Seasonal Variation of Enzymatically Hydrolyzable Organic Phosphorus in Agricultural Drainage Waters

Phosphorus (P) export studies in agricultural watersheds have generally focused on soluble reactive P (SRP), the P fraction generally considered as readily available for biological uptake. However, in some settings, the amount of dissolved organic P (DOP) in agricultural drainage waters can be significant and may represent another important source of P to fuel algal growth. Using the iron-oxide (FeO) strip method and a suite of assays to measure enzymatically hydrolyzable P (EHP), the potential bioavailability of DOP in drainage waters from an agricultural field (Indiana, USA) was investigated to assess the effect of season and hydrologic flow path on DOP attributes. Results showed that DOP accounted for 60%–85% of the total dissolved P (TDP), and that 39% of DOP in surface runoff and 62% of DOP in subsurface tile waters was enzymatically hydrolyzable. Taking the FeO-strip P as another measure of potentially bioavailable P, regression analysis and concentration balance calculations showed that the FeO-strip P mostly included SRP and EHP (monoesterase-, diesterase-, and phytase-hydrolyzable P). For both subsurface and surface pathways, EHP (especially phytase-hydrolyzable P) and FeO-strip P concentration was highly sensitive to dry/wet cycles of soil moisture. Thus, if these dry/wet hydroclimatic events were to become more frequent in the region, as suggested by some climate models, they could accelerate the export of bioavailable P from croplands and further exacerbate water quality degradation in receiving water bodies.