Regulation of N2O emissions from acid organic soil drained for agriculture

Abstract

Organic soils drained for crop production or grazing land are agroecosystems with potentially high but variable emissions of nitrous oxide (N₂O). The present study investigated the regulation of N₂O emissions in a raised bog area drained for agriculture, which is classified as potentially acid sulfate soil. We hypothesised that pyrite (FeS₂) oxidation was a potential driver of N₂O emissions through microbially mediated reduction of nitrate (${\mathrm{NO}}_{\mathrm{3}}^{-}$). Two sites with rotational grass, and two sites with a potato crop, were equipped for monitoring of N₂O emissions and soil N₂O concentrations at the 5, 10, 20, 50 and 100 cm depth during weekly field campaigns in spring and autumn 2015. Further data acquisition included temperature, precipitation, soil moisture, water table (WT) depth, and soil ${\mathrm{NO}}_{\mathrm{3}}^{-}$ and ammonium (${\mathrm{NH}}_{\mathrm{4}}^{+}$) concentrations. At all sites, the soil was acidic, with pH ranging from 4.7 to 5.4. Spring and autumn monitoring periods together represented between 152 and 174 d, with cumulative emissions of 4–5 kg N₂O-N ha⁻¹ at sites with rotational grass and 20–50 kg N₂O-N ha⁻¹ at sites with a potato crop. Equivalent soil gas-phase concentrations of N₂O at grassland sites varied between 0 and 25 µL L⁻¹ except for a sampling after slurry application at one of the sites in spring, with a maximum of 560 µL L⁻¹ at the 1 m depth. At the two potato sites the levels of below-ground N₂O concentrations ranged from 0.4 to 2270 µL L⁻¹ and from 0.1 to 470 µL L⁻¹, in accordance with the higher soil mineral N availability at arable sites. Statistical analyses using graphical models showed that soil N₂O concentration in the capillary fringe (i.e. the soil volume above the water table influenced by tension saturation) was the strongest predictor of N₂O emissions in spring and, for grassland sites, also in the autumn. For potato sites in autumn, there was evidence that ${\mathrm{NO}}_{\mathrm{3}}^{-}$ availability in the topsoil and temperature were the main controls on N₂O emissions. Chemical analyses of intact soil cores from the 0 to 1 m depth, collected at adjacent grassland and potato sites, showed that the total reduction capacity of the peat soil (assessed by cerium(IV) reduction) was much higher than that represented by FeS₂, and the concentrations of total reactive iron (TRFe) were higher than those of FeS₂. Based on the statistical graphical models and the tentative estimates of reduction capacities, FeS₂ oxidation was unlikely to be important for N₂O emissions. Instead, archaeal ammonia oxidation and either chemodenitrification or nitrifier denitrification were considered to be plausible pathways of N₂O production in spring, whereas in the autumn heterotrophic denitrification may have been more important at arable sites.