The effects of stem girdling on biogeochemical cycles within a mixed deciduous forest in eastern Tennessee : II. Soil nitrogen mineralization and nitrification rates.

Nitrogen mineralization and net nitrification rates were 3-7 times greater in soil incubations from a girdled Liriodendron tulipifera (L.) stand than in a control stand. Neither litter nor root extracts had an inhibitory effect on nitrogen mineralization or nitrification rate. A lack of nitrification inhibitors also was demonstrated by the fact that ammonium added to the control stand was completely converted to nitrate upon incubation. Additions of sucrose increased CO2 evolution and decreased nitrogen mineralization and nitrification rates in the girdled plot soil, suggesting that nitrification could be effectively controlled by competition for NH 4+ supplies by heterotrophic soil organisms. CO2 evolution rates during incubation showed that heterotrophic as well as nitrifier activities were greater in the girdled plot soil than in the ungirdled plot soil, but the ratio of C to N mineralized was lower in the girdled plot soil. These results collectively indicate that nitrification is regulated by the availability of NH 4+ in these stands, and that the latter is strongly regulated by heterotrophic demand for N.