Root functional parameters predict fine root decomposability at the community level

Root quality is one of the main drivers of fine root decomposition, an important process controlling soil carbon (C) and nutrient cycling in most terrestrial ecosystems. Root quality is defined by chemical and morphological traits, which differ across species and thus communities. This trait variation is assumed to follow a trade‐off between resource acquisition and conservation (i.e. the root economics spectrum). To what extent root quality or the economics spectrum influence fine root decomposition rates at the community level remains poorly understood, particularly within the context of land use change. Changes in land use induce shifts in plant community composition, which also affect root distribution within the soil profile, resulting in changes in root quality. We hypothesize that at the community level, (i) root decomposability is driven by community root functional parameters (i.e. root traits measured at the community level), (ii) changes in root functional parameters among land use types and with soil depth translate into changes in root decomposability. We collected shallow and deep fine roots (≤ 2 mm) from 20 plant communities across contrasting land use types in seven sites world‐wide, ranging from agricultural crops to natural forests and determined their decomposition rate in standard conditions. Fine root quality was related to known values of functional parameters for these communities, including specific root length (SRL), carbon (C), nitrogen (N) and lignin concentrations. A combination of chemical functional parameters (lignin, C and N concentrations) best explained root decomposition rates at the community level, whereas root economics remained a poorer predictor of decomposability rates. Among land use gradients, roots from agricultural and agroforestry communities decomposed faster than roots from forest sites. Across and within plant communities, a consistently greater decomposability in shallow roots was observed. Both land use and depth effects were explained by changes in root chemical traits at the community level. Synthesis. Our results suggest that the conversion of plant communities from forests to agricultural lands leads to changes in root functional parameters that drastically increase root decomposition rates and may lead to major soil C losses, especially in shallow soil layers.