The age of fine-root carbon in three forests of the eastern United States measured by radiocarbon

Using a new approach involving one-time measurements of radiocarbon (C-14) in fine (<2 min diameter) root tissues we have directly measured the mean a ge of fine-root carbon. We find that the carbon making up the standing stoc k of fine roots in deciduous and coniferous forests of the eastern United S tates has a mean age of 3-18 years for live fine roots, 10-18 years for dea d fine roots, and 3-18 years for mixed live+dead fine roots. These C-14-der ived mean ages represent the time C was stored in the plant before being al located for root growth, plus the average lifespan (for live roots), plus t he average time for the root to decompose (for dead roots and mixtures). Co mparison of the C-14 content of roots known to have grown within I year wit h the C-14 of atmospheric CO2 for the same period shows that root tissues a re derived from recently fixed carbon, and the storage time prior to alloca tion is <2 years and likely <1 year. Fine-root mean ages tend to increase w ith depth in the soil. Live roots in the organic horizons are made of C fix ed 3-8 years ago compared with 11-18 years in the mineral B horizons. The m ean age of C in roots increases with root diameter and also is related to b ranching order. Our results differ dramatically from previous estimates of fine-root mean ages made using mass balance approaches and root-viewing cam eras, which generally report life spans (mean ages for live roots) of a few months to 1-2 years. Each method for estimating fine-root dynamics, includ ing this new radiocarbon method, has biases. Root-viewing approaches tend t o emphasize more rapidly cycling roots, while radiocarbon ages tend to refl ect those components that persist longest in the soil. Our C-14-derived est imates of long mean ages can be reconciled with faster estimates only if fi ne-root populations have varying rates of root mortality and decomposition. Our results indicate that it standard definition of fine roots, as those w ith diameters of <2 mm, is inadequate to determine the most dynamic portion of the root population. Recognition of the variability in fine-root dynami cs is necessary to obtain better estimates of belowground C inputs.