Long-term measurements of NH3 and SO2 fluxes at three contrasting grasslands by time averaged gradients (TAG)

The Time Averaged Gradient (TAG) approach was applied to measure long-term dry net exchange fluxes of NH3 and SO2 to three contrasting temperate grasslands in the UK. The measurements were made at: Auchencorth Moss, an acid peatland site with little grazing; Plynlimon, an upland grassland on mixed soil and significant sheep grazing; Easter Bush, an intensively managed permanent grassland with N fertilisation, used for silage and cattle and sheep grazing. A data series of five years, from 1999 to 2003, for SO2 and a data series of two years, from 2001 to 2003 for NH3 was obtained at Auchencorth Moss. The measurements provided a characterisation of the chemical climate, meteorology, turbulent characteristics, as well as deposition-emission fluxes at the field site. At Plynlimon, fluxes of SO2 and NH3 were measured from June 2000 to September 2003. A refined TAG system was implemented to improve estimates of gradients and reduce uncertainty on the fluxes, and was applied at Easter Bush measuring NH3 fluxes from autumn 2001 to spring 2003, and at Auchencorth Moss from autumn 2004 to date. The TAG systems provided long-term series of average net fluxes of SO2 at -7.04 ng m-2 s-1 and -3.4 ng m-2 s-1 at Plynlimon and Auchencorth Moss, respectively. The data do not permit accurate description of short term fluxes, but provide long term estimates of S dry deposition for comparison between sites. For ammonia the dry deposition fluxes were on average -7.6, -5.98, and -12.2 ng m-2 s-1 for Auchencorth Moss, Plynlimon and Easter Bush respectively, indicating differences in NH3 concentrations and exchange processes. Several discrepancies in NH3 flux between sites reflect the interaction of temperature, rain and different grassland managing regimes.