Atmospheric deposition and sulphur cycling in chalk grasslandA mechanistic model simulating field observations

Abstract

Sulphate fluxes in bulk deposition, throughfall and soil solution were monitored during two years, and integrated within a model describing the cycling of S in a chalk grassland ecosystem. Throughfall fluxes were strongly determined by interceptive properties of the grassland canopy. Seasonal variation in Leaf Area Index resulted in dry deposition velocities for SO₂ varying between 0.1 cm.s⁻¹ (snow cover, almost no aerodynamic resistance) to 0.9–1.8 cm.s⁻¹ in periods with a fully developed canopy. On an annual basis net canopy exchange (assimilation of SO₂ minus foliar leaching) was estimated to be −15% of net throughfall. Simulated soil solution concentrations, being the result of throughfall input, leaching, adsorption, biomass uptake and mineralization, closely fitted actual values (r > 0.92; p > 0.001). Actual and simulated leaching were 1.74 ± 0.03 and 2.00 keq.-ha⁻¹.yr⁻¹, respectively. Sulphur budgets for the soil showed net accumulation from April to October and net losses from October to April. Annual budgets for the ecosystem showed atmospheric input (2.02keq.ha⁻¹.yr⁻¹) and actual output (2.05keq.ha⁻¹.yr⁻¹) to be almost balanced. Apart from increased soil solution concentrations, additional input of sulphate (3.55 keq.ha⁻¹.yr⁻¹) to experimental plots resulted in additional accumulation in the ecosystem of 0.62 keq.ha⁻¹.yr⁻¹