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 depo sition 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 recipro cally transplanted between sites, with different physical conditions, across a gradient of N and S deposition in Europe. The transfer of a s ingle soil to the various sites affected its net nitrification, This w as not simply due to the nitrification of different levels of N deposi tion but was explained by differences in physical climates which influ enced mineralization rates. Variation in the amount of net nitrificati on between soil types at a specific site were explained largely by soi l pH. Within a site all soil types showed similar trends in net nitrif ication over time. Seasonal changes in net nitrification corresponds t o oscillations in temperature but variable time lags had to be introdu ced 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 micr obial 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 s oil fauna-microbial interactions combined with improved moisture condi tions. Losses of NO3- were associated with Ca2+ and Mg2+ in non-acidif ied soil types and with losses of Al3+ in the acidified soils. For sin gle soils the ion equilibrium equation of Gaines-Thomas provided a use ful approximation of Al3+ concentrations in the soil solution as a fun ction of the concentration of Ca2+. The between site deviations from t his predicted equilibrium, which existed for single soils, could be ex plained by differences in throughfall chemistry which affected the tot al ionic strength of the soil solution. The approach of reciprocally t ransferring soil columns highlighted the importance of throughfall che mistry, interacting with the effect of changes in physical climate on forest soil acidification through internal proton production, in deter mining soil solution chemistry. A framework outlining the etiology of forest die-back induced by nitrogen saturation is proposed.