Impact of management practices on nitrous oxide emissions in an irrigated dairy forage rotation.

Abstract

Nitrous oxide emissions from semiarid, irrigated cropping systems are strongly influenced by tillage, nutrient source, and cover cropping, yet their long-term interactive effects remain underexplored. We quantified N₂O emissions from a continuous silage corn (Zea mays) system under factorial combinations of tillage (conventional vs. reduced), nitrogen source (dairy manure vs. synthetic fertilizer), and winter cover cropping (triticale vs. fallow) over 3 years (2021-2023) following 6 years of prior treatment implementation. Dairy manure solids were applied annually in the fall from 2015 through 2020. No further manure applications were made, and from spring 2021 onward, N₂O fluxes were monitored to assess legacy effects. Fluxes were monitored weekly using vented, nonsteady-state chambers. Emissions were episodic, with peak emissions occurring after irrigation onset and during winter months. In 2021, reduced tillage plots produced 25% greater cumulative emissions than CT (3.3 vs. 2.7 kg N₂O-N ha^-1; p = 0.030), though no tillage differences were observed in subsequent years following a field-wide moldboard plowing in spring 2022. Manure-treated plots consistently produced the highest emissions, exceeding synthetic fertilizer treatments by 723%, 267%, and 147% in 2021, 2022, and 2023, respectively (p < 0.0001). Winter cover cropping lowered preplant soil nitrate but did not reduce N₂O losses in manured soils, likely due to continued in-season mineralization. These results show that manure legacy effects persist after applications end and that tillage impacts on emissions are short-lived. Optimizing nutrient use and reducing emissions in semiarid irrigated systems will require integrated management of manure, tillage, and irrigation.