Experimental assessment of N2O background fluxes in grassland systems

In the absence of, or between, fertilization events in agricultural systems, soils are generally assumed to emit N₂O at a small rate, often described as the ‘background’ flux. In contrast, net uptake of N₂O by soil has been observed in many field studies, but has not gained much attention. Observations of net uptake of N₂O form a large fraction (about half) of all individual flux measurements in a long-term time series at our temperate fertilized grassland site. Individual uptake fluxes from chamber measurements are often not statistically significant but mean values integrated over longer time periods from days to weeks do show a clear uptake. An analysis of semi-continuous chamber flux data in conjunction with continuous measurements of the N₂O concentration in the soil profile and eddy covariance measurements suggests that gross production and gross consumption of N₂O are of the same order, and as consequence only a minor fraction of N₂O molecules produced in the soil reaches the atmosphere.