SIMULATING EFFECTS OF VARIABLE NITROGEN APPLICATION RATES ON CORN YIELDS AND NO3 –N LOSSES IN SUBSURFACE DRAIN WATER

Abstract

Using a model as a management tool requires testing of the model against fieldmeasured data prior to itsapplication for solving natural resource problems. This study was conducted to test the Root Zone Water Quality Model(RZWQM98) using four years (1996 to 1999) of fieldmeasured data to simulate the effects of different Napplication rateson corn yields and nitratenitrogen (NO3 N) losses via subsurface drain water. Three Napplication rates (low, medium, andhigh), each replicated three times, were applied to corn in 1996 and 1998 under a randomized complete block design at atiledrained cornsoybean rotation field near Story City, Iowa. No Nfertilizer was applied to soybean in 1997 and 1999.Model calibration and evaluation were based on field measurements of tile flows, NO3 N losses in tile water, andcornsoybean yields. On average, the model simulated tile flow, NO3 N losses in tile water, and yields by showing a percentdifference of 8%, 15%, and 4%, respectively, between measured and simulated values. The simulated yield responsefunction showed that corn grain yields reached a plateau level when the Napplication rate exceeded 200 kgN/ha in 1996and 170 kgN/ha in 1998. These results suggest that RZWQM has the potential to simulate the effects of Napplication rateson corn yields and NO3 N losses with tile water. However, the model overestimated NO3 N losses in subsurface drainagewater during the soybean growth period, which may require further refinements in the Ncycling algorithm in relation toN2 fixation and Nuptake processes.