The effects of H2SO4 and (NH4)2SO4 treatments on the chemistry of soil drainage water and pine seedlings in forest soil microcosms

Abstract. An experiment comparing effects of sulphuric acid and reduced N deposition on soil water quality and on chemical and physical growth indicators for forest ecosystems is described. Six H₂SO₄ and (NH₄)₂SO₄ treatment loads, from 0 – 44 and 0 – 25 kmol_c ha^-1 yr^-1, respectively, were applied to outdoor microcosms of Pinus sylvestris seedlings in 3 acid to intermediate upland soils (calc-silicate, quartzite and granite) for 2 years. Different soil types responded similarly to H₂SO₄ loads, resulting in decreased leachate pH, but differently to reduced N inputs. In microcosms of calc-silicate soil, nitrification of NH₄ resulted in lower pH and higher cation leaching than in acid treatments. By contrast, in quartzite and granite soils, (NH₄)₂SO₄ promoted direct cation leaching, although leachate pH increased. The results highlighted the importance of soil composition on the nature of the cations leached, the SO₄ adsorption capacities and microbial N transformations. Greater seedling growth on calc-silicate soils under both treatment types was related to sustained nutrient availability. Reductions in foliar P and Mg with higher N treatments were observed for seedlings in the calc-silicate soil. There were few treatment effects on quartzite and granite microcosm tree seedlings since P limitation precluded seedling growth responses to treatments. Hence, any benefits of N deposition to seedlings on quartzite and granite soils appeared limited by availability of co-nutrients, exacerbated by rapid depletion of soil exchangeable base cations.

Keywords: acidification, manipulation, nitrogen, ammonium, deposition, soil, drainage, pine, microcosms, forest