NITROGEN-OXIDE FLUX FROM AN AGRICULTURAL SOIL DURING WINTER FALLOW INTHE UPPER COASTAL-PLAIN OF NORTH-CAROLINA, USA

Incorporation of the remaining crop residue, including the root system , of grain (soybean and corn) and fiber (cotton) crops into the soil f ollowing harvest is a common agricultural practice. The crop residue r epresents a substantial portion of nitrogen initially applied as ferti lizer, and thus is a potential source of nitrogen for NO emissions dur ing the winter fallow period. Fluxes of NO and NO2 were measured from fallow fields from February 7 to March 23, 1994, using a dynamic chamb er technique (ambient air as the carrier gas). Average NO flux rates, as a function of previous crop residue, were 9.2 (range -4.2 to 76) ng -N m(-2) s(-1) for soybean, 6.1 (range -11.7 to 110) ng-N m(-2) s(-1) for cotton, and 4.7 (range -0.2 to 40) ng-N m(-2) s(-1) for corn. Maxi mum NO fluxes were observed in midmorning when soil temperatures were lowest. Minimum NO flux occurred after mid-afternoon when soil tempera ture reached a maximum. The decrease in NO flux with increase in soil temperature (5 cm depth) reflected the existence of a NO compensation concentration (i.e., the rate for the NO consumption reactions continu ed to increase with increase in temperature). NO2 deposition was calcu lated for 92% of the data points, with no trend in deposition between the three fields and their corresponding crop residue. These results i ndicate that significant fluxes of NO are generated from fallow agricu ltural fields following incorporation of the residue from the previous crop.