ALBERT

Description: 〈b〉Regulation of N〈sub〉2〈/sub〉O emissions from acid organic soil drained for agriculture: Effects of land use and season〈/b〉〈br〉 Arezoo Taghizadeh-Toosi, Lars Elsgaard, Tim J. Clough, Rodrigo Labouriau, Vibeke Ernstsen, and Søren O. Petersen〈br〉 Biogeosciences Discuss., https//doi.org/10.5194/bg-2019-14,2019〈br〉 〈b〉Manuscript under review for BG〈/b〉 (discussion: open, 0 comments)〈br〉 〈p〉Drained organic soils are extensively used for cereal and high-value cash crop production or as grazing land, but emissions of nitrous oxide (N〈sub〉2〈/sub〉O) are enhanced by the drainage and cultivation. A study was conducted to investigate the regulation of N〈sub〉2〈/sub〉O 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 N〈sub〉2〈/sub〉O emissions. Two sites with rotational grass, and two sites with a potato crop, were equipped for monitoring of N〈sub〉2〈/sub〉O emissions, as well as sub-soil N〈sub〉2〈/sub〉O 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 (FeS〈sub〉2〈/sub〉), as well as total reactive iron (TRFe) and nitrite (NO〈sub〉2〈/sub〉〈sup〉−〈/sup〉). 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 N〈sub〉2〈/sub〉O ranged from around 10 µL L〈sup〉−1〈/sup〉 at grassland sites to several hundred µL L〈sup〉−1〈/sup〉 at potato sites, in accordance with lower soil mineral N concentrations at grassland sites. Total N〈sub〉2〈/sub〉O emissions during 152–174 days were 3–6 kg N〈sub〉2〈/sub〉O-N ha〈sup〉−1〈/sup〉 for rotational grass, and 19–21 kg N〈sub〉2〈/sub〉O-N ha〈sup〉−1〈/sup〉 for potato sites. Statistical analyses by graphical models showed that soil N〈sub〉2〈/sub〉O concentration in the capillary fringe was the strongest predictor for N〈sub〉2〈/sub〉O emissions in spring, and for grassland sites also in the autumn. For potato sites in the autumn, nitrate (NO〈sub〉3〈/sub〉〈sup〉−〈/sup〉) availability in the top soil, together with temperature, were the main controls on N〈sub〉2〈/sub〉O emissions. Pyrite oxidation coupled with NO〈sub〉3〈/sub〉〈sup〉−〈/sup〉 reduction could not be dismissed as a source of N〈sub〉2〈/sub〉O, but the total reduction capacity of the peat soil was much higher than explained by the FeS〈sub〉2〈/sub〉 concentration. The concentrations of TRFe were also much higher than pyrite concentrations, and potentially chemodenitrification could have been a source of N〈sub〉2〈/sub〉O during WT drawdown in spring. The N〈sub〉2〈/sub〉O 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 N〈sub〉2〈/sub〉O emissions.〈/p〉