Seasonal patterns of δ13C in tree roots: using natural abundance measurements to qualify temporal and spatial carbon allocation by root order

Background/Question/Methods

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 (C_B), respired C (C_R) 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: 1^st and 2^nd order roots (< 20 days old) and older, and 3^rd and 4^th order roots (> 20 days). C_R was collected with an adapted cuvette protocol on site; δ¹³C_R 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 C_B using an Isotope Ratio Mass-Spec.

Results/Conclusions

Root order categories showed differences in δ¹³C_B and δ¹³C_R within and between seasons. δ¹³C_B was depleted in 3^rd and 4^th order roots during spring and fall for beech in contrast to 1^st and 2^nd order roots; this trend was also observed in spruce but only during fall measurements. Both beech and spruce δ¹³C_R was depleted in fall compared to spring measurements. Root order also had a significant effect on δ¹³C_R.  In the fall, δ¹³C_R of 1^st and 2^nd order beech roots (-28.45 ^o/_oo) was significantly depleted compared to 3^rd and 4^th 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.