Effects of phosphorus addition on one-dimensional vertical infiltration characteristics of soil water

Phosphorus (P) is one of the macroelements necessary for plant growth, second only to nitrogen (N) in importance. Most studies have shown that phosphorus is easily adsorbed by soil particles or forms insoluble precipitates with some substances in the soil (Fe, Al, and Ca), leading to the less biological availability of phosphorus. A large amount of phosphorus fertilizer is also applied to the soil for the needs of crops in practice. As such, the content of total phosphorus and Olsen-P in farmland soil has continued to rise in recent years in China. For instance, the average available phosphorus content in farmland soil was ranging from only 7.4 to 24.7 mg/kg from the 1980s to 2007. Excessive application of phosphorus fertilizer has posed a great threat to the physical and chemical properties of soil structure. This study aims to reveal the effect of phosphorus addition on the one-dimensional vertical infiltration characteristics of soil water. A series of soil infiltration experiments of adding phosphorus with water (CK, P1, P2, P3, P4) and cultivating soil infiltration experiments with phosphorus (ICK, IP1, IP2, IP3, IP4) were carried out. Five P gradients were set for each experiment, which were 0, 0.075, 0.15, 0.225, 0.3 g/kg. The results showed that: 1) Phosphorus application with water presented no significant effect on the soil infiltration during the infiltration process; 2) The water infiltration capacity was significantly enhanced after the soil was incubated with the phosphorus addition for 90 days. Compared with the control ICK, the cumulative infiltration amount of IP1, IP2, IP3, and IP4 increased by 7.82%, 8.85%, 9.82%, and 11.21%, respectively, whereas, the corresponding infiltration time decreased by 7.77%, 14.56%, 22.33%, and 27.18%, respectively. The cumulative infiltration amount and the migration speed of the wetting front increased significantly, with the increase of the phosphorus concentration. There was an excellent linear relationship between the infiltration parameters and phosphorus concentration in the fitted Kostiakov and the migration distance-time formula of the wetting front; 3) The IP3, and IP4 treatments presented a significantly larger amount of 0.25-2 mm particle size aggregates, compared with the control ICK without phosphorus. The amount increased by 35.9%, and 51.28%, respectively, indicating the addition of phosphorus significantly increased the proportion of large aggregates of 0.25-2 mm in the soil, thus changing the one-dimensional vertical infiltration characteristics of soil water. In summary, the duration of phosphorus addition in the soil was closely related to the change in soil structure. Phosphorus addition also increased the proportion of large aggregates of 0.25-2 mm in the soil, thereby enhancing the infiltration capacity of the soil. It is very necessary to consider the effect of phosphorus fertilizer on soil structure, especially in the case of excessive application of phosphorus fertilizer. Therefore, the phosphorus application can be combined with the soil infiltration capacity, in order to reveal the effect of phosphorus addition on the soil moisture and infiltration capacity. The finding can provide a theoretical basis for the rational and standardized phosphorus application.