The hidden carbon fluxes — unearthing root-related processes in boreal peatland forests

Fine-root production (FRP) and decomposition are critical processes influencing element cycling and carbon (C) balance in boreal peatlands. The aim of this thesis was to estimate FRP across various peatland forests, examine the patterns in, and develop statistical models for estimating, the relationships between FRP and stand and site characteristics, as well as climate variables (I); and to quantify fine-root decomposition rates in various types of drained peatland forests and compare them with corresponding rates in mineral-soil forests (II). FRP was measured using ingrowth cores, covering the 0−50 cm peat profile across 28 drained peatland forest sites in Finland (I). Total site-level FRP values ranged from 30 to 473 g m−2 year−1 of dry mass, with an average of 120 g m−2 year−1, with 76–95% occurring in the 0−20 cm soil layers. Total FRP showed significant variation across different site types and generally declined with decreasing fertility, except for the most fertile site type. Additionally, total FRP tended to be higher in sites with a deeper water table (WT). Stand basal area was the best predictor of total FRP, explaining 16% of the variation at the stand-level. A model incorporating stand basal area and site type explained 47% of the variation in total FRP. Fine-root decomposition was studied using litterbags containing roots from three dominant tree species (Pinus sylvestris, Picea abies, Betula pubescens) and one fern species (Dryopteris carthusiana), covering the 0−30 cm soil profile in six drained peatland and four mineral-soil forest sites in Finland (II). Fine-root decomposition showed significant variation with soil type and nutrient regime. On nutrient-poor sites, the decomposition of fine roots was slower in peat soils compared to corresponding mineral-soil sites, while the opposite was observed in nutrient-rich sites. Consequently, fine-root decomposition was fastest in nutrient-rich peat soils. Sampling depth and root diameter also influenced decomposition rates, with slower rates in deeper soil layers and for larger diameter roots. Among the tree species, P. abies had the slowest decomposition rate. In conclusion, FRP varied significantly across site types, with higher production observed in deeper WT sites and nutrient-rich conditions, while stand basal area emerged as a key predictor. Fine-root decomposition was influenced by soil type and nutrient regime, root diameter, and sampling depth, with faster decomposition in nutrient-rich peat soils and slower rates in deeper layers and for larger roots. These findings provide valuable insights into the interactions between fine-root dynamics and site-specific characteristics, contributing to a better understanding of C cycling in drained peatland forests