Dr. Robert Kröger Mississippi State University

1. Assessing complexity of hydrology, nutrient inputs and phosphorus dynamics within agricultural drainage sediments Dr. Robert Kröger Mississippi State University

2. Ubiquitous features • Conduits for downstream degradation • 50% ~ increase HRT to increase reduction capacity

3. Hydrology Typically ephemeral • Known's: • P in runoff • Sorption/Desorption • VADD reduction • Increase HRT, increase capacity

4. Unknown? • Phosphorus storage and dynamics in drainage ditches • Ephemeral nature and how it influences P contributions to the water column (reservoir effect) • Controlled drainage reduces ephemerality and contributions of P • Dynamics of influent loads and ephemerality to P contributions

5. Central hypothesis Without describing and fully understanding this intermediary step (drainage ditches) between production agriculture and receiving waters, we cannot fully develop predictive phosphorus models or budgets and begin to understand the contribution of P. Increased hydrological variability will increase P flux in agricultural drainage sediments.

6. Objectives • Contribution of field aquatic sediments to P mitigation when P enriched agricultural runoff introduced to the system (field scale flooding event and subsequent drying) • Hydrological variability and altering ephemerality in drainage systems through controlled drainage – effects on P dynamics and biogeochemical variables of sediments (experimental)

7. Objective 1 – Field Experiment N Downstream weir 500 m 300 m Upstream weir 0 m 10 m 40 m 100 m

8. Contour data of wetland • Combined with water level loggers = % inundation • 5 months post amendment 500m 300m 100m 40m 10m 0m

9. Table 1. Percentage of time each transect was inundated 5 months following the simulated storm event • 100m experienced greatest hydrological variability post amendment, and more than likely pre-amendment

10. Methods for Obj. 1 • Water – dissolved inorganic P (filtered, 0.45μm), total inorganic P (nonfiltered) – molybdenum blue (Murphy and Riley, 1962) • Sediment - two forms analyzed for: • Immediately dissolved monomeric orthophosphate – Pw or water extractable P (Sissingh, 1971; Nguyen and Sukias; 2002) • P made available through changes in redox potential – acid ammonium extraction for non-crystalline/amorphous and poorly crystalline Fe and Al bound P (Vaughan et al. 2007)

11. Key Results - Water • Decrease in TIP and DIP concentrations with distance from injection • No correlations between TIP and DIP in the water column – except for 100m site (r2 = 0.864, p <0.05) • Hypothesized due to variable inundation – increased amounts of dissolved orthophosphate desorbing from sediments? • Sediments …

12. A. B.

14. Key Results - Sediment • No statistical difference between NaOH-P and Pox – highlighting the lack of Al-P contribution to P dynamics at this site • Pox data statistically similar concentrations (92 mg L) through time for all sites. • There were no correlations between distance from injection and Pox concentration, but there was a statistical correlation between percent time inundated and Pox concentration • 100m site had significantly higher Pox concentrations (F=5.43, p = 0.001) than all other sites, irrespective of time

15. Objective 2 – Mesocosm ephemerality Laboratory Experiment 3 months 3 weeks 3 days

16. Results: pH 3 Days 6.960 6.876 3 Weeks 6.960 7.075 3 Months 6.960 7.266 3 Months Neutral 3 Weeks 3 Days 3 1 2

17. Ammonium Chloride Soluble and Loosely Bound Phosphorus A Ammonium Fluoride Aluminum Phosphate B Sodium Hydroxide Iron Phosphate C Sodium Citrate, Sodium Bicarbonate, Sodium Dithionate Reductant-Soluble Phosphorus D

18. Management Implications Saturation Point Saturation Point Saturation Point When to dredge?

19. Thanks to AFRI SEED grant • Elizabeth Usborne – MS Student sponsored graduate student with the project • Obj. 1 in review in Ecological Engineering • Obj. 2 in preparation