The effects of ammonium sulphate deposition and root sinks on soil solution chemistry in coniferous forest soils

The effects of enhanced (NH₄₂SO₄deposition on soil solution cation and anion concentrations and annualionic fluxes were followed using a standardised experimental protocolin six European coniferous forests with contrasting soil types, pollutioninputs and climate. Native soil cores containing a ceramic suction cupwere installed in the field, roofed and watered every two weeks withlocal throughfall or local throughfall with added(NH₄)₂SO₄ at 75 kgNH₄⁺-N ha^-1 a^-1. Livingroot systems were established in half of the lysimeters.Untreated throughfall NH₄⁺-N deposition at thesites ranged from 3.7 to 29 kg ha^-1 a^-1Soil leachates were collected at two weekly intervalsover 12 months and analysed for volume, andconcentrations of major anions and cations. Increasesin soil solution NO₃^- concentrations inresponse to N additions were observed after 4–9months at three sites, whilst one sandy soil with highC:N ratio failed to nitrify under any of thetreatments. Changes in NO₃^- concentrationsin soil solution controlled soil solution cationconcentrations in the five nitrifying soils, withAl³⁺ being the dominant cation in the more acidsoils with low base saturation. The acidification responses ofthe soils to the (NH₄₂SO₄additions were primarily related to the ability of thesoils to nitrify the added NH₄⁺. pH and soiltexture seemed important in controllingNH₄⁺ leaching in response to the treatments,with two less acidic, clay/clay loam sites showingalmost total retention of added NH₄⁺, whilstnearly 75% of the added N was leached asNH₄⁺ at the acid sandy soils. The presenceof living roots significantly reduced soil solutionNO₃^- and associated cation concentrations attwo of the six sites. The very different responses of the sixsoils to increased (NH₄)₂SO₄deposition emphasise that the establishment of N critical loadsfor forest soils need to allow for differences in N storagecapacity and nitrification potential.