Subsurface drainage reduces the amount and interannual variability of optimum nitrogen fertilizer input to maize cropping systems in southeast Iowa, USA

Subsurface drainage is widely used to improve field trafficability and crop growth on poorly drained soils. Although drainage causes or exacerbates some environmental challenges, drained croplands are among the most productive in the world and there may be opportunities to better manage drainage for positive environmental outcomes including improved nitrogen use efficiency. The objective of this study was to determine the effects of drainage on cropping system N use. We hypothesized that drainage reduces the agronomic optimum N fertilizer rate (AONR) while increasing grain yield thereby increasing agronomic efficiency (kg grain kg^-1 N at the AONR; AE) while reducing N surplus (i.e., N inputs minus outputs). Using a site in southeast Iowa, USA that included four drainage treatments in both continuous maize and maize-soybean crop rotations with eight N rates (0–392 kg N ha^-1), maize grain yield was measured from 2016 to 2020. The four drainage systems included conventional (1.2 m depth x 18 m spacing), shallow (0.76 m x 12.2 m), controlled (1.2 m x 18 m, with a water table control structure), and a no drainage control. In addition, maize N uptake in the no drain and conventional depth systems was measured from 2016 to 2018. No drainage produced the highest grain yields at the AONR (13.3 ± 1.8 Mg ha^-1), but also the highest AONR (243 ± 155 kg N ha^-1) and lowest AE at the AONR (73 ± 37 kg grain kg^-1 N); moreover, no drainage was the most variable system from year-to-year over all metrics (mean CV 044% ± 20%). In contrast, controlled drainage had the greatest average AE at the AONR (85 ± 28 kg grain kg^-1 N) and the least variability over nearly all metrics from year-to-year (mean CV 25% ± 14%). Conventional drainage had consistently higher maize grain N concentrations than no drainage across N rates, particularly in the continuous maize rotation (6.9 ± 2.8%) and a lower AONR in four of five years compared to No Drainage. Within each rotation at the economic optimum rate, no drainage had higher N surplus than conventional (45 and 19 kg N ha^-1 and 137% and 72% higher for the continuous maize and the maize phase of maize-soybean rotations, respectively). This study demonstrates that drainage reduces the amount and interannual variability of N fertilizer requirements while also reducing interannual variability in yields. Better recognition, understanding, and management of these effects can benefit both productivity and the environment.