长期施用不同磷肥对红壤微团聚体富磷特征及其分散/絮凝行为的影响机制

The formation and stability of soil aggregates in Red Soils are directly affected by the behavior of dispersion-flocculation of mircoaggregates. Long-term application of phosphate (P) fertilizers to red soils can regulate and control soil aggregation structure. The change of soil organic matter and biological active substances has been considered as the main machanism when those phenomenon occurs, but the influence of phosphates on red soil surface charge properties often is ignored. Iron-aluminum oxides are not only the main contributor to positive charges of soils, but also important binding agents of microaggregates. Those microaggregates contain abundant surface charge and have high phosphate adsorption capacity. The surface positive charge decreases with continuous phosphate adsorption; and then the negative surface charge increases as the positive surface charge is neutrlized. Subsequently, the soil particles will be dispersed due to the increasing net negative charge. Red soil is a typical variable charge soil, and the accumulation of phosphates in soils may result in soil microaggregate dispersion. Understanding this relationship may improve the prediction of P function in soil aggregate formation and stability. The objectives of this proposal are to study 1) the physical and chemical characteristics of different sizes of microaggregates under long-term P fertilizer application with five different rates; 2) the properties of surface charges, zero potential charge (ZPC), electrophoretic mobility (EM) and isoelectric point (IEP) on the surfaces of different sizes of microaggregates by electrochemical methods; 3) the change of organic or inorganic colliod materials as affected by phosphate supply levels of the red soil; 4) the microstructure of aggregates formed under different P fertility by using the synchrontron based X-ray micro-computed tomography (SRμ-CT); 5) the critical flocculation concentration (CFC) of phosphate for different sizes of microaggregates. The findings of this study will be beneficial in improving red soil quality, saving limited P resource and minimizing the impact of using P fertilizers on the environment.

施用磷肥对红壤团聚结构的调控作用，通常认为是土壤有机质与生物活性物质变化所导致的，却忽视了磷酸盐对土壤颗粒表面电化学特征的影响作用。由铁铝氧化物胶结形成的微团聚体表面可变电荷丰富、固磷能力强，磷酸盐的吸附固定可以改变其表面电荷的符号与数量，当其表面正电荷被中和、净负电荷数量大于零时，微团聚体分散的可能性增强。本项目将以旱地红壤长期施用不同磷肥的试验地为平台，结合室内实验，1) 明确长期施磷对微团聚体表面电荷特性及其富磷的特征影响；2)借助同步辐射微CT技术，阐明磷酸盐对微团聚体三维微结构特征的影响机制；3)确定土壤悬液磷酸盐临界凝聚浓度阈值，揭示磷酸盐对微团聚体分散-絮凝的作用机制；4)通过综合分析及模型建立，评价并揭示微团聚体分散-絮凝行为在红壤团聚结构稳定及磷素肥力长期定向培育中的作用与贡献。研究结果将深入理解磷肥长期施用的农艺及环境效应，为优化磷素资源、培育红壤磷素肥力提供科学依据。

以长期施肥的红壤旱地为材料，以大小粒级微团聚体为研究对象，分析了长期施肥下红壤旱地中各粒级微团聚体的组成及其物理化学特征，明确了大小粒级微团聚体对红壤磷素的富集与周转、形态转化与生物有效性、吸附与解吸、固持与释放潜能等方面的影响差异与诱因，探讨了长期施磷或磷酸盐吸附对红壤旱地微团聚体分散-凝聚行为的影响作用；基于对大小微团聚体磷饱和度(DPS)与土壤磷储存容量(SPSC)的深入研究，初步构建了红壤磷素高效管理与风险管控的理论模式，研究结果可以为红壤区土壤固存磷的生物高效-环境友好-生态可持续的利用提供理论依据。研究主要结果如下：1) 长期施肥条件下红壤旱地中0.25 -0.05 mm粒级微团聚体为优势粒径、组成比例相对稳定且不受施肥影响，其它粒级受不同施肥的影响差异显著。2)各粒级微团聚体中约有23.4% ~ 70.3%的磷以无机态存在，而有机磷约占16.6 % ~ 27.7 %，其余均为残留态磷; 长期施磷与配施稻秆与猪厩肥可以显著影响各形态磷在大小粒级微团聚体中的分布比例及其有效性，且可以显著增加0.25 -0.05 mm与0.01 - 0.005 mm粒级微团聚体中极有效磷含量；Pi-NaHCO3、Pi-NaOH I和Po-NaOH I是影响红壤旱地微团聚体Bray-P的主要磷组分，但有机磷仍难以直接被作物吸收利用。3) 0.01 - 0.005 mm与< 0.005 mm粒级微团聚体磷Xm、K、MBC和SPR均显著高于0.25 - 0.05 mm与0.05 - 0.01 mm粒级微团聚体；长期施磷或配施猪厩肥可以显著降低红壤旱地大小微团聚体磷的Xm、K、MBC和SPR值。红壤旱地中0.05 - 0.01 mm粒级微团聚体磷的解吸率最高，而< 0.005 mm粒级微团聚体磷的解吸率最低；f-Fe2O3和f-Al2O3是红壤旱地微团聚体磷素吸附能力的关键性因素。4) 随着磷酸盐添加浓度的增加，红壤微团聚体悬液出现明显的先分散后絮凝现象；磷酸盐吸附会显著降低与提高红壤旱地中0.01 - 0.005 mm与2 - 0.25 mm粒径微团聚体的稳定性。；5)按现有施磷量，长期试验地各小区(除厩肥处理外)仍可安全施磷14 ~ 51年；但配施猪厩肥的红壤应立即停止施肥或调整施肥量。6) 基于DPS-SPSC理论构建的土壤磷素管理模式仍需进一步校验并推广应用。