Edge-of-Field Associated Nitrate-N Loss in a Soybean-Corn Rotation

Mitchell J Greve, Purdue University

Abstract

Across the United States corn-belt region substantial quantities of nitrogen (N) fertilizer are applied in both continuous corn (Zea mays L.) and corn grown in rotation with soybean [Glycine max (L.) Merr.]. When compared to continuous corn, corn grown in rotation with soybean typically receives less applied N fertilizer (typically 20-45 kg ha-1 less) than continuous corn due to expected carryover of N from biological N fixation (BNF) by soybean in the preceding year. However, when current N recommendations are followed in both systems, rotational corn has been shown to lose similar or, in some cases, greater amounts of N through subsurface tile lines than continuous corn although the reports in the literature have been inconsistent. In rain-fed systems a key limitation to many previous studies has been an insufficient number of site-years of data to fully characterize management effects across varied environments. Regardless, the development of better management practices to reduce nitrate leaching losses has largely remained focused on managing N applied to corn and soybean’s role in degradation of surface water has been relatively understudied in tile drained agroecosystems. Therefore, the objectives of this study were to use a 23-yr data record to: (1) compare quantities and patterns of N loss in tile drainage water among a soybean-corn rotation fertilized with the recommended preplant N rate, a soybean-corn rotation fertilized with a N reduced rate applied as a sidedress, continuous corn fertilized with the recommended preplant N rate, and an unfertilized, restored prairie as a natural system control, (2) determine whether and when cumulative soybean-corn load losses in drainage water surpassed that of continuous corn, and (3) evaluate the current recommended N credits from the dual perspective of crop productivity and protection of water quality. Established in 1992, the Purdue University Water Quality Field Station has continuously assessed field-scale N cycling and losses in tile drains and the N management of the five treatments examined in this study have been maintained since 1995. Treatments were 135 kg N ha-1 applied in rotational corn as a sidedress at approximately V6 each year (CS-135), 157 kg N ha-1 applied preplant in rotational corn (CS-157), and 180 kg N ha-1 applied preplant in continuous corn (CC180). All corn plots received 23 kg N ha-1 as starter at planting. A restored perennial prairie control with no fertilizer applied (Pgrass) was utilized to compare and discuss the implications of intensively fertilized annual row crops. The 23-yr data record includes N concentration in drainage water, drainflow volume, N load losses in drainflow, grain yield, tissue N concentrations at harvest and N amounts returned to soil in crop residues and removed in grain. Analysis of variance found CS-157 resulted in significantly greater daily flow-proportional N concentrations (23-year mean 11.98 mg L-1) when compared to all other cropping systems (≤ 10.96 mg L-1). No reportable significant differences occurred in mean annual drainage flow volume among the respective cropping systems. Annual N load loss was statistically similar among cropping systems, ranging between 9.88 to 12.32 kg N ha-1 yr-1, and these were all significantly higher than the Pgrass control (1.70 kg N ha-1 yr-1).

Degree

M.Sc.

Subject Area

Agriculture|Agronomy

COinS