Nutrient dynamics along the forest floor – mineral soil continuum

Slow turnover of the forest floor (FF) is often assumed to be related to immobilization of nutrients within the organic matter. However, the FF is also assumed to be an important nutrient source at sites with low nutrient concentrations of the mineral soil. Climate change could threaten FF mediated tree nutrition due to higher turnover rates of the FF. Yet, little is known about on the (de)coupling of nutrient and FF mass turnover and their controls, nor about future responses to a changing climate.

Within the research unit FOREST FLOOR (FOR 5315) we aim to identify processes that control the relevance of the FF for tree nutrition as compared to the mineral topsoil. We test the hypotheses that (1) at nutrient poor sites the turnover of C is lower than the turnover of P and N, (2) admixture of minerals to FF material and subsequent biotic formation of mineral-organic associations decreases the pool of organically bound P+N and of easily mobilizable P+N, and (3) that in Norway spruce (Picea abies), FF will contribute more to N and P uptake than in maple (Acer pseudoplatanus), while beech (Fagus sylvatica) will take an intermediate position.

Within a natural P and temperature gradient (12 sites), we will test our hypotheses by studying availability to plants and mobilization kinetics of the macronutrients from FF and A horizons using ion exchange resins and membranes. Preliminary tests showed a good suitability of exchanger-resin application to assess P – availability. Moreover, we will benefit from litter bag experiments, and analyze the mobilization of N from labeled litter in a mesocosm approach. Our results will reveal the role of FF for the nutrition of beech, maple and spruce depending on the nutrient status of the mineral soil as well as how climate change is impacting tree nutrition services of the FF.