Significance of nitrification and vegetation uptake in proton budgets in forest surface soil

Abstract

We calculated the annual proton budgets of surface soils (O and A horizons) in a deciduous forest (Df site) and a coniferous forest (Cf site) of Volcanogenous Regosols in Tomakomai, Hokkaido, Japan. To estimate the nitrification and nutrient uptake by vegetation, we took soil leachates by using tension-free lysimeters under the conditions excluding and including vegetation uptake. It was suggested that the method of determining the vegetation, uptake by subtracting the net flux in the horizon with vegetation uptake from the net flux in the horizon without vegetation uptake was appropriate, after comparing the calculation results obtained using growth increment in terms of nitrogen uptake. Nitrate was the major nitrogen source accounting for 97 to 96% of the total nitrogen uptake due to the remarkable nitrification activity of the Tomakomai soil. The total H+ source amounted to 13.1 and 11.5 kmolc ha-1 y-1 at the Df site and Cf site, respectively. The inflow amount of H+ from throughfall and stemflow to the O horizon was negligible and the internal H+ sources in the O and A horizons accounted for 99% of the total H+ sources at the Df site and 98% at the Cf site. Base cation uptake by vegetation and nitrification were quantitatively important H+ sources. On the other hand, weathering and ion exchange of base cations as well as NO3- uptake by vegetation were the major H+ sinks. The H+ outflow from surface soil was negligible.