Negative background nitrous oxide emissions in agricultural and natural lands

Negative BNE was observed in pasture land-use and in peatland, various forests, and wetland ecosystems (n = 29) with the majority of observations coming from pasture (n = 12) and peatland (n = 11). This is consistent with Chapuis–Lardy et al. (2007) who also reported soil N₂O consumption in unfertilized agricultural and natural ecosystems. There were no significant differences in negative BNE across land-use or ecosystem types and the median values ranged from -0.05 to -0.33 kg N₂O−N ha⁻¹ yr⁻¹. The pasture sites exhibiting negative BNE were located in humid, temperate regions in soils with typically high organic matter (SOC > 24 mg kg^-1) and near neutral pH (mean of 6.9 for this pasture data subset; data not shown). The wide soil C:N ratios, N-limited status, and/or intermittent anaerobic conditions in these several terrestrial ecosystems showing negative BNE could also interact to cause the observed uptakes of atmospheric N₂O by these soils. Furthermore, with particular focus on peatland and wetland sites, soil N₂O consumption could be stimulated by anaerobic conditions, low mineral N, and readily available organic C forms as energy sources for denitrifying microbial activity leading to increased reduction of N₂O to dinitrogen (Bremner 1997; Chapuis–Lardy et al. 2007).

 The large majority of the BNE data compiled in our meta-analysis was positive (efflux). The frequency of negative BNE values (influx) observed in our meta-analysis was only 3.2% of the total number of collated BNE data (n = 907). Moreover, observed negative BNE values were one order of magnitude lower than typical BNEA and BNEN values. Based on the magnitude and frequency of negative BNE, this meta-analysis suggests that contribution of negative BNE to global N₂O budget as a sink of atmospheric N₂O can be minor.