Nitrogen-Enhanced Mineralization of Soil Organic Phosphorus in a Midwestern Floodplain Soil

Abstract:

In the highly agricultural Midwestern United States, the sequestration of phosphorus (P) in floodplain soils through microbial immobilization is critical to reducing P losses to downstream water bodies. This process, however, may be coupled with nitrogen (N) availability. As an essential component of enzyme production, higher N availability may boost the microbial production and resulting activity of phosphatase, the enzyme that mediates the mineralization of organic P into bioavailable orthophosphate. Increased phosphatase activity could subsequently reduce net P immobilization. The objective of this study was to evaluate whether inorganic N inputs enhance or inhibit P immobilization in an Illinois floodplain soil. Accordingly, the soil was amended with nitrate (NO₃-N) and ammonium (NH₄-N), and P dynamics were monitored during a 40-day incubation. The addition of NH₄-N significantly boosted acid and alkaline phosphatase activity relative to the control. Likewise, labile P increased in the NH₄-N treatments while microbial and organic P decreased, suggesting that increased mineralization occurred. Ammonium-induced mineralization was further evidenced in the NMR spectra, in which the NH₄-N treated samples consistently had the lowest proportions of organic P species and the highest proportions of inorganic orthophosphate. Meanwhile, the addition of NO₃-N promoted P mineralization to a lesser degree, as evidenced by more modest increases in labile and soluble P relative to the control. In this presentation, the extent of P mineralization will also be modeled using zero-order and two-compartment kinetic models. While floodplain soils have a great potential to sequester anthropogenic P, high availability of inorganic N, especially NH₄-N, promotes greater phosphatase activity and P mineralization, potentially reducing P retention in floodplain soils.