ORGANIC-MATTER ACCUMULATION AND NITROGEN MINERALIZATION DURING SECONDARY SUCCESSION IN HEATHLAND ECOSYSTEMS

Five series of plots in Dutch heathlands were selected in which the above-ground biomass and the litter (L) and humus (FH) layer had been removed 5-50 yr ago. Amounts of organic matter in the L and FH layers increased with age while the communities were dominated by Calluna vulgaris or Erica tetralix. Where dwarf shrubs were replaced by Molinia caerulea, amount of soil organic matter did not further increase. Both above-ground and below-ground biomass in the communities dominated by dwarf shrubs increased with age to c1300 and 800 g m-2, respectively, but were much lower in Molinia-dominated plots (400 and 500 g m-2). Annual N mineralization remained at a low level, or even decreased, during the first 10 yr after turf removal. Thereafter the mineralization rate increased with an increasing amount of organic matter in the L and FH layers. Amount of organic matter in the L and FH layers and the biomass percentage of Molinia together explained 84% of the observed variance in the annual N mineralization. Molinia apparently has a positive effect on the mineralization rate. In communities dominated by Calluna or Erica all the N that entered the ecosystem appeared to accumulate in the plant biomass and the soil organic matter; losses of N from the ecosystem appeared to be negligible. In Molinia-dominated communities part of the N that entered the system was lost to deeper soil layers, to the atmosphere or to herbivores. -from Author