CONSERVATION OF MINERAL NITROGEN IN RESTORED SOILS AT OPENCAST COAL-MINE SITES .2. THE EFFECTS OF INHIBITION OF NITRIFICATION AND ORGANIC AMENDMENTS ON NITROGEN LOSSES AND SOIL MICROBIAL BIOMASS

Soils stored in stockpiles during opencast mining operations accumulat e significant quantities of mmonium (of the order of 200 mug NH4+-N g- 1 soil) within the predominantly anaerobic cores of mounds. Upon stock pile dismantling and land restoration, this NH4+-N is rapidly oxidized to NO3--N, which is readily lost from newly restored soil ecosystems by leaching and denitrification. Experiments were set up to examine ho w these significant reserves of mineral N might be conserved in such s ituations. Application of the nitrification inhibitor dicyandiamide wa s successful in minimizing NO3--N lost by leaching, though large conce ntrations of NH4+-N were detected in drainage waters. Straw incorporat ion decreased nitrate leaching by up to 40%; biomass C was some 40% gr eater in straw-amended than in unamended soils after 14 weeks, though biomass N was similar in both. Addition of nitrogen-free organic mater ials (glucose, starch and cellulose) produced different results, with glucose amendment showing the greatest reduction in nitrate leaching i n the short term (due to an apparent stimulation of denitrification) w hereas addition of cellulose resulted in the most effective conservati on of nitrogen over 14 weeks; this was due, at least in part, to uptak e of mineral N by the soil microbial biomass.