ARS | Publication request: Assessment of In-Stream Phosphorus Dynamics in Agricultural Drainage Ditches

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 28, 2009
Publication Date: June 1, 2009
Citation: Smith, D.R. 2009. Assessment of In-Stream Phosphorus Dynamics in Agricultural Drainage Ditches. Science of the Total Environment. Science of the Total Environment 407(2009):3883-3889.

Interpretive Summary: The intensive row crop agricultural systems in the Midwestern United States can enrich surface waters with nutrients. This project was conducted to evaluate the in-stream processing of P in agricultural ditches. Phosphorus injection studies were conducted at seven sites along three drainage ditches (735 to 10,600 ac drainage area), and one site on a third order stream that receives the discharge from the agricultural ditches (46,900 ac drainage area). We found that as the velocity of water increases in the ditch, the mean distance for P to be taken up by sediments increases. We also found that the amount of indirect drainage is positively correlated with the P uptake length, while the amount of closed depressions in the watershed is inversely correlated with the P uptake velocity. Land use also played an important role in the amount of P sediments were able to remove from the water column. From this study, we learn that there is a dynamic relationship between the sediment and water column with respect to P transport. Further research may shed light on how we can manage agricultural drainage ditches to maximize P sequestration from the water column, thereby minimizing downstream delivery of this nutrient. The impact of this research is to inform other researchers and policy makers of these results and potential methods to minimize eutrophication of downstream water bodies.

Technical Abstract: The intensive row crop agricultural systems in the Midwestern United States can enrich surface waters with nutrients. This project was conducted to evaluate the in-stream processing of P in agricultural ditches. Phosphorus injection studies were conducted at seven sites along three drainage ditches (298 to 4,300 ha drainage area), and one site on a third order stream that receives the discharge from the agricultural ditches (19,000 ha drainage area). Phosphorus uptake lengths (Snet) ranged from 40 to 1,900 m, and P uptake rates (U) ranged from 0.4 to 52 mg m-2 hr-1. Phosphorus Snet was correlated with water velocity (R2 = 0.54, P < 0.05), indirect drainage in the watershed (R2 = 0.92, P < 0.001), and the amount of small grains, forest, urban land, alfalfa and corn (R2 = 1.00, P < 0.0001). Agricultural drainage ditches actively process nutrients and could potentially be managed to optimize this processing.