Seasonal patterns of δ13C in tree roots: using natural abundance measurements to qualify temporal and spatial carbon allocation by root order
During a plant’s lifetime, a majority of assimilated carbon (C), ~ 20-80%, is shuttled to belowground organs in three dominant forms: structural, stored reserves, and labile. This study evaluates whether adult Norway spruce and European beech trees growing interspecifically use different pools of C for lower and higher order root growth during two periods of peak production: early spring and late fall. Using a natural abundance approach; bulk (CB), respired C (CR) and C-fraction (labile, starch, lipid, and structural) δ13C was measured in roots born during three weeks of observation in spring and fall of 65-year-old Picea abies and Fagus sylvatica trees. Roots were separated based on branching order and age: 1st and 2nd order roots (< 20 days old) and older, and 3rd and 4th order roots (> 20 days). CR was collected with an adapted cuvette protocol on site; δ13CR was analyzed using GCMS coupled to a Quadrupole Mass-Spec, and C-fractions were separated from freeze-dried root samples using an isotope-sensitive extraction protocol and analyzed alongside CB using an Isotope Ratio Mass-Spec.
Root order categories showed differences in δ13CB and δ13CR within and between seasons. δ13CB was depleted in 3rd and 4th order roots during spring and fall for beech in contrast to 1st and 2nd order roots; this trend was also observed in spruce but only during fall measurements. Both beech and spruce δ13CR was depleted in fall compared to spring measurements. Root order also had a significant effect on δ13CR. In the fall, δ13CR of 1st and 2nd order beech roots (-28.45 o/oo) was significantly depleted compared to 3rd and 4th order roots (-27.47 o/oo), while the opposite was true of spruce roots in the spring (-24.57 vs. -26.4 o/oo). Observed seasonal and root order differences in δ13C suggests that both species utilize at least two pools of C in varying proportions at different times of the year for fine root growth and maintenance. We conclude that natural abundance measurements of 13C/12C are sufficient for discriminating between C pools used in fine root growth and maintenance.