Gross ammonification, gross nitrification and N net mineralization in temperate forest soils at low temperatures

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The temperature dependency of N turnover at low soil temperatures is poorly known. In the past, N mineralization in temperate forest soils at low soil temperatures in the dormant season was often considered negligible, which is questioned by recent findings. Here, we will study rates of in situ N net mineralization throughout a full year in a beech and a spruce forest ecosystem to quantify the relevance of winterly processes at the annual scale. In laboratory experiments with undisturbed soil samples at constantly low temperatures (+8, +5, +2, -1, -4 °C) we will investigate the temperature dependency of gross ammonification, gross nitrification and immobilization in different soil horizons from both sites using the 15N dilution technique. To test the hypothesis that substrate quantity and quality influence the temperature dependency of these processes, different substrates (NH4 in case of gross nitrification; glycine, proline and 2,6-pyridindicarbonic acid in case of gross ammonification) will be added. Temperature dependencies will be quantified using the Arrhenius equation. The results of this project will be of special relevance when predicting effects of future climate change on the N cycle in forest soils.