Phytolith solubility in paddy soil

水稻和小麦等主要作物从土壤溶液中积极吸附大量溶解的硅（Si），这些硅（Si）沉淀在植物中，形成无定形的二氧化硅体，即“植物岩”。硅营养对植物的抗逆性起着至关重要的作用。秸秆循环利用对水稻土壤硅循环和生物有效性的影响取决于土壤中植物岩的溶解度。实验室研究表明，从植物凋落物中提取的新鲜植物岩溶出率很高。然而，这些数据似乎与表土和考古遗址中有时大量储存的植物岩相矛盾。拟建项目计划系统研究土壤老化过程中决定植物岩溶解度的因素。据推测，随着时间的推移，植物岩溶解度会随着(i)活性表面积的减少和（ii）植物岩表面无机和有机涂层的形成（表面“钝化”）而降低。我们进一步假设富铁涂层的形成是由水稻土壤中氧化还原条件的周期性变化加强的；因此，氧化还原循环应加速植物岩表面的钝化。利用扫描电镜（SEM）和x射线光电子能谱（XPS）等多种分析方法，研究了不同条件下稻秆中含有植物岩的矿物袋在植物岩老化过程中表面的化学和形态变化。将在实验室实验中研究从矿物袋中提取的植物岩的溶解度，并将测试表面化学成分参数（例如铁和铝浓度）与植物岩溶解度之间的关系。该项目将展示是否/如何通过使用XPS等现代技术对活性硅酸盐矿物表面的化学变化进行精确的纳米级表征，从而提高对土壤中硅通量的了解。它还将通过清除或回收收获残留物，加强对人类对土壤中硅有效性的影响的理解。

Major crop species such as rice and wheat actively adsorb large quantites of dissolved silicon (Si) from soil solution, which precipitates in the plants forming amorphous silicon dioxide bodies, the `phytoliths`. Silicon nutrition plays a crucial role in stress resistance of plants. The impact of straw recycling on Si cycling and bioavailability in rice soils depends on solubility of phytoliths in soil. Laboratory studies showed high dissolution rates for fresh phytoliths extracted from plant litter. These data, however, seem to contradict the sometimes large storage of phytoliths in topsoil and in archaeological sites. The proposed project plans to systematically study factors determing solubility of phytoliths during ageing in soil. It is hypothesized that phytolith solubility decreases over time due to (i) decreases in active surface area and (ii) formation of inorganic and organic coatings on the phytoliths surfaces (surface `passivation`). We furthermore assume that formation of iron-rich coatings is intensified by periodic changes in redox conditions in rice soils; the redox cycles should thus accelerate the passivation of phytolith surfaces. Mineral bags containing phytoliths extracted from rice straw will be buried in soil under different conditions, and the chemical and morphological changes at phytolith surfaces during phytolith ageing will be followed using a variety of analytical methods, including scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The solubility of the phytoliths retrieved from the mineral bags will be studied in laboratory experiments, and relations between parameters on chemical composition of the surfaces (e.g., Fe- and Al-concentrations) and phytolith solubility will be tested. The project will demonstrate whether/how an improved understanding of Si fluxes in soils can be gained by precise nanometer-scale characterization of chemical alterations at surfaces of reactive silicate minerals using modern techniques such as XPS. It will also enhance understanding on human impact on Si availability in soil by removing or recycling of harvest residues.