EFFECTS OF EXPERIMENTAL ACIDIFICATION ON C AND N MINERALIZATION IN FOREST SOILS

Diluted H2SO4 and S powder were used to simulate the effects of acidic deposition on C and N mineralization. Short-term effects were studied after treatment of litter, humus and mineral soil materials in the laboratory, while long-term effects were studied in humus and mineral soil layers from two field sites. The acid additions were terminated 4 years before the start of the study on one site and 11 years before the study on the other. The acid treatment always reduced C mineralization when pH (H2O) was reduced. Despite a total recovery of pH and base saturation in the site treated 11 years before sampling, C mineralization still remained suppressed, indicating long-term effects on the soil organisms. The acid treatment caused an initial increase in net N mineralization followed by a decrease with time. Mineralized N from killed soil organisms could explain the initial increase in net N mineralization. In the '4-year-site', the net N mineralization decreased almost in parallel with the C mineralization, while in the '11-year-site', the net N mineralization was considerably higher than expected from the C mineralization data. A possible explanation is that the acid treatment in the latter site changed the microbial community in such a way that organic matter fractions with high C:N ratios were less readily decomposed than those with low C:N ratios. The acid treatment had increased the pool of organic C in the upper soil horizons in the 11-year-site but this had not yet occurred in the 4-year-site. Independent data showed increased tree growth during the period of acid treatment, indicating a direct growth response to the increase in N supply.