Conservation of mineral nitrogen in restored soils at opencast coal mine sites: II. The effects of inhibition of nitrification and organic amendments on nitrogen losses and soil microbial biomass

Soils stored in stockpiles during opencast mining operations accumulate significant quantities of ammonium (of the order of 200 μg NH₄⁺-N g^?1 soil) within the predominantly anaerobic cores of mounds. Upon stockpile dismantling and land restoration, this NH⁺₄-N is rapidly oxidized to NO^?₃-N, which is readily lost from newly restored soil ecosystems by leaching and denitrification. Experiments were set up to examine how these significant reserves of mineral N might be conserved in such situations. Application of the nitrification inhibitor dicyandiamide was successful in minimizing NO₃^?-N lost by leaching, though large concentrations of NH₄⁺-N were detected in drainage waters. Straw incorporation decreased nitrate leaching by up to 40%; biomass C was some 40% greater in straw-amended than in unamended soils after 14 weeks, though biomass N was similar in both. Addition of nitrogen-free organic materials (glucose, starch and cellulose) produced different results, with glucose amendment showing the greatest reduction in nitrate leaching in the short term (due to an apparent stimulation of denitrification) whereas addition of cellulose resulted in the most effective conservation of nitrogen over 14 weeks; this was due, at least in part, to uptake of mineral N by the soil microbial biomass.