Summary• Characterization of turnover times of fine roots is essential to understanding patterns of carbon allocation in plants and describing forest C cycling. We used the rate of decline in the ratio of 14 C to 12 C in a mature hardwood forest, enriched by an inadvertent 14 C pulse, to investigate fine-root turnover and its relationship with fine-root diameter and soil depth.• Biomass and ∆ 14 C values were determined for fine roots collected during three consecutive winters from four sites, by depth, diameter size classes ( < 0.5 or 0.5 -2 mm), and live-or-dead status.• Live-root pools retained significant 14 C enrichment over 3 yr, demonstrating a mean turnover time on the order of years. However, elevated ∆ 14 C values in dead-root pools within 18 months of the pulse indicated an additional component of live roots with short turnover times (months). Our results challenge assumptions of a single live fine-root pool with a unimodal and normal age distribution.• Live fine roots < 0.5 mm and those near the surface, especially those in the O horizon, had more rapid turnover than 0.5 -2 mm roots and deeper roots, respectively.

How ecosystems adapt to climate changes depends in part on how individual trees allocate resources to their components. A review of research using tree seedlings provides some support for the hypothesis that some tree species respond to exposure to drought with increases in root : shoot ratios but little change in total root biomass. Limited research on mature trees over moderately long time periods (2-10 yr), has given mixed results with some studies also providing evidence for increases in root : shoot ratios. The Throughfall Displacement Experiment (TDE) was designed to simulate both an increase and a decrease of 33% in water inputs to a mature deciduous forest over a number of years. Belowground research on TDE was designed to examine four hypothesized responses to long-term decreases in water availability ; (1) increases in fine-root biomass, (2) increases in fine root : foliage ratio, (3) altered rates of fine-root turnover (FRT), and (4) depth of rooting. Minirhizotron root elongation data from 1994 to 1998 were examined to evaluate the first three hypotheses. Differences across treatments in net fine-root production (using minirhizotron root elongation observations as indices of biomass production) were small and not significant. Periods of lower root production in the dry treatment were compensated for by higher growth during favorable periods. Although not statistically significant, both the highest production (20 to 60% higher) and mortality (18 to 34 % higher) rates were found in the wet treatment, resulting in the highest index of FRT. After 5 yr, a clear picture of stand fine-root-system response to drought exposure has yet to emerge in this forest ecosystem. Our results provide little support for either an increase in net fine-root production or a shift towards an increasing root : shoot ratio with long-term drought exposure. One possible explanation for higher FRT rates in the wet treatment could be a positive relationship between FRT and nitrogen and other nutrient availability, as treatments have apparently resulted in increased immobilization of nutrients in the forest floor litter under drier conditions. Such hypotheses point to the continued need to study the interactions of water stress, nutrient availability and carbon-fixation efficiency in future long-term studies.

Use of minirhizotrons in forested ecosystems has produced considerable information on production, mortality, distribution, and the phenology of root growth. But installation of minirhizotrons severs roots and disturbs soil, which can cause root proliferation in perennial plants. We compared the magnitude and vertical distribution of root growth observations in a mature hardwood forest during the growing season immediately after minirhizotron installation with observations more than two years later. We also compared the vertical root growth distribution during these two different years with the preinstallation distribution of fine root biomass. Before minirhizotron installation and again two years later, about 74% of fine root biomass was in the upper 30 cm of soil, but immediately after installation, 98% of the root elongation was in the upper 30 cm. Large differences in the quantity of root elongation were observed across different slope positions in the minirhizotron data from the first growing season (approximately four times greater on the upper slope as the lower slope). Such differences with slope position were not seen in the later minirhizotron data, nor in the preinstallation fine root biomass data. The evidence suggests that the minirhizotron data collected immediately after installation can be biased by disturbance of roots and soil during installation, which result in excessive root proliferation, particularly near the soil surface. Root proliferation appears to be the result of a response to both root pruning and to nutrient release in microsites near the newly installed minirhizotron.

[1] Fine root (<2 mm) cycling rates are important for understanding plant ecology and carbon fluxes in forests, but they are difficult to determine and remain uncertain. This paper synthesizes minirhizotron and isotopic data and a root model and concludes that (1) fine roots have a spectrum of turnover times ranging from months to many years and (2) the mean age of live root biomass (A) and the mean age of roots when they die (i.e., their turnover time (t)) are not equal. We estimated A and t of fine roots in three forests using the root model Radix. For short-lived roots, we constrained t with existing minirhizotron data; for long-lived roots, we used new radiocarbon measurements of roots sampled by diameter size class and root branch order. Long-lived root pools had site mean t of 8-13 y and 5-9 y when sampled by diameter and branch order, respectively. Mean turnover times across sites were in general not significantly different as a function of branch-order, size class, or depth. Our modeling results indicate that ∼20% of fine root biomass has turnover times of about a year, and ∼80% has decadal turnover times. This partitioning is reflected in our predicted mean ages of ∼9 y and turnover times of ∼3 y. We estimate that fine root mortality contributes between 38 and 104 g C m −2 y −1 to soil in these forests. These estimates are 20 to 80% of previous estimates in these and similar forests, in part because we explicitly account for the large portion of fine-root biomass with decadal cycling rates. Our work shows that both fast and slow cycling roots must be modeled jointly to account for the heterogeneous nature of fine-root dynamics.Citation: Gaudinski, J. B., M. S. Torn, W. J. Riley, T. E. Dawson, J. D. Joslin, and H. Majdi (2010), Measuring and modeling the spectrum of fine-root turnover times in three forests using isotopes, minirhizotrons, and the Radix model, Global Biogeochem. Cycles, 24, GB3029,

