Drying and wetting cycles of O horizon in forest soils have not received much attention, partly due to methodological limitations for nondestructive monitoring of the O horizon water content. The objective of this study was to determine the importance of moisture limitations in the O horizon of a temperate forest on summertime soil respiration. We measured soil respiration in three replicated plots in a mixed deciduous forest at Harvard Forest, Massachusetts, weekly from May to October 2001. Direct Current (DC) half-bridge sensors that had been calibrated using destructive samples of the Oi and Oe/ Oa horizons were placed in the Oi and Oe/Oa horizons to record hourly changes of gravimetric water contents. Soil temperature explained 47% of the variation in soil respiration using the Arrhenius equation. The residuals of the temperature model were linearly correlated with gravimetric water content of the Oi horizon (r2 = 0.72, P < 0.0001) and Oe/Oa horizon (r2 = 0.56, P < 0.001), indicating that temporal variation in soil respiration can be partly explained by water content of the O horizon. Additionally, a laboratory study was performed to evaluate drying/wetting cycles of the O horizon at constant temperature. Even small simulated rainfall amounts of 0.5 mm significantly increase CO2 flux from dry O horizon within a few minutes. The duration of CO2 pulses increased with the amount of applied water, lasting from a few hours to days. A strong correlation between CO2 release and water content of the O horizons demonstrates a clear regulatory role of litter water content on decomposition within the O horizons.

Using a new approach involving one-time measurements of radiocarbon (¹⁴C) in fine (<2 mm 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 ¹⁴C-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 ¹⁴C content of roots known to have grown within 1 year with the ¹⁴C 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 ¹⁴C-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 a 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.