Processes contributing to nitrite accumulation and concomitant N<inf>2</inf>O emissions in frozen soils
Citation
Wertz, S., Goyer, C., Burton, D.L., Zebarth, B.J., Chantigny, M.H. (2018). Processes contributing to nitrite accumulation and concomitant N2O emissions in frozen soils. Soil Biology & Biochemistry, [online] 126 31-39. http://dx.doi.org/10.1016/j.soilbio.2018.08.001
Plain language summary
Agricultural fields are an important source of nitrous oxide, a greenhouse gas responsible for the destruction of the ozone layer and accelerating climate changes. It is well known that significant nitrous oxide emissions can be observed from frozen soils during winter and at spring thaw. Several soil processes can be responsible for production of nitrous oxide emissions from soils including denitrification, nitrification, dissimilatory nitrate reduction to ammonium and chemical denitrification. Nitrite is a highly reactive compound that rarely accumulates in soil however; nitrite was reported to accumulate in frozen soils. However, what soil processes resulted in nitrite accumulation in soil and the possible role of nitrite accumulation in contributing to elevated nitrous oxide emissions were unclear. The objectives of this study were to determine the soil processes leading to nitrite accumulation and its contribution to nitrous oxide emissions in frozen soils under controlled conditions. Soils were incubated for 21 days under frozen (-2°C) or unfrozen (+0.5°C) conditions. The contribution of nitrification and denitrification processes to NO2- accumulation and the contribution of nitrite to nitrous oxide emissions were measured. Nitrite progressively accumulated over 21 days in soils incubated at -2°C however, in soils incubated at +0.5°C, nitrite concentration increased temporarily than declined thereafter. Greater nitrous oxide emissions were observed at the beginning of the incubation at +0.5°C compared with -2°C however, nitrous oxide emissions were greater at -2°C compared with +0.5°C at the end of the experiment (day 21). The results indicated that (i) denitrification was the major process leading to soil nitrite accumulation and simultaneous production of nitrous oxide under both temperatures; and that (ii) nitrous oxide emissions were more sustained in frozen soils where nitrite concentrations were greater compared to unfrozen soils.
Abstract
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 °C) or unfrozen (+0.5 °C) conditions. Soils were amended with either 15N-NH4+ or 15N-NO3 to quantify the contribution of nitrification and denitrification processes to NO2− accumulation, respectively. Production of 15N-N2O was measured to determine the contribution of NO2− accumulation to N2O emissions. Nitrite progressively accumulated over 21 days in soils incubated at −2 °C to concentrations up to 8.87 μg N g−1 dry soil. However, in soils incubated at +0.5 °C, NO2− concentration increased from day 0 to day 7 (up to 3.61 μg 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 °C (average of 0.0135 μg N g−1 dry soil h−1) compared with −2 °C (average of 0.0045 μg N g−1 dry soil h−1) from day 2 to day 14. However, N2O emissions were 3.5 fold greater at −2 °C compared with +0.5 °C on day 21. In soils at −2 °C, 97.40–99.75% of the NO2− and over 99.56% of N2O were derived from NO3−. In soils incubated at +0.5 °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, 15N 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.