Natural organic matter control on silicate interactions with iron oxides and silicon phytoavailability

在过去的几年里，硅对植物营养的重要性，特别是对水稻的重要性，已经被越来越多的人认识到。硅的吸收提高了生物产量和对病原菌的抗逆性。硅是通过原生矿物和次生矿物的风化而释放出来的。同样在风化过程中形成的成壤铁氧化物是硅酸(H4SiO4)的重要吸附剂，因此，可能控制植物有效硅。虽然H4SiO4吸附铁氧化物的基本机制相对较好，但天然有机物(OM)的可能影响尽管在所有土壤中都是丰富的，但没有得到太多的关注。矿物表面与溶解的OM或适当的有机涂层的吸附竞争对氧化铁相保留H4SiO4的影响在很大程度上是未知的。因此，人们对OM对硅的植物利用率的影响知之甚少。特别是在氧化还原条件具有强烈反差的土壤中，如沉水水稻种植，关于铁氧化物、硅和有机质之间的相互作用的研究很少。因此，我们的项目将首次测试天然OM对铁氧化物保留硅的影响以及对硅的植物可利用性的影响。在不同环境条件下的批量实验的基础上，我们将首先借助表面灵敏的X射线光电子能谱(XPS)研究在OM存在下H4SiO4的吸附和解吸以及OM对表面硅形态(单体到聚合硅)的影响。在随后的微体和中体实验中，我们将研究在OM存在下硅与铁氧化物结合对水稻吸收硅的影响。为了测试植物是否以及在多大程度上可以利用铁氧化物结合的硅，特别考虑到OM的作用和主要的硅表面物种，将进行定义的硅-铁氧化物相的微观实验。在有天然土壤的盆栽中，我们将通过测定硅的释放/固定、硅表面物种的变化和植物吸收来阐明在不同氧化还原条件下的这些过程。这些实验结果将有助于我们更好地了解天然有机质对土壤中硅循环的影响。

Over the last years, the importance of silicon (Si) for plant nutrition, especially for rice, has been increasingly recognized. Uptake of Si enhances biomass yields and stress tolerance against pathogens. Silicon is released by weathering of primary and secondary minerals. Pedogenic Fe oxides, likewise formed during weathering, are important sorbents of silicic acid (H4SiO4), and thus, potentially control plant-available Si. While the basic mechanisms of H4SiO4 sorption to Fe oxides are relatively well understood, possible effects of natural organic matter (OM), despite being abundant in all soils, did not receive much attention. The effects of sorption competition with dissolved OM or due organic coatings on mineral surfaces on the retention of H4SiO4 by Fe oxide phases are largely unknown. Consequently, the influence of OM on the phytoavailability of Si is poorly understood. Particularly in soils with strongly contrasting redox conditions, as in submerged rice cropping, little research has been conducted regarding the interactions among Fe oxides, Si and OM. Our project will, therefore, test for the first time the effects of natural OM on the Si retention by Fe oxides and the consequences for the phytoavailability of Si. Based on batch experiments under different environmental conditions, we will first study the adsorption and desorption of H4SiO4 in presence of OM and the impact of OM on the surface Si speciation (monomeric to polymeric Si) by relying on surface-sensitive X-ray photoelectron spectroscopy (XPS). We will study the consequences of the Si binding to Fe oxides in presence of OM for the Si uptake by rice plants in subsequent micro- and mesocosm experiments. Microcosm experiments will be conducted with defined Si-Fe oxide phases in order to test whether and to which extent Fe oxide-bound Si can be utilized by plants, with special consideration to the role of OM and the prevailing Si surface species. In mesocosms with natural soil, we will elucidate these processes under variable redox conditions by determining the release / immobilization of Si, the changes in Si surface species, and the plant uptake. The results of these experiments will promote our understanding of the impact of natural OM on the Si cycling in soils.