Nutrient concentrations in fine roots

Fine roots are an important source and sink for nutrients in terrestrial biogeochemistry. We examined the following hypotheses for fine root nutrients by analyzing data from 56 published studies: (1) that there is a general, inverse relationship of fine root nutrient concentrations with root diameter, and (2) that retranslocation of nutrients out of fine roots is minimal. We analyzed nutrient concentrations of roots ≤5 mm in diameter as a function of root diameter and root status (live, dead, and undifferentiated), including a comparison for coniferous and broad-leaved trees. For fine roots <2 mm in diameter, average C:N and C:N:P ratios were 43:1 and 522:12:1, significantly narrower than for 2-5 mm roots (79:1 and 920:12:1). Live roots <2 mm in diameter contained significantly more N, P, and Mg and less C than did roots 2-5 mm in diameter, but no significant differences were observed for K or Ca. Mean N and P concentrations were 11.0 and 0.9 g/kg, respectively, for live roots <2 mm diameter, compared to 6.5 and 0.6 g/kg in roots 2-5 mm in diameter. Mean N concentrations in live and dead fine roots were identical and may imply little retranslocation of root N with senescence, but conflicting evidence from Ca:N ratios highlights the need for further research. These results have practical implications for various ecological methods and for the representation of roots in biogeochemical models.