Agricultural drainage ditches mitigate phosphorus loads as a function of hydrological variability

Area of Science:

• Environmental Science
• Agricultural Science
• Hydrology

Background:

• Nonpoint source phosphorus (P) loading from agricultural lands, particularly in the Mississippi watershed, exacerbates downstream aquatic ecosystem degradation.
• Enriched runoff contributes to coastal water eutrophication and Gulf of Mexico hypoxia, impacting marine environments.

Purpose of the Study:

• To assess the phosphorus mitigation capacity of agricultural drainage ditches under no-till cotton farming during natural and variable rainfall events.
• To quantify the effectiveness of these ditches in reducing inorganic P loads entering receiving waters.

Main Methods:

• Monthly sampling of two experimental ditches for total inorganic P concentrations in baseflow and stormflow over two years.
• Utilized phosphorus concentrations, Manning's equations (considering vegetation changes), and NRCS hydrographs to calculate storm P loads.
• Analyzed baseflow regressions and percentage reductions to understand ditch P dynamics (sink/source).

Main Results:

• Drainage ditches acted as both sinks and sources for dissolved inorganic P and particulate P throughout the year.
• Storm event loads transported 5.5% of the annual applied fertilizer into the ditches.
• Annually, the ditches reduced the maximum inorganic effluent P load by 43.92% +/- 3.12%.

Conclusions:

• Agricultural drainage ditches demonstrate a fair potential for mitigating phosphorus loads.
• Further research into controlled drainage systems is recommended to enhance the P mitigation capacity of these agricultural features.