Linking Carbon fractionation and Mineralization Potential in Young Alder and Oak Trees’ Rhizosphere Soil During the Growing Season

Abstract Predicting soil carbon sequestration after reforesting former farmland requires understanding how tree types and seasons influence fast- and slow-cycling carbon in the rhizosphere. This study investigated the impact of species and seasonal shifts on the distribution and mineralization of labile and stable carbon pools of young forests. We compared a young (~ 5-year) N₂-fixing alder ( Alnus glutinosa ) with a non-N₂-fixing oak ( Quercus robur ) across the growing season, expecting mineral-associated organic carbon (MAOC) to dominate total soil organic carbon (SOC), and C mineralization potential to peak in spring. We fractionated rhizosphere soils into particulate organic carbon (POC; >53 μm) and MAOC ( 70% of SOC). Most of the soil carbon and nitrogen was stored in the fine fraction than the coarse fraction, highlighting the strong mineral association. The POC/SOC ratio varied with species and season (alder > oak; autumn peak), whereas MAOC/SOC was comparatively invariant. Although MAOC exhibited a larger potentially mineralizable C, POC mineralized > 25 times faster per unit C, with autumn consistently highest. Alder soils exhibiting a higher intensity of organic functional group peaks, particularly in the light fraction and POC, and show stronger C = O (carbonyl) signals. Overall, this finding suggests that POC drives the seasonal C mineralization, while MAOC provides the stable backbone of C storage at this newly established forest, potentially enhancing long-term C sequestration from the outset.