ALBERT

Description: 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 (NO3-). 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 NO3- and ammonium (NH4+) 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 NO3- 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.

Description: Drained organic soils are extensively used for cereal and high-value cash crop production or as grazing land, but emissions of nitrous oxide (N2O) are enhanced by the drainage and cultivation. A study was conducted to investigate the regulation of N2O emissions in a raised bog area drained for agriculture. The area has been classified as potentially acid sulfate soil, and we hypothesised that pyrite oxidation was a potential driver of N2O emissions. Two sites with rotational grass, and two sites with a potato crop, were equipped for monitoring of N2O emissions, as well as sub-soil N2O concentrations at 5, 10, 20, 50 and 100 cm depth, during spring and autumn 2015. Precipitation, air and soil temperature, soil moisture, water table (WT) depth, and soil mineral N were recorded during weekly field campaigns. In late April and early September, intact cores were collected to 1 m depth at adjacent grassland and potato sites for analysis of soil properties, which included acid volatile sulfide (AVS) and chromium-reducible sulfur (CRS) to quantify, respectively, iron monosulfide (FeS) and pyrite (FeS2), as well as total reactive iron (TRFe) and nitrite (NO2−). Soil organic matter composition and total reduction capacity was also determined. The soil pH varied between 4.7 and 5.4. Equivalent soil gas phase concentrations of N2O ranged from around 10 µL L−1 at grassland sites to several hundred µL L−1 at potato sites, in accordance with lower soil mineral N concentrations at grassland sites. Total N2O emissions during 152–174 days were 3–6 kg N2O-N ha−1 for rotational grass, and 19–21 kg N2O-N ha−1 for potato sites. Statistical analyses by graphical models showed that soil N2O concentration in the capillary fringe was the strongest predictor for N2O emissions in spring, and for grassland sites also in the autumn. For potato sites in the autumn, nitrate (NO3−) availability in the top soil, together with temperature, were the main controls on N2O emissions. Pyrite oxidation coupled with NO3− reduction could not be dismissed as a source of N2O, but the total reduction capacity of the peat soil was much higher than explained by the FeS2 concentration. The concentrations of TRFe were also much higher than pyrite concentrations, and potentially chemodenitrification could have been a source of N2O during WT drawdown in spring. The N2O emissions associated with rapid soil wetting and WT rise in autumn were consistent with biological denitrification. Soil N availability and seasonal WT changes were important controls of N2O emissions.