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

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 (P-o) 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.

Automated summary

This study quantified the different soil phosphorus (P) pools in organic soils under controlled drainage and pump drainage systems. The results showed that calcium bound P was the largest P pool in these soils, acting as a sink, while aluminum and iron bound P were driving forces for P movement in the soil. Fertilizer and root P content were significantly related to changes in drainage water quality.