CONSERVATION OF MINERAL NITROGEN IN RESTORED SOILS AT OPENCAST COAL-MINE SITES .1. RESULTS FROM FIELD STUDIES OF NITROGEN TRANSFORMATIONS FOLLOWING RESTORATION

One of the characteristics of soils located within the cores of stockp ile storage mounds at opencast mine sites is their accumulation of amm onium-nitrogen. Two areas of restored land were constructed from soil stockpiled for 3 years; one consisted of mound-surface ('aerobic zone' ) soil, and the other of deeply buried ('anaerobic zone') soil. In tha t constructed from mound-surface soil, concentrations of both ammonium - and nitrate-nitrogen remained fairly stable throughout the first 6 m onths of restoration at about 12-20 mug g-1, but in the site construct ed from deeply buried soil, concentrations of ammonium-N decreased fro m an initial high of 160 to 14 mug N g-1 soil after 14 weeks, and incr eased again to 42 mug N g-1 soil by week 29. In contrast, concentratio ns of nitrate-nitrogen at the latter site increased from an initial 9 mug to a maximum recorded level of 77 mug N g-1 soil by week 14, befor e subsiding to 9 mug N g-1 soil by week 29. Nitrate was considered to have been lost from the restored soils by a combination of leaching an d denitrification, as no vegetation was established at these sites. Af ter a short-term stimulation following restoration, soil microbial bio mass levels remained fairly constant, though soils (up to 3 years afte r restoration) were characterized by a very small ratio of biomass C:o rganic C.