# Regulation of N 2O emissions from acid organic soil drained for agriculture

Organic soils drained for crop production or grazing land are agroecosystems with potentially high but variable emissions of nitrous oxide (inline-formulaN2O). The present study investigated the regulation of inline-formulaN2O emissions in a raised bog area drained for agriculture, which is classified as potentially acid sulfate soil. We hypothesised that pyrite (inline-formulaFeS2) oxidation was a potential driver of inline-formulaN2O emissions through microbially mediated reduction of nitrate (inline-formula $M7inlinescrollmathmlchem{\mathrm{normal NO}}_{normal 3}^{-}$ 25pt16ptsvg-formulamathimga02883d0956e7dc256b9fe9fffa70b09 bg-16-4555-2019-ie00001.svg25pt16ptbg-16-4555-2019-ie00001.png ). Two sites with rotational grass, and two sites with a potato crop, were equipped for monitoring of inline-formulaN2O emissions and soil inline-formulaN2O 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 inline-formula $M10inlinescrollmathmlchem{\mathrm{normal NO}}_{normal 3}^{-}$ 25pt16ptsvg-formulamathimgdd23f13eb24280cbe650be4567ce8571 bg-16-4555-2019-ie00002.svg25pt16ptbg-16-4555-2019-ie00002.png and ammonium (inline-formula $M11inlinescrollmathmlchem{\mathrm{normal NH}}_{normal 4}^{+}$ 24pt15ptsvg-formulamathimg8cff18dc7544e09830abea500d71300b bg-16-4555-2019-ie00003.svg24pt15ptbg-16-4555-2019-ie00003.png ) 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 inline-formulaN2O-N hainline-formula−1 at sites with rotational grass and 20–50 kg inline-formulaN2O-N hainline-formula−1 at sites with a potato crop. Equivalent soil gas-phase concentrations of inline-formulaN2O at grassland sites varied between 0 and 25 inline-formulaµL Linline-formula−1 except for a sampling after slurry application at one of the sites in spring, with a maximum of 560 inline-formulaµL Linline-formula−1 at the 1 m depth. At the two potato sites the levels of below-ground inline-formulaN2O concentrations ranged from 0.4 to 2270 inline-formulaµL Linline-formula−1 and from 0.1 to 470 inline-formulaµL Linline-formula−1, in accordance with the higher soil mineral N availability at arable sites. Statistical analyses using graphical models showed that soil inline-formulaN2O concentration in the capillary fringe (i.e. the soil volume above the water table influenced by tension saturation) was the strongest predictor of inline-formulaN2O emissions in spring and, for grassland sites, also in the autumn. For potato sites in autumn, there was evidence that inline-formula $M28inlinescrollmathmlchem{\mathrm{normal NO}}_{normal 3}^{-}$ 25pt16ptsvg-formulamathimgda99f0b0265c157ce21f9580f34f8fd2 bg-16-4555-2019-ie00004.svg25pt16ptbg-16-4555-2019-ie00004.png availability in the topsoil and temperature were the main controls on inline-formulaN2O 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 inline-formulaFeS2, and the concentrations of total reactive iron (TRFe) were higher than those of inline-formulaFeS2. Based on the statistical graphical models and the tentative estimates of reduction capacities, inline-formulaFeS2 oxidation was unlikely to be important for inline-formulaN2O emissions. Instead, archaeal ammonia oxidation and either chemodenitrification or nitrifier denitrification were considered to be plausible pathways of inline-formulaN2O production in spring, whereas in the autumn heterotrophic denitrification may have been more important at arable sites.

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Taghizadeh-Toosi, Arezoo / Elsgaard, Lars / Clough, Tim J. / et al: Regulation of N2O emissions from acid organic soil drained for agriculture. 2019. Copernicus Publications.

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