LAUSR

Abstract Data from forest inventories provide information on C stocks under various tree species. However, based on the literature, the effects of tree species can be insignificant due to variation caused by the influence of site-related factors. The aim of this study was to explore the effects of soil texture, pHH2O, elevation, and dominant tree species on soil carbon (C) stocks in forest soils of Poland using data mining. Data were obtained from 468 investigation sites, spread over the entire area of Poland, representing forest sites with five dominant tree species (Scots pine, Norway spruce, Silver fir, deciduous oak and European beech), different elevations (lowlands, uplands, and mountains) and variable soil texture. We analyzed the effects of these characteristics using generalized additive models (GAM). Total soil-profile (up to 100 cm) C stocks (on average 26 t ha−1 in the organic horizon and 66 t ha−1 in the mineral soil) in forest soils of Poland were slightly lower than those reported previously for European forests. Simple ANOVA results implied that soils of pine stands contained significantly less stored C than soil under the other coniferous species (spruce and fir) and under deciduous oak and beech. The GAM analysis enabled quantification of the overall contribution of variables to variation in C stocks, namely fine-fraction (FF; silt and clay) content (15%), pHH2O (13%), altitude (8%), and dominant tree species (3%). The FF content positively affected C stocks (up to 10 t C ha−1 particularly in sandy soils (80–100% sand). The effect of elevation was most evident in the organic horizon and 0–10 cm mineral layer, which resulted in an increase of ∼40 t C ha−1 from 300 to 1000 m above sea level. The GAM analysis also showed that in all horizons the C stocks were lowest in soil pHH2O range 4.5–5.5. Decreased pHH2O or increased pHH2O resulted in an increase of 10–40 t C ha−1 in C stocks. Tree species alone explained about 3% of variation in C stocks, but a large portion of the effects of pHH2O may be attributable to the dominant tree species.