Nitrogen mineralization in heathland ecosystems dominated by different plant species

Net N mineralization rates were measured in heathlands still dominated by ericaceous dwarf shrubs (Calluna vulgaris or Erica tetralix) and in heathlands that have become dominated by grasses (Molinia caerulea or Deschampsia flexuosa). Net N mineralization was measured in situ by sequential soil incubations during the year. In the wet area (gravimetric soil moisture content 74-130%), the net N mineralization rates were 4.4 g N m-2 yr-1 in the Erica soil and 7.8 g N m-2 yr-1 in the Molinia soil. The net nitrification rate was negligibly slow in either soil. In the dry area (gravimetric soil moisture content 7-38%), net N mineralization rates were 6.2 g N m-2 yr-1 in the Calluna soil, 10.9 g N m-2 yr-1 in the Molinia soil and 12.6 g N m-2 yr-1 in the Deschampsia soil. The Calluna soil was consistently drier throughout the year, which may partly explain its slower mineralization rate. Net nitrification was 0.3 g N m-2 yr-1 in the Calluna soil, 3.6 g N m-2 yr-1 in the Molinia soil and 5.4 g N m-2 yr-1 in the Deschampsia soil. The net nitrification rate increased proportionally with the net N mineralization rate suggesting ammonium availability may control nitrification rates in these soils. In the dry area, the faster net N mineralization rates in sites dominated by grasses than in the site dominated by Calluna may be explained by the greater amounts of organic N in the soil of sites dominated by grasses. In both areas, however, the net amount of N mineralized per gram total soil N was greater in sites dominated by Molinia or Deschampsia than in sites dominated by Calluna or Erica. This suggests that in heathlands invaded by grasses the quality of the soil organic matter may be increased resulting in more rapid rates of soil N cycling.