Regulation of N₂O emissions from acid organic soil drained for agriculture: Effects of land use and season

Organic soils are extensively under agricultural management for cereal and high-value cash crop production or as grazing land. Drainage and tillage is known to promote emissions of nitrous oxide (N2O), however, a previous monitoring program found that, in addition to effects of land use, annual N2O emissions from fields with rotational grass and potato showed distinct seasonal patterns. A new study was therefore conducted to investigate the regulation of N2O emissions in an area with raised bog and which was previously classified as a potentially acid sulfate soil. Four sites, i.e., two sites with rotational grass and two with a potato crop, were equipped for weekly monitoring of soil surface N2O emissions and sub-soil N2O concentrations to 1 m depth during spring and autumn 2015. Also, various environmental variables (precipitation, air and soil temperature, soil moisture, water table (WT) depth, and soil mineral N) were recorded. In late April and early September 2015, intact cores to 1 m depth were further collected at adjacent grassland and potato sites and analysed for pH, EC, nitrite (NO2−), total reactive Fe (TRFe), acid volatile S (AVS) and chromium-reducible S (CRS). The soil pH varied between 4.6 and 5.5. Total N2O emissions during 152–174 days were 4–10 kg N2O ha−1 for rotational grass, and 30–32 kg N2O ha−1 for arable sites with a potato crop. Soil N2O concentrations ranged from around 10 µL L−1 at grassland sites to several hundred µL L−1 at 50–100 cm depth at sites with potato. This reflected lower soil mineral N concentrations at grassland sites where probably competition from plants for available N was effective. Fertilisation had no immediate effect on N2O emissions, but effects appeared in connection with rainfall where the WT also rose toward the soil surface and N2O accumulated in the soil profile at all sites. Graphical models showed that the strongest predictor for N2O emissions from both grassland and potato sites in spring, and grassland sites in autumn, was soil N2O concentration near the WT depth. In contrast, for potato sites in autumn, nitrate (NO3−) in the top soil, together with temperature, controlled N2O emissions. The distribution of TRFe and NO2− in soil profiles suggested that chemodenitrification in the capillary fringe could be a significant source of N2O during WT drawdown in spring, while N2O emissions associated with the rapid soil wetting and WT rise in autumn may be attributed to biological denitrification. The concentration of TRFe in soil profiles was related to soil organic carbon, and much higher than concentrations of AVS, and thus iron oxides/hydroxides rather than iron sulfides were probably the source of TRFe. Controlling seasonal WT dynamics and soil mineral N accumulation appear to be important controls of N2O emissions in acid organic soil used for agriculture.