Diurnality of soil nitrous oxide (N2O) emissions

Abstract:

Soil emissions of nitrous oxide (N₂O) are important contributors to the greenhouse gas balance of the atmosphere. Agricultural soils contribute ~65% of anthropogenic N₂O emissions. Understanding temporal and spatial variability of N₂O emissions from agricultural soils is vital for closure of the global N₂O budget and the development of mitigation opportunities. Recent studies have observed higher N₂O fluxes during the day and lower at night. Understanding the mechanisms of such diurnality may have important consequences for our understanding of the N cycle. We tested the hypothesis that diurnal cycles are driven by root carbon exudes that stimulate denitrification and therefore N₂O production. Alternatively, we considered that the cycle could result from higher afternoon temperatures that accelerate soil microbial activity. We removed all plants from a corn field plot and left another plot untouched. We measured soil N₂O emissions in each plot using a standard static chamber technique throughout the corn growing season. And also compared static chamber results to ecosystem level N₂O emissions as measured by eddy covariance tower equipped with an open-path N₂O sensor. We also measured soil and air temperatures and soil water and inorganic N contents. Soil N₂O emissions followed soil inorganic N concentrations and in control plot chambers ranged from 10 µg N m^-2 hr^-1 before fertilization to 13×10³ after fertilization. We found strong diurnal cycles measured by both techniques with emissions low during night and morning hours and high during the afternoon. Corn removal had no effect on diurnality, but had a strong effect on the magnitude of soil N₂O emissions. Soil temperature exhibited a weak correlation with soil N₂O emissions and could not explain diurnal patterns. Further studies are underway to explore additional mechanisms that might contribute to this potentially important phenomena.