Impact of Weeds and Nitrogen on Nitrous Oxide Emissions.

In crop production systems where nitrogen (N) is added as a fertilizer, undesirable nitrous oxide (N₂O) emissions increase as rates of N increase and soil moisture increases, especially in anaerobic conditions. Management practices that influence the amount of N and water in the soil, therefore, can have a direct impact on N₂O emissions. This research investigates how a preemergence plus postemergence (PRE + POST) herbicide program and a POST-only herbicide program impact N₂O emissions at two different N rates. Compared to a PRE + POST system, we hypothesized that weeds in a POST-only management system will reduce N₂O emissions while growing by reducing soil N and water, but later increase emissions as weeds decay and increase soil moisture and encourage N cycling.  Gas samples were collected from a series of non-crop greenhouse and field studies. To simulate weed growth in PRE + POST and POST-only management systems, three non-crop greenhouse studies used a 2x2 factorial treatment structure of weed density (0 or 100 plants m^-2) and N rate (0 or 200 kg N ha^-1). Experimental units were sprayed with glyphosate when weeds were 10-15 cm tall. The non-crop field study, which was conducted in summers 2013 and 2014, was a 2x2 of weed management (PRE + POST vs. POST-only) and N rate (0 or 225 kg ha^-1).  There was no significant interaction of weed management by N rate in the combined analysis of the greenhouse studies (p=0.1626) or the field studies (p=0.4539). However, treatments with N addition had higher N₂O emissions than those without N in both the greenhouse (p=<0.0001) and the field (p=<0.0001).  Weeds significantly increased total N₂O emissions (p=0.0004) and emissions after POST application (p=0.0003) in the greenhouse studies, but had no significant effect in the field studies.