Late spring urea application increased apparent carbon dioxide equivalence emissions but fall and summer applications did not.

Synchronizing nitrogen (N) fertilizer application with plant N uptake, as opposed to applying earlier, is believed to improve N use efficiency while simultaneously reducing nitrate leaching, ammonia volatilization, and greenhouse gas emissions. However, little research has been conducted to confirm that this is true. Therefore, this study's objective was to determine the effect of urea application date (early fall, mid-fall, late fall, late spring, early summer, and midsummer) and rate (0 and 224 kg N ha^-1) on the apparent carbon dioxide equivalence (CO_2ea). This completely randomized field experiment was conducted six times (dates) where each treatment was replicated four times. Soil temperature and moisture along with greenhouse gases (N₂O, CO₂, and CH₄,) were measured every 4 h for 21 days at Aurora, SD, in 2017 and 2018. N₂O and CH₄ were expressed in carbon dioxide equivalence, and all three gases were summed to CO_2ea. Delaying the fertilizer from early to late fall decreased CO_2ea from 69,375 to 18,420 g CO_2ea (ha × day)^-1. During late spring through midsummer, CO_2ea were similar for the three application dates and averaged 101,500 g CO_2ea (ha × day)^-1. This study showed that the lowest CO_2ea values for the 3 weeks following the application were associated with applying urea in the fall after the temperatures had decreased to less than 10°C or waiting until early summer. This research suggests that full-season assessments of fall, spring, and summer urea application dates on CO_2ea are warranted.