Resin extractions from forest floors reveal tree specific adaptation to the phosphorus status of the mineral soil in European beech forests

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 in the mineral soil. Yet, little is known about the availability of nutrients present in the FF and how it is related to FF turnover.

Within the DFG-funded Research Unit FOREST FLOOR we identify processes that control the relevance of the FF for tree phosphorus (P) nutrition as compared to the mineral topsoil in European beech (Fagus sylvatica) forests with admixtures of spruce (Picea abies) and maple (Acer pseudoplatanus). We quantified plant-available P at lab conditions using resin exchangers in the FF and the mineral topsoil up to 20 cm along four elevation-related temperature gradients of different soil P status. We hypothesize that the FF gains in importance for P nutrition with decreasing P status of the mineral soil.

Results show that along the P gradient with high mean annual temperature (MAT: 9-10 °C), highest resin extractable P (P_resin) concentrations within one site are found in the mineral soil. On the P gradient with low MAT (5-6.1 °C) however, highest P_resin concentrations are found in the FF and also in SOM rich mineral soil, especially at the silicate site with low total P concentrations. This translates into surprisingly high P_resin values under beech (P-poor site Kandel, Black forest FF: 290 ± 200 µg P/g soil, Ah horizon: 410 ± 110 µg P/g soil), which corresponds to twice the amount extracted by citric acid. P_resin concentration under beech clearly differed from plots under spruce and maple with mostly 100 µg P/g soil at all temperature classes for P poor mineral soils. However, this high P availability in the FF was not observed for carbonate sites.

In conclusion, the innovative resin extraction method provided new insights regarding P nutrition in beech forests compared to citric acid as reference method. High P availability under beech at Kandel despite low MAT and low P status of the mineral soil indicate a tight recycling of P resources via the FF and SOM rich mineral soil. Our results show that the effect of increasing MAT on P availability depends on the P status of the mineral soil. These findings suggest a crucial role of the FF for beech forest P nutrition and its potential vulnerability under climate change.