Impacts of Agricultural Tile Drainage on Water, Phosphorus, and Sediment Losses.

Eutrophication in Lake Champlain has been a concern since 1979, with phosphorus (P) concentrations identified as the major cause. The objective of this study was to determine whether altering the primary pathway of agricultural runoff from the surface to the subsurface utilizing tile drainage would affect the forms and quantity of exported P. Four field plots (22.9 x 45.7 m) were established in a low soil test P (0.86 mg kg^-1) corn field (Zea mays, L.) in Chazy, NY and assigned to a tile-drained (TD) or naturally-drained (UD) treatment. Data were collected April 21, 2014 through June 30, 2015. Continuous water flow from each hydrologic pathway was measured and total P (TP), soluble reactive P (SRP), unreactive P (UP), and sediment (TSS) loads were calculated. Event-based loads and cumulative losses were calculated.  No significant differences in cumulative TP exports were found between treatments (UD = 230.9 g ha^-1; TD = 233.9 g ha^-1).  Approximately 55% more SRP and 158% more TSS was exported by UD (130.8 g ha^-1; 168.8 kg ha^-1) than TD (84.2 g ha^-1; 65.5 kg ha^-1).  Unreactive P exports from TD (149.7 g ha^-1) were 50% greater than UD (100.1 g ha^-1). Two runoff events were responsible for the majority of P and TSS losses. A storm in May 2014 generated the greatest individual-event sediment losses in both treatments, contributing 65 and 67% of the cumulative losses from TD and UD, respectively.  This event was also responsible for 40% of UP losses from TD.  A 3-day rain/snowmelt event in December 2014 resulted in 61 and 84% of all SRP losses for TD and UD, respectively. The results of the study indicate that at this experimental site, tile drainage did not have a negative impact on water quality with respect to P, relative to a naturally drained field.