我国典型农田土壤不同Olsen-P水平下磷有效性的差异及化学机制

Phosphorus (P) is a limiting nutrient for crop production. Soil P extracted with Olsen method (Olsen-P) is widely used to represent soil available P. Research from my group have shown that the relationship between crop yield and soil Olsen-P follows the "S"-type curve, indicating that the availability of ΔOlsen-P is different in soils with different Olsen-P levels. However, the difference of P availability in soils with different Olsen-P levels and corresponding mechanisms are still not clear yet. This project, based on the long-term field experiments of three typical soil types in China (Black soil, Fluvo-aquite soil, and Loess soil), collects soil samples with great difference in Olsen-P levels but with similar other soil properties, and carry out the pot trial with wheat. Relationship between Olsen-P and crop yield is investigated. Based on the relationship, difference in soil P availability with different Olsen-P levels and three thresholds of Olsen-P for the 5%, 50%, and 95% of maximum crop yield (P5, P50, and P95) are quantified. Then, the mechanisms leading to the difference in soil P availability are investigated from the perspective of inorganic P forms and P adsorptive capacity at P5, P50, and P95. P adsorptive capacity are characterized by indexes, such as adsorption strength coefficient and desorption ratio, by using both the traditional adsorption-desorption test and the sequential extractions using NaHCO3 solution. The results would rich the knowledge on the interaction between soil and P and the surface chemical mechanism of P availability, and thus provide theoretical basis for activating soil P and increasing P use efficiency.

磷是植物生长发育必需的大量营养元素；Olsen-P作为土壤有效磷的指标而被广泛应用。申请者课题组通过我国农田长期试验发现，作物产量对Olsen-P的响应不是直线是类“S”型曲线，暗示着不同Olsen-P水平下土壤磷的有效性不同，但其有效性的差异及机制尚不清楚。针对这一科学问题，本项目以我国3种典型农田土壤（黑土、潮土和塿土）长期试验为基础，采集土壤性状相似仅Olsen-P 差异较大的土壤，结合盆栽试验揭示小麦产量对Olsen-P的响应关系，确定最高相对产量5%，50%和95%的Olsen-P临界值（阈值）及其磷有效性差异；利用磷形态分级方法明确不同Olsen-P阈值下土壤无机磷的形态变化，采用吸附-解吸试验及连续浸提方法探明土壤对磷的吸持能力，从土壤化学方面揭示不同Olsen-P水平下土壤磷有效性的差异机制。研究结果将丰富磷有效性的土壤表面化学机制，为土壤磷活化和高效利用提供科学依据。

长期磷肥施用使农田土壤Olsen-P显著提高，了解合理施肥条件下磷肥利用率土壤Olsen-P的定量关系及土壤磷形态、吸附-解吸特征对于指导磷肥合理施用、提高磷肥利用率和降低土壤磷的淋失风险至关重要。本项目以农田土壤肥料长期定位试验为基础，选择东北黑土、华北潮土和西北塿土3种典型土壤，结合盆栽/微区试验，确定我国小麦、玉米产量对Olsen-P的响应关系确定最高相对产量5%，50%和95%的Olsen-P临界值（阈值）及其磷有效性差异，从土壤磷形态和土壤磷吸持能力两方面揭示不同Olsen-P水平下土壤磷有效性的差异及机制。结果表明：（1）华北潮土Olsen-P对冬小麦和夏玉米相对产量的P5、P50和P95阈值分别为0.3、3.6、15.4 mg kg-1和0.1、4.8、7.8 mg kg-1；西北塿土Olsen-P对冬小麦和夏玉米相对产量的P5、P50和P95阈值分别为0.1、1.3、5.5 mg kg-1和0.2、2.1、9.1 mg kg-1；东北黑土玉米对Olsen-P响应不显著。（2）文献数据整合分析结果表明在Olsen-P>20 mg kg-1的土壤上施用>90 kgP2O5 hm-2的磷肥对小麦产量无提高，在其他情况下磷肥施用对小麦产量的提高均具为正影响。（3）潮土中无机磷组分含量的大小顺序基本为：Ca10–P > Ca8-P > O-P > Fe–P ~ Al–P > Ca2-P。Ca-P 是无机磷的主要组分，约占其73%–81%。Ca2-P和Ca8-P的含量及占比均随着土壤Olsen-P的增加而增加，每增加1 mg kg-1 的Olsen-P，Ca8-P增加4.3 mg kg-1。除O-P和 Ca10–P，其他组分与Olsen-P呈显著的线性正相关（P < 0.01）。（4）不同Olsen-P水平土壤对磷的吸附均符合Langmuir方程，吸附能常数（k）和最大缓冲能力（MBC）与初始土壤Olsen-P呈显著线性负相关，但最大吸附量（Qm）与初始土壤Olsen-P关系不明显。经过16次连续提取不同Olsen-P水平土壤提取的磷均达到了平衡，平衡值随Olsen-P水平的提高而提高；累积释磷量和提取次数符合密切里西方程，拟合的最大释磷量（释磷容量）为12~368 mg kg-1。释磷容量与初始Olsen-P呈显著的直线正相关关系（P < 0.01）。