潮土磷素累积流失风险及环境阈值

潮土是中国分布比较广、施肥强度大的典型耕作土壤，潮土中磷素累积与流失对区域水环境的污染风险不容忽视。该研究在潮土面积最大的河南省采集磷素水平不同的典型潮土作为供试土壤，采用人工模拟降雨及土柱模拟试验方法，通过测定土壤中Olsen-P和溶解态活性磷CaCl2-P含量以及径流或淋滤液中各形态磷浓度，研究了潮土中磷素随地表径流和下渗流失特征，并通过分段线性模型对潮土的磷素环境阈值进行拟合。结果表明：1）不同形态磷在潮土土壤剖面中均有一定程度的累积，土壤Olsen-P和CaCl2-P含量表现为高磷最大，中磷次之，低磷最小，而磷吸持指数值表现为低磷最大，中磷次之，高磷最小。从磷素的剖面分布来看，低磷和中磷水平潮土Olsen-P和CaCl2-P含量随着土壤深度的增加而降低，而高磷水平的潮土Olsen-P和CaCl2-P含量在20～40 cm土层含量最高。2）不同磷水平潮土径流中总磷（TP）、可溶性总磷（TDP）和颗粒磷（PP）浓度和流失量大小表现为高磷最高，中磷和低磷水平土壤次之，潮土径流流失以PP为主。3）低磷和中磷水平潮土淋滤液中的各形态磷浓度和流失量随着土层深度的增加而降低，而在高磷水平的潮土淋滤液中，20～40 cm土层淋滤液中磷浓度和流失量要显著高于其他土层，在整个土壤剖面磷素浓度随着土层深度的增加呈现先上升后下降的趋势，潮土淋滤流失以TDP为主，其中，高磷和低磷水平潮土以可溶性有机磷占主导，而中磷水平潮土以钼酸盐反应磷（MRP）占主导。4）通过分段回归模型将不同含磷水平潮土的水溶性磷与土壤中Olsen-P含量进行拟合，得出潮土土壤磷素环境阈值为24.65 mg/kg，研究还表明径流和渗漏液中TP浓度与土壤CaCl2-P含量呈显著正相关，因此可通过测定CaCl2-P来预测并判断土壤磷素流失风险。

Accumulative loss risk of phosphorus and its environmental threshold in fluvo-aquic soil

Abstract: Fluvo-aquic soil is a typical cultivated soil with the widely distribution and high intensity of fertilization in China. However, the accumulation and loss of phosphorus in fluvo-aquic soil cannot be ignored as the ecological changes. In this study, the typical fluvo-aquic soils were collected as the test soils with different phosphorus levels (high, medium, and low) in Henan province, China, in order to explore the characteristics of phosphorus loss from fluvo-aquic soil induced by surface runoff and leaching. Simulation experiments of an artificial rainfall and a soil column were conducted to measure the content of Olsen-P and soluble active phosphorus (CaCl2-P) in soils, and the concentrations of different forms of phosphorus in runoff or leaching. In a split-line model, the relationship between soil Olsen-P content and TP in solution was used to evaluate P risk from black soil in surface runoff and leaching. The results showed that: 1) The contents of Olsen-P and CaCl2-P in fluvo-aquic soil profiles were in the order of high, medium and low phosphorus level, whereas, the PSI value were in the order of low, medium and high phosphorus level. In the profile distribution of phosphorus, the content of Olsen-P and CaCl2-P in fluvo-aquic soil with low and medium phosphorus level decreased with increasing soil profile, while the content of Olsen-P and CaCl2-P in fluvo-aquic soil with high phosphorus level was the highest in the 20-40 cm soil layer. 2) The concentration and losses of Total Phosphorus (TP), Total Dissolved Phosphorus (TDP) and particulate phosphorus (PP) in the runoff from fluvo-aquic soils with different phosphorus levels were in the order of high > medium > low phosphorus level, and the PP dominated in fluvo-aquic soil runoff. 3) The concentration and loss of various forms of phosphorus in the leachate of low- and medium-phosphorus level from fluvo-aquic soil decreased with the increasing soil depth. However, in the leachate of high phosphorus level, the concentration and loss of phosphorus in the 20~40 cm soil profile were significantly higher than those of in other soil profiles, where the phosphorus concentration in the whole profile increased first and then decreased with the increasing soil profile, whereas, the profile of high phosphorus level presented the distribution pattern that upper and lower was low, and middle was high. The leaching loss of fluvo-aquic soil was dominated by TDP. The high- and low-phosphorus fluvo-aquic soil was dominated by Dissolved Organic Phosphorus (DOP), while the medium-phosphorus fluvo-aquic soil was dominated by Molybdate Reactive Phosphorus (MRP). 4) The relationship between CaCl2-P and Olsen-P were characterized using split-line models, identifying the thresholds at 24.65 Olsen-P mg/kg for the fluvo-aquic soil. It was also pointed out that the TP concentration in runoff and leachate was positively correlated with the CaCl2-P content in soil, and that CaCl2-P can be used to predict the phosphorus concentration of water discharged from soil. The outcomes derived from these experimental conditions regarding the determination of P loss in runoff by the artificially simulated rainfall experiment. The P loss in leaching by the soil column method and P environmental thresholds also need further verification in the field. This finding can help to assess phosphorus loss from fluvo-aquic soil under different phosphorus levels, and further to provide a scientific basis for water environment protection and soil phosphorus management in fluvo-aquic soil areas.