Organic matter accumulation and nitrogen mineralization during secondary succession in heathland ecosystems.

SUMMARY (1) Five series of plots in heathlands were selected in which the above-ground biomass and the litter (L) and humus (FH) layer had been removed between one and fifty years ago. Secondary succession in these heathlands was studied by comparing the amounts of soil organic matter, above-gound biomass, below-ground biomass and the annual nitrogen mineralization in plots of different ages. (2) The 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 the amount of soil organic matter did not further increase. (3) Both above-ground and below-ground biomass in the communities dominated by dwarf shrubs increased with age to about 1300 and 800 g m-2, respectively, but were much lower in the Molinia-dominated plots (c. 400 and 500 g m-2). (4) The annual nitrogen mineralization remained at a low level, or even decreased, during the first ten years after turf removal. Thereafter the mineralization rate increased with an increasing amount of organic matter in the L and FH layers. Multiple regression analysis revealed that the 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 nitrogen mineralization. It is proposed that Molinia has a positive effect on the mineralization rate. (5) In communities dominated by Calluna or Erica all the nitrogen that entered the ecosystem appeared to accumulate in the plant biomass and the soil organic matter, whereas losses of nitrogen from the ecosystem appeared to be negligible. In Moliniadominated communities part of the nitrogen that entered the system was lost to deeper soil layers, to the atmosphere or to herbivores.

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