Nitrous Oxide Emissions from Irrigated Cotton in the Southwest USA: Assessment and Mitigation Strategies.

English Abstract:

Nitrogen fertilizer is an important source of the greenhouse gas nitrous oxide (N₂O) in irrigated cropping systems. However, little data exists for row crops in the desert southwest of North America, where seasonal irrigation amounts on farms can exceed 1.2 m. The objective of these studies was to assess the effect of N fertilizer rates on N₂O emissions from furrow- and sprinkler-irrigated cotton (Gossypium hirsutum) in Central Arizona and to test some mitigation management options. Cotton was planted from late April to May 1 from 2012 to 2014 on 1-m wide beds. Nitrogen fertilizer as liquid urea ammonium nitrate (320 g N kg^-1) varied from 0 to 233 kg N ha^-1 per season. There were two split applications of N fertilizer in surface irrigation, and three splits under the sprinkler. Emissions were measured weekly with 1-L vented chambers placed for 24 minute periods in the bottom of furrows. Fifty-ml samples were taken at 0, 12, and 24 minutes and analyzed for N₂O with an ECD-GC. During the cotton growing season N₂O emissions were measured from May to August. Nitrous oxide emissions were not agronomically significant, i.e. < than 0.5 % of applied N, but increased several-fold with the addition of N fertilizer. In 2012 and 2013 with furrow irrigation, knifing-in of N fertilizer resulted in lower N₂O emissions than fertigating into the header irrigation line. In 2014 under the sprinkler, the addition of the enhanced efficiency N source Agrotain Plus to N fertilizer had inconsistent mitigation effects on N₂O emissions. Emissions were positively correlated with soil moisture and temperature. Nitrous oxide fluxes ended 2-3 d after irrigation events that were as high as 125 mm in furrow irrigation, and declined during the season, as plant N uptake progressed. Agrotain Plus apparently breaks down quickly in the desert environment, making its use an N₂O emission inhibitor not consistent.