EFFECTS OF 3 RATES OF ATMOSPHERIC NITROGEN DEPOSITION ENRICHED WITH N-15 ON LITTER DECOMPOSITION IN A HEATHLAND

The decomposition and nitrogen dynamics of litter were investigated at three rates of N deposition, using N-15 enriched (NH4)2SO4 applied in artificial rain (0.5, 2.0 and 8.0 g N m-2 in 11 months). The experiment was done in the field under transparent roofs, on leaf litter from Calluna vulgaris (L.) Hull, Molinia caerulea (L.) Moench and Deschampsia flexuosa (L.) Trin. These leaf litters differed in their contents of nitrogen, carbon and recalcitrant carbon compounds. All the litters increased in their delta-N-15 value markedly, the increases being greater as the rate of N deposition increased. However, the rates at which Molinia or Calluna leaves decomposed remained unaffected. The decomposition rate of Deschampsia leaved increased only temporarily at the highest rate of N deposition. Deschampsia leaves decomposed fastest; this suggests that the amount of carbon usuable by micro-organisms determines whether an additional amount of N will accelerate the decomposition process. The changes in the delta-N-15 values and total N contents of the litter indicated that Calluna and Deschampsia litter simultaneously retain and release N. The N released originated at least partly from the initial litter. In contrast, all the N (including the N that was retained from the rain) seemed to remain bound in the litter-microbial biomass complex of Molinia.