Abstract. The effects of elevated CO 2 on nutrient cycling and selected belowground processes in the closed-canopy sweetgum plantation were assessed as part of a free-air CO 2 enrichment (FACE) experiment at Oak Ridge, Tennessee. We hypothesized that nitrogen (N) constraints to growth response to elevated CO 2 would be mitigated primarily by reduced tissue concentrations (resulting in increased biomass production per unit uptake) rather than increased uptake. Conversely, we hypothesized that the constraints of other nutrients to growth response to elevated CO 2 would be mitigated primarily by increased uptake because of adequate soil supplies. The first hypothesis was not supported: although elevated CO 2 caused reduced foliar N concentrations, it also resulted in increased uptake and requirement of N, primarily because of greater root turnover. The additional N uptake with elevated CO 2 constituted between 10 and 40% of the estimated soil mineralizeable N pool. The second hypothesis was largely supported: elevated CO 2 had no significant effects on tissue concentrations of P, K, Ca, or Mg and caused significantly increased uptake and requirement of K, Ca, and Mg. Soil exchangeable pools of these nutrients are large and should pose no constraint to continued growth responses. Elevated CO 2 also caused increased microbial biomass, reduced N leaching and increased P leaching from O horizons (measured by resin lysimeters), reduced soil solution NH þ 4 , SO 2À 4 , and Ca 2þ concentrations, and increased soil solution pH. There were no statistically significant treatment effects on soil nutrient availability as measured by resin capsules, resin stakes, or in situ incubations. Despite significantly lower litterfall N concentrations in the elevated CO 2 treatment, there were no significant treatment effects on translocation or forest floor biomass or nutrient contents. There were also no significant treatment effects on the rate of decomposition of fine roots. In general, the effects of elevated CO 2 on nutrient cycling in this study were not large; future constraints on growth responses imposed by N limitations will depend on changes in N demand, atmospheric N deposition, and soil mineralization rates.

Characterizing the use of carbon (C) reserves in trees is important for understanding regional and global C cycles, stress responses, asynchrony between photosynthetic activity and growth demand, and isotopic exchanges in studies of tree physiology and ecosystem C cycling. Using an inadvertent, whole-ecosystem radiocarbon ( 14 C) release in a temperate deciduous oak forest and numerical modeling, we estimated that the mean age of stored C used to grow both leaf buds and new roots is 0.7 y and about 55% of new root growth annually comes from stored C. Therefore, the resultant mean age of C used to grow new root tissue is ~0.4 y. In short, new roots contain a lot of stored C but it is young in age. Additionally, the type of structure used to model stored C input is important. Model structures that did not include storage, or that assumed stored and new C mixed well (within root or shoot tissues) before being used for root growth, did not fit the data nearly as well as when a distinct storage pool was used.Consistent with these whole-ecosystem labeling results, the mean age of C in new-roottissues determined using "bomb-14 C" in three additional forest sites in North America and Europe (one deciduous, two coniferous) was less than 1-2 y. The effect of stored reserves on estimated ages of fine roots is unlikely to be large in most natural abundance isotope studies.However, models of root carbon dynamics should take stored reserves into account, particularly for pulse labeling studies and fast-cycling roots (< 1 y).2

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Summary• We used an inadvertent whole-ecosystem 14 C label at a temperate forest in Oak Ridge, Tennessee, USA to develop a model (Radix1.0) of fine-root dynamics. Radix simulates two live-root pools, two dead-root pools, non-normally distributed root mortality turnover times, a stored carbon (C) pool, and seasonal growth and respiration patterns.• We applied Radix to analyze measurements from two root size classes (< 0.5 and 0.5-2.0 mm diameter) and three soil-depth increments (O horizon, 0-15 cm and 30-60 cm).• Predicted live-root turnover times were < 1 yr and ∼10 yr for short-and long-lived pools, respectively. Dead-root pools had decomposition turnover times of ∼2 yr and ∼10 yr. Realistic characterization of C flows through fine roots requires a model with two live fine-root populations, two dead fine-root pools, and root respiration. These are the first fine-root turnover time estimates that take into account respiration, storage, seasonal growth patterns, and non-normal turnover time distributions.• The presence of a root population with decadal turnover times implies a lower amount of belowground net primary production used to grow fine-root tissue than is currently predicted by models with a single annual turnover pool.