Species-specific root exudation drives Carbon and Nitrogen Dynamics in Young Reforested Alder and Oak Forests

Root exudates are key regulators of rhizosphere processes, which control nitrogen (N) and carbon (C) cycling in the rhizosphere that underpin forest ecosystem functioning. However, how species and season-specific exudate quantity and quality influence these processes remain poorly understood. We compared exudates quantity and quality from an N₂-fixing alder (Alnus glutinosa), with a non-N₂-fixing oak (Quercus robur) tree across the growing season and its implications for N and C dynamics in the rhizosphere of a young forest (c.5 years old). We also scanned and recorded root traits of relevance to nutrient acquisition. We investigated how tree species and seasonality affect root exudate composition, soil N transformation, and mineralization potential of soil organic matter (SOM) among fast- and slow-cycling SOM pools. To isolate exudate effects, we complemented field observations with controlled additions of artificial exudates cocktails to soils mimicking natural concentrations and C:N ratios.

Root exudates were collected in situ as in Philip et al. (2008). Exudate quantity and composition differed markedly between species and varied seasonally. Oak exudates exhibited substantially greater exudation rates, including 57.0% higher carbon (p = 0.005) and 64.5% higher nitrogen exudation (p< 0.001). In contrast, alder exudates had a 15.9% higher C:N ratio than oak across the growing season, indicating lower organic C quality and reduced lability. Furthermore, oak exhibited a more acquisitive root strategy than alder, with higher specific root length (+125.8%, p = 0.016) and root tissue density (+86.8%, p = 0.186). Root exudation peaked in summer and declined in autumn, tracking seasonal photosynthetic activity. Exudates metabolomic analyses showed dominance of secondary metabolite biosynthesis pathways, followed by amino acid metabolism, which was more pronounced in alder, whereas oak exudates were characterized by enhanced aromatic compound degradation, likely reflecting stronger microbial processing in the oak rhizosphere.

Artificial root exudate addition showed that oak exudates, characterized by lower C:N ratios and higher carbon (C) inputs, stimulated stronger microbial nutrient cycling responses than alder exudates, increasing soil respiration by up to 1.74-fold, microbial biomass C by 1.62-fold, microbial biomass N by 11-fold, and gross N mineralization by fourfold. N Mineralization rates increased with exudate concentration and incubation time, with the strongest responses under oak exudates. However, net nitrification declined at high exudate inputs, likely due to microbial immobilization of N and gaseous N losses.

Carbon fractionation revealed that mineral-associated organic carbon (MAOC) dominated soil C and N stocks (>90%), whereas particulate organic carbon (POC) varied seasonally and between species (alder > oak; autumn maximum). Despite its smaller pool size given that this restored forest stand was 5 years old at the time of sampling, POC mineralized over 25 times faster per unit C than MOAC overall.

Overall, root exudate quantity and quality regulate microbial activity, nutrient retention, and C–N coupling in young forest soils, with consequences for productivity, carbon sequestration, and ecosystem–climate feedbacks.