Phosphorus fate, transport and management on subsurface drained agricultural organic soils: a review

Large quantities of mineral phosphorus (P) fertilizer are often applied to intensively cultivated organic soils. Although erosion and runoff can contribute to loss of P, the large amount of fertilizer applied causes a rapid build-up of this nutrient, resulting in the downward movement of excess P in the soil profile and subsequent loss through tile drainage water. For arable organic soils, these losses often occur through subsurface tile drains, a common requirement to maintain a favorable air-water balance in the crop root zone, as well as to prevent soil subsidence. As such, subsurface drainage is a major pathway for agricultural P loss, contributing to persistent eutrophication of rivers, lakes, and estuaries globally. Although studies have been conducted on P mitigation within organic soils, application of drainage water management (DWM) as a P mitigation strategy in these soils, has not been extensively studied. The objective of this paper is to address this gap in knowledge by reviewing previous studies on P losses from subsurface drained agricultural organic soils while evaluating potential mitigation strategies. Specifically, this paper assesses the unique properties of organic soils that could influence P fate and transport, such as the distribution of P pools within the soil pools; variable pore geometry, hydrophobicity, and shrinkage; P loads exiting tile drains; and DWM practices in mitigating P losses. It is concluded that P retention is affected by the dynamic nature of soil water movement in organic soils and that substantial P loads enter surrounding water bodies via subsurface drainage effluent. There is evidence that DWM is an effective best management practice in the abatement of subsurface P losses.