Retention of¹⁵N-Labeled Fertilizer in an Illinois Prairie Soil with Winter Rye

Human production of reactive nitrogen (N), particularly for agriculture, has led to consequences for air and water pollution. Winter annual cover crops show promise for reducing N losses from grain agroecosystems. We applied 15N-labeled ammonium sulfate at corn (Zea mays L.) planting in May 2009 to a long-term corn and soybean (Glycine max [L.] Merr.) variety trial on an Illinois Mollisol. We tracked the fate of the labeled fertilizer in treatments comparing winter rye (Secale cereal) cover to the typical winter bare fallow (BF). We measured fertilizer recovery in corn at harvest in the fall, in rye biomass the following spring (May 2010), and in a range of heterogeneous soil organic matter (SOM) pools at both sampling dates. In the spring, total recovery of added 15N in crops and soil was low (37–45%). Due to unfavorable conditions for cover crop (CC) establishment and an unusually rainy October, rye growth missed the optimal window for 15N recovery and little tracer 15N was recovered in the rye. However, the CC significantly reduced soil inorganic N (SIN) pools in the spring (11.1 kg N ha-1 in the BF treatment compared with 1.9 kg N ha-1 in the CC treatment) by an amount similar in magnitude to total N uptake by rye biomass (23.7 kg N ha-1), suggesting that a key role of CCs is scavenging inorganic N mineralized from SOM pools. This study reflected current challenges for incorporating CCs into grain rotations with corn varieties that have longer growing seasons.