Nitrogen and carbon transformations before and after clear-cutting in repeatedlyN-fertilized and limed forest soil

Nitrogen and carbon transformations were monitored in a Norway spruce (Picea abies L.) stand in the summer before clear-cutting, and for the following three summers. During 30 y before the clear-cutting the stand had been repeatedly limed (total 6 t limestone ha−1), fertilized with N (total about 900 kg N ha−1), and both treatments were combined. Aerobic incubation experiments in the laboratory showed that, before clear-cutting, nitrification took place only in the soil that had been both limed and N-fertilized. Clear-cutting increased soil pH and net formation of mineral N, and initiated nitrification in all soils. These effects were observed throughout the study period. The only exception was the soil that had been both limed and N-fertilized, where the effect of clear-cutting on these N transformations was negligible or even suppressive. Generally, the greatest response in N transformations to clear-cutting was observed in the control soil. There was a small increase in microbial biomass C and N, and C mineralization in the first summer after clear-cutting. Net formation of mineral N correlated positively with pH at a lower pH range (pH 3.9–4.9) and negatively at a higher pH range (pH 4.9–6.9). C mineralization correlated positively with microbial biomass C, but there was no linear relationship between net formation of mineral N and microbial biomass N. C mineralization and net N mineralization were not correlated.