水旱轮作下红壤性水稻土中磷与铁的形态转化及其耦合机制

Phosphorus (P) is one of the essential macronutrients for the growth of plant. In soils, P can be easily fixed by soil matrix, and P bioavailability is a main factor that limits the growth of crop in many agricultural soils. Rice-upland crop rotation system is the major farming systems along Yangtze River basin and over southern regions in China. Strong changes in hydrothermal conditions and redox potential of soil affect the speciation, transport, bioavailability of P. Many reported studied focused on the speciation transformation of iron (Fe) and bioavailability of P in upland rotation system, but the soil chemical processes that affect Fe and P transformation are less involved and the corresponding microscopic mechanisms remain unclear. In this proposal, reddish paddy soil in rice-upland crop rotation system was chose as the object of study. The transformation of P speciation was illuminated based on traditional fractionation method combined with solution 31P nuclear magnetic resonance (NMR) spectroscopy and P K-edge X-ray absorption spectroscopy. The alteration of iron oxides’ speciation and structure were investigated by solution extraction and mössbauer spectroscopy. The aims are to 1) to reveal the main factors and mechanisms controlling the speciation transformation and bioavailability of P; and 2) to identify the role of iron in transformation, activation and fixation of P, and the coupling mechanism between P and Fe. The results of this project will provide the important scientific basis for in-depth understanding of soil chemical processes of P cycling in upland rotation system, rational application of P fertilizer and efficient nutrient regulation.

磷是植物生长发育必需的大量营养元素之一，易被土壤基质固定，其生物有效性常常是众多农田土壤中作物生长的主要限制因子。水旱轮作是我国长江流域和南方地区主要耕作制度，水热条件和氧化还原电位的强烈变化影响土壤磷的形态、迁移和生物有效性。已有较多关于水旱轮作体系下铁形态变化和磷有效性的研究，但水旱轮作体系中影响铁和磷形态转化的土壤化学过程却较少涉及，相应的微观机制仍不明确。本项目拟选取水旱轮作下典型红壤性水稻土为对象，在传统的磷分级提取基础上采用液态31P核磁共振波谱和磷K边X射线吸收光谱阐明磷化学形态动力学变化，用溶液提取和穆斯堡尔谱探究土壤铁氧化物的形态和结构的变化，揭示水旱轮作体系土壤磷形态转化和生物有效性的主控因素及机制，明确铁在磷素转化、活化与固定中作用及其耦合关系，为深入理解水旱轮作下磷素循环的土壤化学过程、磷肥的合理施用与养分高效管理提供依据。

水旱轮作是我国长江流域和南方地区主要耕作制度，水热条件和氧化还原电位的强烈变化影响土壤磷的形态、迁移和生物有效性。本研究选取湖南祁阳红壤土为对象，在实验室模拟水旱轮作条件进行土壤培养，以期探讨不同淹水-落干条件下土壤中磷和铁的形态转化机制。研究发现淹水使红壤土pH增加并趋于中性，使Eh降低。对高磷土壤pH和Eh影响较大的均是添加抗坏血酸淹水组（H-AA）＞自然淹水组（H-CK）＞通氮淹水组（H-N2）。淹水过程磷的有效性主要受铁铝氧化物结合磷形态转化的影响。淹水在平衡期内会使土壤磷的有效性短暂升高，在之后的持续淹水中，磷的有效性逐渐降低。淹水在平衡期先短暂增加了Al-P和Ca-P，在淹水的过程中Al-P和Ca-P向Fe-P和O-P转化。在落干的过程中Al-P降低，O-P增加。抗坏血酸能促进土壤无机磷向稳定态转化。薄膜梯度扩散技术（DGT）的结果表明，随着淹水时间的延长，DGT提取的有效磷浓度增加，土壤淹水过程中pH升高可能是促进土壤供磷能力增加的重要原因。形态分析表明淹水后土壤中Fe(Ⅱ)铁和无定型铁显著增加，结晶铁含量降低。淹水增加了土壤铁的活化度，落干则降低。腐殖酸对淹水土壤pH和Eh的影响比生物炭大，对土壤铁的活化作用也强于生物炭。Hedley分级结果表明：有机物的添加在平衡期内显著增加了土壤磷的有效性，但是淹水过程使磷的有效性逐渐降低，其中腐殖酸对土壤磷的活化作用强于生物炭。