Seasonally distinct sources of N2O in acid organic soil drained for agriculture as revealed by N2O isotopomer analysis
Acid organic soils drained for agriculture are hotspots for nitrous oxide (N2O) emissions. Previous studies have indicated that water table (WT) depth and nitrogen (N) availability are important environmental controls, however, little is known about pathways leading to N2O emission. We investigated a raised bog drained for agriculture (pH 4.6 to 5.5) with, respectively, rotational grass and a potato crop in the experimental year and subplots with and without N fertilisation. The emissions and isotopic composition of N2O were monitored during spring and autumn periods with significant changes in WT depth, and in the availability of ammonium (NH4+) and nitrate (NO3-). Nitrous oxide emissions were determined at 6-h intervals using eight automated chambers interfaced with a laser instrument for N2O isotopomer analysis. A threshold of 600 ppb N2O (final concentration) was adopted to stabilise estimates of site preference (SP) and d15Nbulk of emitted N2O against the background of N2O in ambient air. Nitrous oxide emissions varied consistently with land use, slope position and time of day and year, but with little effect of N fertilisation. Isotopic signatures of N2O from the grassland site could not be investigated due to generally low emissions. In the potato field, in total 255 and 266 flux measurements exceeded the 600 ppb threshold in unfertilised and fertilised subplots, respectively. Both N2O emissions and isotopic signatures responded to WT dynamics, individual rain events, and soil N availability. Across eight individual periods, the d15Nbulk of N2O emitted from the soil varied between - 55 and + 18%, while site preference (SP) varied between - 4 and + 25%. The first and the last period (shoulder seasons) were both N limited, and here isotopic signatures of N2O were similar and distinctly different from signatures in midseason periods with mineral N accumulation. Nitrifier denitrification and chemodenitrification were proposed as potential pathways to N2O production in shoulder seasons, whereas in mid-season periods the isotopic signatures could be associated with either ammonia oxidation or fungal denitrication (late spring), or with bacterial denitrification (early autumn). The interpretation of isotopic signatures of N2O was confounded by the possible effect of N2O reduction. The study provides evidence that both sources and pathways of N2O emissions from managed organic soil vary dynamically with WT changes and N availability.