Nitrate, Ammonium, and Phosphate Fluxes to Subsurface Drains In the Eastern Cornbelt: N Source and Rotation

Subsurface drainage systems in cropland create pathways for nutrient and contaminant movement into surface water, but the relative contribution of diverse cropping practices to the nitrate (NO₃^-), ammonium (NH₄⁺), and orthophosphate (PO₄^3-) fluxes is still not well documented. This study was conducted to assess management effects on nutrient mass fluxes. From 1998 through 2006, we monitored flow, NO₃^-, NH₄⁺, and PO₄^3- in drainage water from lysimeter plots (216 m²) planted to corn cropped continuously (CC) or in rotation with soybean (CS), and fertilized with urea-ammonium nitrate (UAN), or with liquid swine manure injected in spring (SM) or fall (FM). Soybean (SC) in rotation with CS and restored prairie grass (PG) were also included. Across years, corn yields in CS and SC with UAN averaged 9648 and 9201 kg ha^-1 respectively; the difference was significant in some years. SM and FM reduced mean yields to 8629 and 8429 kg ha^-1, respectively. Soils receiving UAN averaged roughly 8-fold greater NO₃^- fluxes than PG (19.9 vs. 2.5 kg N ha^-1 yr^-1); losses from CC, CS and SC were relatively similar. Nitrate fluxes from CCFM were both substantially higher (33.3 kg N ha^-1 yr^-1) and more variable (17 to 48 kg ha^-1 yr^-1) than all other treatments including CCSM (mean loss 18.7 kg ha^-1 yr^-1). This pronounced timing effect reflects the abundant drainage events that occur during the fallow period. Major, discrete snow melt and rainfall events also appeared to control magnitude of daily fluxes for all three nutrients. With respect to NH₄⁺ and PO₄^3-, only manured soils recorded high but episodic/pulse fluxes in certain hydrological years. Compared to the average of all other treatments, CCFM raised PO₄^3- flux (23 vs. 441 g P ha^-1 yr^-1), while CCSM resulted in higher NH₄⁺ flux (217 vs. 680 g P ha^-1 yr^-1).