Effects of air pollutant-temperature interactions on mineral-N dynamics and cation leaching in reciplicate forest soil transplantation experiments

Increased emissions of nitrogen compounds have led to atmospheric deposition to forest soils exceeding critical loads of N over large parts of Europe. To determine whether the chemistry of forest soils responds to changes in throughfall chemistry, intact soil columns were reciprocally transplanted between sites, with different physical conditions, across a gradient of N and S deposition in Europe. The transfer of a single soil to the various sites affected its net nitrification. This was not simply due to the nitrification of different levels of N deposition but was explained by differences in physical climates which influenced mineralization rates. Variation in the amount of net nitrification between soil types at a specific site were explained largely by soil pH. Within a site all soil types showed similar trends in net nitrification over time. Seasonal changes in net nitrification corresponds to oscillations in temperature but variable time lags had to be introduced to explain the relationships. With Arrhenius' law it was possible to approximate gross nitrification as a function of temperature. Gross nitrification equalled net nitrification after adaptation of the microbial community of transplanted soils to the new conditions. Time lags, and underestimates of gross nitrification in autumn, were assumed to be the result of increased NH₄ ⁺ availability due either to changes in the relative rates of gross and net N transformations or to altered soil fauna-microbial interactions combined with improved moisture conditions. Losses of NO₃/^- were associated with Ca²⁺ and Mg²⁺ in non-acidified soil types and with losses of Al³⁺ in the acidified soils. For single soils the ion equilibrium equation of Gaines-Thomas provided a useful approximation of Al³⁺ concentrations in the soil solution as a function of the concentration of Ca²⁺. The between site deviations from this predicted equilibrium, which existed for single soils, could be explained by differences in throughfall chemistry which affected the total ionic strength of the soil solution. The approach of reciprocally transferring soil columns highlighted the importance of throughfall chemistry, interacting with the effect of changes in physical climate on forest soil acidification through internal proton production, in determining soil solution chemistry. A framework outlining the etiology of forest die-back induced by nitrogen saturation is proposed.