CHANGES IN SOIL ORGANIC-MATTER AND NET NITROGEN MINERALIZATION IN HEATHLAND SOILS, AFTER REMOVAL, ADDITION OR REPLACEMENT OF LITTER FROM ERICA-TETRALIX OR MOLINIA-CAERULEA

The effects of different litter input rates and of different types of litter on soil organic matter accumulation and net N mineralization were investigated in plant communities dominated by Erica tetralix L. or Molinia caerulea (L.) Moench. Plots in which the litter on the soil had repeatedly been removed were compared with plots in the same plant community in which litter had been added to the soil. In another treatment, litter was removed and replaced by litter from the other plant community. Net N mineralization was measured in situ after 5 years. Less soil organic matter and soil N was found in plots in which litter had been removed, compared with control plots, or plots to which litter had been added, but these differences were significant for the Erica sp. soils only. Plots in which litter had been replaced and control plots did not differ significantly in the amount of soil organic matter. However, in both plant communities, the differences agreed with the faster decomposition rate of Molinia sp. litter compared with Erica sp. litter. The gravimetric soil moisture content was correlated positively with the amount of soil organic matter, both in the Erica sp. soils and the Molinia sp. soils. Net N mineralization rates (g N m-2) differed significantly between treatments for Erica sp. soils but no for Molinia sp. soils. For Erica sp. soils, net N mineralization rates increased with increasing amounts of soil organic matter and soil N. Replacing the litter with Molinia sp. litter (which differs in chemical composition) had no clear additional effect on the net N mineralization rate.