Carbon Residence Time in Fine Roots Across Root Branching Orders Following a Long-Term Labelling Experiment

Determining the residence time of carbon (C) in the root structures is a very important trait for understanding plant C allocation mechanisms and root contributions to ecosystem processes. Carbon allocation and residence time are difficult to measure in the root system under field conditions, because of the lack of reliable methods to determine root production and mortality. Traditional classification of fine roots (< 2 mm diameter) is comprised of 5-to-6 branching orders, with considerable heterogeneity in morphology, chemistry and potentially function among other traits. In this study, we determined a number of easily measured root traits and the mean residence time of C in fine roots dissected by root branching order of L. styraciflua following a long-term (10 yr) isotope labeling field experiment. With increasing root order, fine roots increase in diameter and decrease in N concentration. A negative power-law function describes the relationship between root diameter and root N concentration. The carbon isotope composition of roots sampled two and three years following the cessation of the C isotope labelling indicated the residence time of C in individual root branching orders. We found considerable variation in the isotope signature between samples, which indicated spatial heterogeneity in the residence time of C at the tree to landscape scale. When examining the difference in the isotope signatures between root orders within a sample, C residence times increased with each increase in root branching order. Together, these results imply that while some roots may die in clusters of lower order roots, or ephemeral root modules, other factors also influence production and mortality of individual lower order roots. Finally, knowing the residence time of C in the root system gives an idea of allocation of C within root types at the whole-plant level and facilitates the scaling of root processes at the ecosystem level.