Formation of mineral N (NH4+, NO3—) during mineralization of organic matter from coal refuse material and municipal sludge

In 1994, the 14 hectare plateau of a coal refuse bank in Landsweiler‐Reden (Southwest‐Germany) was covered with a mixture consisting of 80% (v/v) refuse materials, 10% (v/v) composted wood and 10% (v/v) of sewage sludge as part of a reclamation project. The amount of sludge dry matter applied was approximately 450 Mg ha⁻¹ to a depth of 2 m. The approximate amount of nitrogen (N) applied with the substrate was 20 Mg ha⁻¹ (total N). From April 1996 until November 1997, contents of mineral nitrogen and nitrogen mineralization were monitored down to a depth of 2 m. Nitrogen mineralization was monitored by means of a modified buried bag procedure using a retrievable cylindrical receptacle. The contents of NH₄⁺‐ and NO₃⁻‐N were largest at the beginning of the observation period, reaching a peak value of 650 kg ha⁻¹ in May 1996. Then, mineral N stabilized in 1997 at a level of 200 kg N ha⁻¹, with the soil profile below 150 cm contributing about 75% to this amount. Net nitrogen mineralization was characterized by the same depth distribution. Other than in surface horizons, mineralization activity at the bottom of the profile continued into 1997 with the same intensity as in 1996. Variability among replicate buried bag incubations was high (CV > 100% on several occasions). Nitrogen loss through leaching was estimated at 630 kg N ha⁻¹ over the observation period, averaging at 360 kg N ha⁻¹ a⁻¹. The reclamation procedure used in this study may have the potential to contaminate ground water in hydrologically sensitive areas.