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 (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.
Natural Nitrogen Isotope Ratios as a Potential Indicator of N2O Production Pathways in a Floodplain Fen
