Winter cereal rye cover crop decreased nitrous oxide emissions during early spring

Despite differences between the cover crop growth and decomposition phases, few greenhouse gas (GHG) studies have separated these phases from each other. This study's hypothesis was that a living cover crop reduces soil inorganic N concentrations and soil water, thereby reducing N2O emissions. We quantified the effects of a fall‐planted living cereal rye (Secale cereale L.) cover crop (2017, 2018, 2019) on the following spring's soil temperature, soil water, water‐filled porosity (WFP), inorganic N, and GHG (N2O‐N and CO2–C) emissions and compared these measurements to bare soil. The experimental design was a randomized complete block, where years were treated as blocks. Rye was fall planted in 2017, 2018, and 2019, but mostly emerged the following spring. The GHG emissions were near‐continuously measured from early spring through June. Rye biomass was 1,049, 428, and 2,647 kg ha–1 in 2018, 2019, and 2020, respectively. Compared to the bare soil, rye reduced WFP in the surface 5 cm by 29, 15, and 26% in 2018, 2019, and 2020 and reduced soil NO3–N in surface 30 cm by 53% in 2019 (p = .04) and 65% in 2020 (p = .07), respectively. Rye changed the N2O and CO2 frequency emission signatures. It also reduced N2O emissions by 66% but did not influence CO2–C emissions during the period prior to corn (Zea mays L.) emergence (VE). After VE, rye and bare soils N2O emissions were similar. These results suggest that nitrous oxide (N2O‐N) sampling protocols must account for early season impacts of the living cover. Core Ideas Rye reduced N2O‐N emissions 66% during the period prior to corn emergence. Nitrous oxide emission reductions were attributed to a decrease in soil water and inorganic N. Rye changed the N2O‐N and CO2–C frequency emissions signatures. These findings show that sampling protocols must account for early season growth.