104B Base Grants – Seasonal and Hydrological Controls of Bioavailability

The water quality impact of phosphorus (P) exported from croplands in the US Midwest is well documented. Monitoring efforts have generally focused on the inorganic P (P i ) fractions, a preference largely dictated by the assumption that P i is immediately available to algae. However, in some settings, the amount of organic P (P o ) loss can be significant, and may represent another P source that can sustain algal growth in receiving water bodies. At the present, the bioavailability of P o is largely unknown.

Our monitoring activities (2015-2019) at two Central Indiana agricultural watersheds have shown that P o was the dominant form of P exported, ranging between 63 and 92% of total P load. In light of these observations, we investigated the bioavailability of P o in drainage waters, and examined the effect of hydrologic flow path (surface runoff vs subsurface tile discharge) and season on the biochemical attributes of P o . The pool of bioavailable P was determined using the filter strip method (retention of P i + P o fractions available to algae on FeO-impregnated filters). Enzyme assays were conducted to quantify the enzymatically-hydrolysable fractions of P o or EHP (monester, diester, phytate) in drainage waters. Initial results have shown that EHP concentration was generally higher in the summer, and consistently higher (2-fold) in tile waters than in surface runoff. We plan to: (i) continue these measurements through summer 2020 to assess annual variability, and (ii) conduct additional analysis using NMR spectroscopy to gain further insight into the speciation and dynamics of P o in agricultural waters. Overall, these initial results indicate that a sizable portion of the dissolved P o pool, once presumed to be not bioavailable, can in fact be hydrolyzed and converted to P i , fueling algal growth.