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 atmosphericdeposition to forest soils exceeding critical loads of N overlarge parts of Europe. To determine whether the chemistry offorest soils responds to changes in throughfall chemistry, intactsoil columns were reciprocally transplanted between sites, withdifferent physical conditions, across a gradient of N and Sdeposition in Europe.The transfer of a single soil to the various sites affected itsnet nitrification. This was not simply due to the nitrificationof different levels of N deposition but was explained bydifferences in physical climates which influenced mineralizationrates. Variation in the amount of net nitrification between soiltypes at a specific site were explained largely by soil pH.Within a site all soil types showed similar trends in netnitrification over time. Seasonal changes in net nitrificationcorresponds to oscillations in temperature but variable time lagshad to be introduced to explain the relationships. WithArrhenius law it was possible to approximate gross nitrificationas a function of temperature. Gross nitrification equalled netnitrification after adaptation of the microbial community oftransplanted soils to the new conditions. Time lags, andunderestimates of gross nitrification in autumn, were assumed tobe the result of increased NH₄⁺ availability due either tochanges in the relative rates of gross and net N transformationsor to altered soil fauna-microbial interactions combined withimproved moisture conditions.Losses of NO₃^- were associated with Ca²⁺and Mg²⁺ in non-acidified soil types and with losses ofAl³⁺ in the acidified soils. For single soils the ionequilibrium equation of Gaines-Thomas provided a useful approximationof Al³⁺ concentrations in the soil solution as a functionof the concentration of Ca²⁺. The between site deviationsfrom this predicted equilibrium, which existed for single soils, couldbe explained by differences in throughfall chemistry which affectedthe total ionic strength of the soil solution.The approach of reciprocally transferring soil columnshighlighted the importance of throughfall chemistry, interactingwith the effect of changes in physical climate on forest soilacidification through internal proton production, in determiningsoil solution chemistry. A framework outlining the etiology offorest die-back induced by nitrogen saturation is proposed.