Soil N and C transformations in two forest clear-cuts during three years after mounding and inverting

Abstract

Transformations of nitrogen and carbon in the humus layer were studied in central Finland on two clear-cut forest sites for 3 yr after spot mounding and inverting together with an untreated control. In laboratory-incubation experiments, samples from the humus layer were measured for rates of C mineralization, net N mineralization and net nitrification, and the amounts of C and N in the microbial biomass were determined. Soil solutions were collected for 4 yr with suction lysimeters. Overall, site preparation did not affect the rate of net N mineralization significantly (per kg organic matter). In the first growing season, however, the mean rate of net N mineralization was higher in both site preparation treatments than in the untreated control. At first, net nitrification was negligible in all treatments, but in the second year it increased in both site preparation treatments. On the other site, C mineralization was lowest with inverting, but the amounts of microbial C and N were highest. This indicates a shortage of easily mineralizable C sources in inverted spots. In the soil solution collected below the humus layer, increased concentrations of NO3-N and total N were found in both site-preparation treatments but, during the fourth year, overall concentrations of N had declined. Key words: Boreal forest, clear-cut, N transformations, site preparation, soil solution