Dynamics of fine-root decomposition and its response to site nutrient regimes in boreal drained-peatland and mineral-soil forests

Fine roots may contribute significantly to soil organic matter pool in forest ecosystems; however, their decomposition is often overlooked in studies on litter decomposition and carbon (C) and nutrient cycling. To address this gap, we conducted a five-year litterbag experiment encompassing three representative tree species (Pinus sylvestris, Picea abies, Betula pubescens), and one fern species (Dryopteris carthusiana) across various boreal peatland forest types, comparing them with corresponding rates in upland forests on mineral soils. Litterbags were recovered annually, and mass remaining was first characterized by three different model types with varying complexity. Based on this preliminary screening, we chose for the final analyses a double-exponential model, which examined parameters A, i.e. the proportion of material in the slow-decomposing pool, k1, the mass loss rate of the slow-decomposing pool, and k2, the rate of mass loss in the fast-decomposing pool. Fine-root decomposition exhibited significant variation with soil type and nutrient regime. In mineral soil, lower k1 values indicated slower decomposition in more nutrient-rich sites. Conversely, in peat soil, higher k1 values indicated faster decomposition in more nutrient-rich sites. Soil depth and root diameter emerged as influential factors, with deeper layers and larger diameter roots exhibiting slower decomposition rates. Species-specific effects were also significant, with D. carthusiana exhibiting the lowest A value, indicating faster initial decomposition compared to tree species. Among the tree species, differences in A value were minor, with variation observed primarily in k1 value, where P. abies had the lowest rate. No significant effects on k2 value were observed. These findings underscore the complex interplay between species characteristics, soil type, site nutrient regimes, and root morphology in determining fine-root decomposition dynamics in boreal forests. Importantly, our results show that soil type must be considered when modeling decomposition dynamics.