Processes contributing to nitrite accumulation and concomitant N2O emissions in frozen soils

Nitrous oxide (N2O) emissions from frozen soils have been observed during winter. Concomitantly, nitrite (NO2-) accumulations were reported in frozen soils and may have contributed to elevated N2O emissions. The objectives of this study were to determine the processes leading to NO2- accumulation and its contribution to N2O emissions in frozen soils. Soil microcosms were incubated for 21 days under frozen (-2 degrees C) or unfrozen (+0.5 degrees C) conditions. Soils were amended with either N-15-NH4+ or N-15-NO3 to quantify the contribution of nitrification and denitrification processes to NO2- accumulation, respectively. Production of N-15-N2O was measured to determine the contribution of NO2- accumulation to N2O emissions. Nitrite progressively accumulated over 21 days in soils incubated at -2 degrees C to concentrations up to 8.87 mu g N g(-1) dry soil. However, in soils incubated at + 0.5 degrees C, NO2- concentration increased from day 0 to day 7 (up to 3.61 mu g N g(-1) dry soil) and declined thereafter. At both temperatures, NO2- accumulation was associated with a progressive decrease in NO3- concentration while NH4+ concentration remained relatively stable over time. Greater N2O emissions were observed at +0.5 degrees C (average of 0.0135 mu g N g(-1) dry soil h(-1)) compared with -2 degrees C (average of 0.0045 mu g N g(-1) dry soil h(-1)) from day 2 to day 14. However, N2O emissions were 3.5 fold greater at -2 degrees C compared with +0.5 degrees C on day 21. In soils at -2 degrees C, 97.40-99.75% of the NO2- and over 99.56% of N2O were derived from NO3-. In soils incubated at +0.5 degrees C, the proportions of NO2- and N2O derived from NO3- slightly decreased while the proportions of NO2- and N2O derived from NH4+ increased up to 4.3% and 4.7%, respectively, from day 2 to day 21. At both temperatures, N-15 enrichments of N2O and NO2- were similar. The results indicated that (i) denitrification was the major process leading to soil NO2- accumulation and concomitant production of N2O under both temperatures; and that (ii) N2O emissions were more sustained in frozen soils where NO2- concentrations were greater compared to unfrozen soils.