Major nutrients and acidity: budgets and trends at four remote boreal stands in Finland during the 1990s.

Major nutrients and acidity were studied at six plots in four Integrated Monitoring catchments in Finland, located along a south-north deposition gradient. The quality and quantity of bulk precipitation, throughfall, soil water, and litterfall were monitored during the period 1989-1997. The results showed a marked decline in SO4 and H+ concentrations overall. Trends in throughfall ANC values were positive, while those in soil water were mostly negative. The increase in throughfall ANC values indicates recovery from acid deposition. The adsorption of base cations and release of organic acids was implicated in the decrease in soil water ANC trends. For all variables, the changes tended to be the greatest in the most southerly catchment, which received the most deposition loads. Mass balance budgets showed that total deposition inputs were greater than soil leaching outputs (at 40 cm) for NO3, NH4 and SO4, and were attributed to nutrient uptake and cycling processes. In contrast, for Ca, Mg, K and Na, and S (Valkea-Kotinen3 and Vuoskojärvi2 plots only), inputs were less than outputs, indicating net leaching. Throughfall + litterfall inputs to the forest floor showed that the organic layer is particularly important in the retention of Ca, Mg and N. Sulfur was retained in the lower soil layers, and attributed to Al and Fe hydroxide adsorption in the B horizon.