Soil solution nitrogen and cations influenced by (NH4)2SO4 deposition in a coniferous forest

The effects of chronically enhanced (NH4)2SO4 deposition on ion concentrations in soil solution and ionic fluxes were investigated in a Picea abies plot at Grizedale forest, NW England. Soil cores closed at the base and containing a ceramic suction cup sampler were ‘roofed’ and watered every 2 weeks with bulk throughfall collected in the field. Treatments consisted of the inclusion of living roots from mature trees in the lysimeters and increasing (NH4)2SO4 deposition (NS treatment) to ambient + 75 kg N ha−1 a−1. Rainfall, throughfall and soil solutions were collected every 2 weeks during 18 months, and analysed for major cations and anions. NO3− fluxes significantly increased following NS treatment, and were balanced by increased Al3+ losses. Increased SO42− concentrations played a minor role in controlling soil solution cation concentrations. The soil exchange complex was dominated by Al and, during the experimental period, cores of all treatments ‘switched’ from Ca2+ to Al3+ leaching, leading to mean Ca/Al molar ratios in soil solution of NS treated cores of 0.24. The experiment confirmed that the most sensitive soils to acidification (through deposition or changing environmental conditions) are those with low base saturation, and with a pH in the lower Ca, or Al buffer ranges.