Linking soil phosphorus pools to drainage water quality in intensively cropped organic soils

Abstract

Higher than average fertilization rates, as applied to mineral soils, are often recommended for cultivated organic soils (>20% organic matter), which over time, have led to phosphorus (P) pollution into receiving water bodies via subsurface tile drainage. Limited studies have documented the P pools within organic soils or their link to tile drainage water quality. This study quantified the different soil P pools found in organic soils under two water management practices: controlled drainage (CD) and pump drainage (PD) systems. Phosphorus availability in these soils was assessed by measuring inorganic P (Pi) and organic P (Po) via sequential fractionation; available P via Bray-1 P testing; and microbial P, along with crop root total P uptake. Drainage water samples were also analyzed for total P (TP) and dissolved reactive P (DRP). The results identified calcium (Ca) bound P as the largest P pool in these organic soils. Its correlation with other P pools suggests that it acts as a P sink in these soils. The correlation analysis further suggests that aluminum (Al) and iron (Fe) bound P is a driving force for P movement in the soil, as it had the most significant relationship with the P parameters in both the soil and drainage water. The regression analysis of TP found that the fertilizer and root P content were significantly related to the changes in drainage water quality at both sites. A P balance indicated that more fertilizer was being applied each year than was being used by the crops or released into the water, causing an accumulation of legacy P in the soil. Overall, the soil P pools affect the fluctuations of P concentration in the tile drainage water quality, featuring the complex relationship of the soil-water P dynamic.