Edge-of-field Associated Nitrate-N Loss in a Soybean-corn Rotation
Across the United States corn-belt region substantial quantities of nitrogen (N) fertilizer are applied in both continuous corn (Zea maysL.) 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-1less) 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-1applied preplant in rotational corn (CS-157), and 180 kg N ha-1applied preplant in continuous corn (CC-180). All corn plots received 23 kg N ha-1as 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-1yr-1, and these were all significantly higher than the Pgrass control (1.70 kg N ha-1yr-1). When corn and soybean years in rotational systems were analyzed separately for leaching losses, CS-157 was significantly higher than CS-135 and CC-180 (14.70, 10.85 and 11.88 kg N ha-1, respectively) whereas losses by SC-157 and SC-135 were similar averaging 12.26 and 12.13 kg N ha-1, respectively. Nitrogen treatment did not impact either corn or soybean mean yields. We concluded that soybean BNF production may be a major driver in N load loss in rotational corn when compared to continuous corn and further reductions in load losses from rotational systems will require a focus on managing soybean-derived N. Lastly, future research should include monthly or seasonal assessment of N load losses to better target practices at vulnerable times of nutrient loss.