Nitrogen availability and nematode populations in soil and litter after gap formation in a semi-natural beech-dominated forest

Abstract

Small-scale spatial variation in N availability, substrate induced respiration (SIR) and population dynamics of nematodes in mineral soil and beech ( Fagus sylvatica L.) litter were studied along two transects through a newly formed gap and surrounding forest in a semi-natural mixed deciduous forest in Denmark for three years after gap formation. In the litter, C/N ratio dropped, while decomposition rates and numbers of nematodes were stimulated in the gap as compared to under closed canopy. In contrast, the number of nematodes in soil were inhibited in the gap for two years after gap formation, but gradually recovered in the third year. Recovery was most marked in the northern part of the gap centre where dense regeneration of ash ( Fraxinus excelsior L.) occurred. Omnivorous and predatory nematodes became more prevalent one year after gap formation. Effects on trophic groups of nematodes had disappeared by the end of the study period. The maturity index for nematodes was low one year after gap formation and was still lower in the gap than in the surrounding forest at the end of the study period. SIR in soil was lowest in the gap. Results thus indicate a stimulation of decomposers in the litter layer due to formation of the gap and a contrasting inhibition followed by recovery of decomposers in the soil, possibly governed by changes in C input from living roots. Soil C/N ratio, content of soil organic matter (SOM) and monthly nitrification and net N mineralization rates were not affected by a change in forest development phases caused by gap formation. However, seasonal trends and large spatial variation in net N mineralization and nitrification rates were apparent. Soil NH 4–N concentrations dropped markedly at the onset of the growing season in the second year and remained lower in the gap than in the closed forest throughout the growing season, possibly reflecting an increased N demand of the ash regeneration.