Alien red oak affects soil organic matter cycling and nutrient availability in low-fertility well-developed soils

Invasive alien species can dramatically change the litter and organic matter decomposition rate, nutrient cycling and availability, thus threatening the ecosystem functionality. We assessed the effect of red oak (QR) introduction on low fertility well-developed soils, originally covered by Quercus robur L. (QC). We determined litter and soil organic matter composition and decomposition rate by combining morphological features with C-13 NMR spectroscopy, NaClO oxidation and soil respiration. Total and available nutrients were also determined. The sites showed different humus forms: Dysmull-Hemimoder in QC and Mor in QR. The Oi horizons had a similar composition, but the higher presence of tannins and alkyl C/O -aEuro parts per thousand alkyl C and aryl C/O -aEuro parts per thousand alkyl C ratios in QR indicated that litter was less degradable. This was confirmed by soil respiration tests, with a higher preservation of the NaClO resistant fraction along the profile, mainly due to selective accumulation of alkyl components. This was accompanied by high retention of phosphorus in the organic horizons and drastic reduction of both total and available P in the mineral horizons. Calcium was strongly affected too. In these well-developed soils red oak changed organic matter dynamics, reduced P availability and cation biocycling, leading the ecosystem functionality towards a no-return threshold.