Fine root classification matters: nutrient levels in different functional categories, orders and diameters of roots in boreal Pinus sylvestris across a latitudinal gradient

Background: Fine roots and their symbionts are the key drivers of processes such as nutrient cycling and belowground productivity. Their function depends on position in a branching hierarchy, with absorptive roots (responsible for resource acquisition), and transport roots. Aim: We examined how the different diameter cutoffs for root classification mirror concentrations of 11 different nutrients, plus Al in functionally different fine roots of Pinus sylvestris based on anatomical/ functional categorization and in relation to their biomass along a latitudinal gradient. Results: The C:N:P ratios for pooled roots ≤2 mm in diameter reflected the nutrient concentrations of transport roots. In contrast, the C:N:P ratios for all absorptive roots was best reflected by nutrient ratios in the 1st root order, and not by those in root <2 mm. These patterns were more clear along decreasing mean annual temperature, despite increasing contribution of absorptive roots in the fine root biomass. Higher biomass fraction of transport roots did not always mirror higher concentration of element accumulated. Narrowing the root diameter cutoffs to 0.5 mm provided more informative data about nutrient concentrations in the absorptive root pool than <2 mm cut-off. Conclusions: By analyzing nutrient concentration using ≤2 mm cut-off it is impossible to properly understand nutritional properties of roots with different functional attributes. Given that the diameter of fine root orders vary considerably even within a single species between stands, the diameter cutoffs for absorptive roots should be based on the characteristics of the studied species, and not on a fixed, universal diameter cutoff.