Many alpine forests are severely depleted of nutrients by extensive logging and by grazing of cattle and wildlife. Fertilization may be a remedy, but nitrate leaching may pose problems for the groundwater. However, use of slow-release fertilizers may avoid this problem, whilst improving the Ca, Mg, K, and P status of these soils. Intact soil cores (11 cm diameter, 40 cm depth) from a mixed forest and a Norway spruce stand in the Northern Calcareous Alps of Austria were used in a laboratory experiment to study the effects of adding organic fertilizers and recycling products on patterns of nitrate and ammonium release. The amended (corresponding to 300 kg N ha-1) and control cores were incubated for 29 weeks at 15°C. Soil water (retrieved 5 cm below the soil surface) and leachate were analysed for nitrate and ammonium in regular intervals. After the incubation, soil microbial biomass, basal respiration and nitrogen mineralization were determined. Some of the organic fertilizers, especially those that had undergone composting processes (compost of organic waste, compost of bark + sewage sludge, compost of sawdust + sewage sludge, Biovin®) and Biosol®, caused only minor increases of nitrate and ammonium in the soil water and leachate. Others, like uncomposted sewage sludge products (Primafert® and sewage sludge + shale), resulted in elevated (up to 150 mg nitrate-N1-1) nitrate concentrations in the soil water and in the leachate. Fertilization with N-rich fertilizers (Biosol®, Primafert®, sewage sludge + shale) resulted in significant decreases in microbial biomass and basal respiration at the end of the 6-month incubation. Microbial biomass and basal respiration were not affected by the other organic fertilizers.
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