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

Abstract

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 age of fine-root carbon. We find that the carbon making up the standing stock of fine roots in deciduous and coniferous forests of the eastern United States has a mean age of 3-18 years for live fine roots, 10-18 years for dead fine roots, and 3-18 years for mixed live+dead fine roots. These C-14-derived mean ages represent the time C was stored in the plant before being allocated for root growth, plus the average lifespan (for live roots), plus the average time for the root to decompose (for dead roots and mixtures). Comparison of the C-14 content of roots known to have grown within I year with the C-14 of atmospheric CO2 for the same period shows that root tissues are derived from recently fixed carbon, and the storage time prior to allocation is <2 years and likely <1 year. Fine-root mean ages tend to increase with depth in the soil. Live roots in the organic horizons are made of C fixed 3-8 years ago compared with 11-18 years in the mineral B horizons. The mean age of C in roots increases with root diameter and also is related to branching order. Our results differ dramatically from previous estimates of fine-root mean ages made using mass balance approaches and root-viewing cameras, 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, including this new radiocarbon method, has biases. Root-viewing approaches tend to emphasize more rapidly cycling roots, while radiocarbon ages tend to reflect those components that persist longest in the soil. Our C-14-derived estimates of long mean ages can be reconciled with faster estimates only if fine-root populations have varying rates of root mortality and decomposition. Our results indicate that it standard definition of fine roots, as those with diameters of <2 mm, is inadequate to determine the most dynamic portion of the root population. Recognition of the variability in fine-root dynamics is necessary to obtain better estimates of belowground C inputs

Mean age of carbon in fine roots from temperate forests and grasslands with different management

“…Plants fixing atmospheric CO 2 record its 14 C signature, once data are corrected for mass-dependent isotopic fractionation (Stuiver and Polach, 1977). The precision of 14 C measurements using accelerator mass spectroscopy (AMS; ±2-3 ‰) combined with the recent rate of 14 C decline of ∼ 4-5 ‰ per year (Levin et al, 2010) enables us to use radiocarbon as a tool for determining the average time elapsed between C fixation and its incorporation into root tissues (Gaudinski et al, 2001). Accordingly, 14 C investigations can be used to estimate average fine root C ages rather than the direct turnover time of root systems.…”

“…Here we use 14 C to estimate root C age of fine roots samples in 27 grasslands and 27 forest plots with different management in three regions in Germany, with a steady-state model implemented by Gaudinski et al (2001). Because part of the overall C age might reflect plant allocation of older carbon to the root system, we use the term "fine root C age" instead of fine root age.…”

“…We further derived the mean C age of fine roots, which represents the time C was stored in the plant before being allocated for root growth, plus the root average lifespan. For our composite root samples, we chose a steady-state model implemented by Gaudinski et al (2001). This method includes the 14 C concentration of atmospheric CO 2 over the past n years, where n represents the average age of the root composite sample assuming that any variation in ages of the root mixture is normally distributed around the mean (Gaudinski et al, 2001).…”

“…An 1. Example of the model we used to determine the mean age of fine root C from the 14 C signature of the fine roots (Gaudinski et al, 2001). …”

“…Furthermore, our ability to observe directly and quantify roots in situ is limited (Majdi et al, 2005;Trumbore and Gaudinski, 2003) Estimates of C allocation to root growth and maintenance have been based on the assumption that fine roots turn over approximately annually (Jackson et al, 1997). However, measurements of isotopes in fine roots demonstrated that both the 14 C age (Gaudinski et al, 2001) and the incorporation rate of a continuous 13 C label (Matamala et al, 2003) in fine root C were inconsistent with an annual turnover. The various observations can be reconciled by assuming that fine roots are not a single homogeneous pool (Gaudinski et al, 2010;Guo et al, 2008;Strand et al, 2008;Tierney and Fahey, 2002;Trumbore, 2009).…”