外源低分子量水溶性有机物（LMW-DOM）可提高土壤磷的有效性，而前期文献对其作用的研究主要集中在根际与非根际土壤的络合增溶、竞争吸附等非生物机制。本申请以红壤菜地磷形态转化及其生物有效性的研究为核心，以LMW-DOM为调控手段，在系统深入研究非生物机制基础上，进一步重点研究磷素转化功能微生物、微生物生物量磷周转、异化铁还原溶解等微生物机制，以及生物-非生物耦合机制。首先采用土壤培养实验，在γ射线灭菌与非灭菌条件下研究不同种类LMW-DOM对不同施磷量、含水量红壤菜地磷形态转化的影响，分别揭示非生物机制和微生物机制；其次采用盆栽根袋试验，在不种植与种植蔬菜条件下研究代表性LMW-DOM对特定施磷量、含水量红壤菜地磷生物有效性的影响，揭示生物-非生物耦合机制。预期成果可为提出可行的土壤磷有效性调控手段提供理论依据。

红壤菜地铁铝氧化物对磷素固定引起磷素有效性降低并日益成为蔬菜生长发育的限制因子。外源低分子量水溶性有机物(LMW-DOM)可提高土壤磷素有效性，而前人研究主要集中在竞争吸附、络合增溶等非生物机制方面。本项目以红壤磷素形态转化及其有效性研究为核心，以LMW-DOM为调控手段，进一步重点研究磷素转化功能微生物、异化铁还原溶解等微生物机制。结果表明，灭菌/非灭菌条件下红壤磷素有效性因LMW-DOM种类、水分条件不同而存在差异，即灭菌条件下，好气环境更有利于提高红壤磷素有效性，可能是由于参与红壤磷素转化的酶活性在好气条件下更高，而非灭菌条件下，LMW-DOM的一级解离常数或还原度与淹水红壤速效磷之间均呈正相关关系，表明这些性质参数值越大，对LMW-DOM通过非生物机制或/和微生物机制来提高磷素有效性就越有利；LMW-DOM施入红壤后，柠檬酸钠和谷氨酸钠可将碳酸氢钠提取态有机磷矿化后向易利用的树脂交换态磷转化，部分抵消了无定形铁吸附对磷素释放的负效应，而葡萄糖则将其向难利用的氢氧化钠提取态磷转化；添加LMW-DOM后引起红壤水溶性有机碳、pH、Eh等性质差异且与多种形态磷组分相关性良好，同时Fe(III)还原和磷活化的增加使特定时间点Fe(II)累积量与多种形态磷组分含量相对应，表明这些影响磷素吸附-解吸、铁氧化-还原反应的关键化学性质共同决定了红壤磷素形态转化；对细菌16S rRNA基因、真菌rRNA基因、phoD基因和Geobacteraceae 16S rRNA基因丰度的分析以及基于细菌群落的非度量多维尺度分析显示，尽管LMW-DOM种类和水分条件存在差异，Rhizobium、Mesorhizobium、Paenibacillus、Bosea、Stenotrophomonas、Enterobacter、Achromobacter、Alkaliphilus、Pseudomonas、Bacillus、Streptomyces是各处理分解有机磷、参与再分配的优势菌属，它们在不同体系仍然相似。总而言之，LMW-DOM施入红壤后影响淹水/好气/干湿交替条件下红壤磷素形态转化，特别是影响淹水条件下Fe(Ⅲ)还原溶解引起的磷素形态转化。因此，本研究对识别LMW-DOM调控红壤磷素有效性的非生物与微生物协同响应，拓展对铁还原耦合磷素循环以及磷素转化功能微生物作用机理的认识具有重要意义。