Seasonal and Hydrological Controls of Bioavailability

Project Title: Seasonal and hydrological controls of bioavailability of organic phosphorus in agricultural drainage waters

Principal Investigator(s): Dr. Pierre-André Jacinthe, Indiana University Purdue University Indianapolis, Department of Earth Sciences

Dates: March 1, 2020 – February 28, 2021

Total Federal Funds:  Total Non-Federal Funds:

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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.