Water Quality in Walnut Creek Watershed: Nitrate‐Nitrogen in Soils, Subsurface Drainage Water, and Shallow Groundwater

Abstract

Abstract Nonpoint source contamination of surface and groundwater resources with nitrate‐N (NO 3 ‐N) has been linked to agriculture across the midwestern USA. A 4‐yr study was conducted to assess the extent of NO 3 ‐N leaching in a central Iowa field. Water flow rate was monitored continuously and data were stored on an internal datalogger. Water samples for chemical analysis were collected weekly provided there was sufficient flow. Twelve soil cores were collected in spring, early summer, mid‐summer, and after harvest for each of the 4 yr. Nitrate‐N concentrations in shallow groundwater exhibited temporal trends and were higher under Clarion soil than under Okoboji or Canisteo soil. Denitrification rates were two times higher in Okoboji surface soil than in Clarion surface soil and the highest denitrificafion potential among subsurface sediments was observed for deep unoxidized loess. Soil profile NO 3 ‐N concentrations decreased with depth and were the same below 30 cm for fertilized corn ( Zea mays L.) and soybean ( Glycine max L. Merr.). Nitrate‐N concentrations 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 pattern of NO 3 ‐N concentrations in subsurface drainage water was not related to the timing of fertilizer N application or the amount of fertilizer N applied. Total NO 3 ‐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 drainage water NO 3 ‐N was lost when crop plants were not present (November‐May), except in 1993. Our results indicate that NO 3 ‐N losses to subsurface drainage water occur primarily as a result of asynchronous production and uptake of NO 3 ‐N in the soil and the presence of large quantifies of potentially mineralizable N in the soil organic matter.