Water quality in Walnut Creek watershed: Nitrate-nitrogen in soils, subsurface drainage water, and shallow groundwater

Nonpoint source contamination of surface and groundwater resources with nit rate-N (NO3-N) has been linked to agriculture across the midwestern USA. A 4-yr study was conducted to assess the extent of NO3-N leaching in a centra l Iowa field. Water now rate was monitored continuously and data were store d on an internal datalogger. Water samples for chemical analysis were colle cted weekly provided there was sufficient now. Twelve soil cores were colle cted in spring, early summer, mid-summer, and after harvest for each of the 4 yr. Nitrate-N concentrations in shallow groundwater exhibited temporal t rends and were higher under Clarion soil than under Okoboji or Canisteo soi l. Denitrification rates were two times higher in Okoboji surface soil than in Clarion surface soil and the highest denitrification potential among su bsurface sediments was observed for deep unoxidized loess Soil profile NO3- N concentrations decreased with depth and were the same below 30 cm for fer tilized corn (Zea mays L.) and soybean (Glycine max L. Merr.). Nitrate-N co ncentrations in subsurface drainage water exceeded 10 mg L-1 for 12 mo and were between 6 and 9 mg L-1 for 32 mo during the 4-yr study. The temporal p attern of NO3-N concentrations in subsurface drainage water was not related to the timing of fertilizer N application or the amount of fertilizer N ap plied. Total NO3-N losses to subsurface drains were greatest in 1993 (51.3 kg ha(-1)) and least in 1994 (4.9 kg ha(-1)). Most of the subsurface draina ge water NO3-N was lost when crop plants were not present (November-May), e xcept in 1993, Our results indicate that NO3-N losses to subsurface drainag e water occur primarily as a result of asynchronous production and uptake o f NO3-N in the soil and the presence of large quantities of potentially min eralizable N in the soil organic matter.