Mechanisms underlying episodic nitrate and phosphorus leaching from poorly drained agricultural soils

Poorly drained depressions within tile-drained croplands can have disproportionate environmental and agronomic impacts, but mechanisms controlling nutrient leaching remain poorly understood. We monitored nitrate and soluble reactive phosphorus (SRP) leaching using zero-tension soil lysimeters across a depression to upland gradient over 2 years in a corn-soybean (Zea mays L.-Glycine max [L.] Merr.) field in Iowa. We also measured stable isotopes (δ¹⁵N and δ¹⁸O) of nitrate to examine its sources and transformations. SRP concentrations peaked during winter and early spring after phosphorus (P) fertilization (mean = 3 mg P L^-1), with highest values in the depression, and SRP was relatively stable thereafter (mean = 0.3 mg P L^-1) irrespective of periods of high soil moisture that led to widespread iron (Fe) reduction across the field. During a near-average precipitation year, nitrate stable isotopes indicated direct leaching of fertilizer nitrate within days of application, followed by nitrification of fertilizer ammonium and several weeks of denitrification in depressional soils. Nevertheless, nitrate concentrations remained high (mean = 28 mg N L^-1) in the depression despite strong isotopic evidence for denitrification (>48% N removal). During a wet year, nitrate concentrations were lower in the depression than upland and nitrate isotopes were highly variable, consistent with nearly complete nitrate removal by denitrification in the depression and significant denitrification in upland soils. We conclude that poorly drained depressional soils can potentially decrease nitrate leaching via denitrification under sustained wet conditions, but they inconsistently denitrify and are vulnerable to high nitrate and SRP losses when soils are not saturated, especially following fertilization.