The Effects of Soil Water Content on Distribution of Mobile Phosphorus Concentration in Soil

Abstract

【Background and objective】 Concentration of mobile phosphorus (P) in soil affects not only plant growth but also water quality because of its potential leaching. Most studies on soil P focused on topsoil while overlooking subsoil which is equally important. The objective of this paper is to study the dynamics of mobile P at different soil depths and its driving forces in attempts to help improve P uptake by plants while in the meantime reducing its leaching. 【Method】 Column experiments filled with soil taken from Sanjiang plain in China were conducted with average volumetric soil water content controlled at 50% (M1), 75% (M2) and 100% (M3) respectively. In each treatment, we measured the changes in mobile phosphorus in the depths of 0~10 cm (topsoil) 10~20 cm (middle soil) and 20~30 cm (subsoil). We also analyzed its relationship with total organic carbon (TOC), total Fe, total Al and total Ca in soil solution; the experiments lasted 84 days. 【Result】 When soil water content was 50%, soil solution was not extractable from the topsoil and the middle soil, and only was limited water extracted from the subsoil. When soil water content exceeded 75%, soil solution was more mobile for extraction in which the P concentration in the middle soil was found to be the highest, 1.3~1.7 times that in the topsoil and 5.2~8.2 times that in the subsoil. The impact of soil water on P concentration varied with soil depth and was ranked in the following order: In the topsoil, M3 (1.27 mg/L) > M2 (0.82 mg/L), in the middle soil M3 (1.60 mg/L) > M2 (1.40 mg/L), and in the subsoil M3 (0.31 mg/L) > M2 (0.17 mg/L) >M1 (0.13 mg/L). A stepwise regression analysis indicated that P concentration in the soil solution was related to TOC, total Fe, total Al, and total Ca in soil solution, with the correlations varying with soil water and soil depth. These suggested that improving soil water management could shift dissolution and/or decomposition of P in different forms in the soil, and can thus bed used to manage soil P. Regardless of soil water content, P concentration in the soil was much higher than the critical P concentration, 0.1 mg/L, which could trigger eutrophication in surface water. 【Conclusion】 Increasing soil water could result in a quick release of P from soil to the overlying surface water, triggering eutrophication as a result. Care thus must be taken when temporally returning dry farmland to wetland.

Keywords

• water management
• soil solution
• phosphorus
• sanjiang plain
• returning farmland to wetland