Farmers looking to maximize soil conservation benefits of a rye cover crop (Secale cereale L.) (RCC) may choose to delay termination closer to corn (Zea mays L.) planting. However, delaying RCC termination may reduce corn yield due to nitrogen (N) immobilization and seedling disease. The objective of this trial was to evaluate corn growth and yield in response to in‐furrow (IF) fertilizer and fungicide following a RCC and across different RCC termination timings. A field study was established at three locations in Kentucky in 2019 and 2020 to evaluate corn response to two RCC termination timings (21 d before corn planting [early terminated] and 1 d after corn planting [postplant terminated]) and three IF starter treatments (fertilizer, fungicide, and fertilizer + fungicide). A postplant‐terminated RCC resulted in greater rye shoot biomass, early‐season (Apr–May) soil moisture, and preplant soil inorganic N compared with an early‐terminated RCC. Also, a postplant‐terminated RCC reduced corn stand by an average of 31% at two of three locations and reduced corn yield by an average of 15.7% across locations. The inclusion of IF fertilizer, fungicide, or fertilizer + fungicide did not improve corn yield at any location, and no interaction between RCC termination timing and IF starter was observed. Overall, our results suggest IF fertilizer and/or fungicide does not ameliorate corn stand and yield reductions following a postplant‐terminated RCC. In addition, farmers should look to terminate a RCC earlier (14–21 d before planting) to reduce potential corn stand and yield loss.

Nitrogen loss from cultivated soils threatens the economic and environmental sustainability of agriculture. Nitrate (NO3−) derived from nitrification of nitrogen fertilizer and ammonified soil organic nitrogen may be lost from soils via denitrification, producing dinitrogen gas (N2) or the greenhouse gas nitrous oxide (N2O). Nitrate that accumulates in soils is also subject to leaching loss, which can degrade water quality and make NO3− available for downstream denitrification. Here we use patterns in the isotopic composition of NO3− observed from 2012 to 2017 to characterize N loss to denitrification within soils, groundwater, and stream riparian corridors of a non-irrigated agroecosystem in the northern Great Plains (Judith River Watershed, Montana, USA). We find evidence for denitrification across these domains, expressed as a positive linear relationship between δ15N and δ18O values of NO3−, as well as increasing δ15N values with decreasing NO3− concentration. In soils, isotopic evidence of denitrification was present during fallow periods (no crop growing), despite net accumulation of NO3− from the nitrification of ammonified soil organic nitrogen. We combine previous results for soil NO3− mass balance with δ15N mass balance to estimate denitrification rates in soil relative to groundwater and streams. Substantial denitrification from soils during fallow periods may be masked by nitrification of ammonified soil organic nitrogen, representing a hidden loss of soil organic nitrogen and an under-quantified flux of N to the atmosphere. Globally, cultivated land spends ca. 50% of time in a fallow condition; denitrification in fallow soils may be an overlooked but globally significant source of agricultural N2O emissions, which must be reduced along-side other emissions to meet Paris Agreement goals for slowing global temperature increase.

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