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

The effects of enhanced (NH4)(2)SO4 deposition on soil solution cation and anion concentrations and annual ionic fluxes were followed using a standardised experimental protocol in six European coniferous forests with contrasting soil types, pollution inputs and climate. Native soil cores containing a ceramic suction cup were installed in the field, roofed and watered every two weeks with local throughfall or local throughfall with added (NH4)(2)SO4 at 75 kg NH4+-N ha(-1) a(-1). Living root systems were established in half of the lysimeters. Untreated throughfall NH4+-N deposition at the sites ranged from 3.7 to 29 kg ha(-1) a(-1). Soil leachates were collected at two weekly intervals over 12 months and analysed for volume, and concentrations of major anions and cations. Increases in soil solution NO3- concentrations in response to N additions were observed after 4-9 months at three sites, whilst one sandy soil with high C:N ratio failed to nitrify under any of the treatments. Changes in NO3- concentrations in soil solution controlled soil solution cation concentrations in the five nitrifying soils, with Al3+ being the dominant cation in the more acid soils with low base saturation. The acidification responses of the soils to the (NH4)(2)SO4 additions were primarily related to the ability of the soils to nitrify the added NH4+. pH and soil texture seemed important in controlling NH4+ leaching in response to the treatments, with two less acidic, clay/clay loam sites showing almost total retention of added NH4+, whilst nearly 75% of the added N was leached as NH4+ at the acid sandy soils. The presence of living roots significantly reduced soil solution NO3- and associated cation concentrations at two of the six sites. The very different responses of the six soils to increased (NH4)(2)SO4 deposition emphasise that the establishment of N critical loads for forest soils need to allow for differences in N storage. capacity and nitrification potential.