Quantitative aspects of heterogeneity in soil organic matter dynamics in a cool‐temperate Japanese beech forest: a radiocarbon‐based approach

Abstract

Soil is the largest carbon reservoir in terrestrial ecosystems; it stores twice as much carbon as the atmosphere. It is well documented that global warming can lead to accelerated microbial decomposition of soil organic carbon (SOC) and enhance the release of CO 2 from the soil to the atmosphere; however, the magnitude and timing of this effect remain highly uncertain due to a lack of quantitative data concerning the heterogeneity of SOC biodegradability. Therefore, we sought to identify SOC pools with respect to their specific mean residence times (MRTs), to use those SOC pools to partition soil respiration sources, and to estimate the potential response of the pools to warming. We collected surface soil and litter samples from a cool-temperate deciduous forest in Japan, chemically separated the samples into SOC fractions, estimated their MRTs based on radiocarbon ( 14 C) isotope measurements, and used the data to construct a model representing the soil as a complex of six SOC pools with different MRT ranges. We estimate that a minor, fast-cycling SOC pool with an MRT of less than 10 years (corresponding to the O horizon and recognizable plant leaf fragments in the A1 horizon) is responsible for 73% of annual heterotrophic respiration and 44% of total soil respiration. However, the predicted response of these pools to warming demonstrates that the rate of SOC loss from the fast-cycling SOC pool diminishes quickly (within several decades) because of limited substrate availability. In contrast, warming will continue to accelerate SOC loss from slow-cycling pools with MRTs of 20-200 years over the next century. Although using a 14 C-based approach has drawbacks, these estimates provide quantitative insights into the potential importance of slow-cycling SOC dynamics for the prediction of positive feedback to climate change.

Radiocarbon based assessment of soil organic matter contribution to soil respiration in a pine stand of the Campine region, Belgium

“…It is very likely that the used 14 C based approach tends to underestimate the fluxes from mineral horizons. In fact, these are mixtures of SOC with a range of turnover times from a few years to several centuries (Gaudinski et al 2000;Trumbore 2000;Koarashi et al 2009). Looking at the 14 C signature of the humic acid and the unextractable SOC from the A1 mineral horizon (0-8 cm) of the same stand obtained by Chiti et al (2009), both fractions in the 0-4 cm have a positive Δ 14 C of 125.6‰ and 3.5‰, respectively, while in the 4-8 cm depth both of them have a highly negative Δ 14 C, −90.2‰ and −137.0‰, respectively.…”

Radiocarbon and stable carbon isotopic insights into provenance and cycling of carbon in Lake Superior

“…This calculation indicates that approximately 34% of the DIC originated from soil respiration. This estimate, much less well constrained as a result of variable radiocarbon ages within soil OM fractions and soil horizons (Crow et al 2009;Koarashi et al 2009), and the estimate from d 13 C values both indicate that around 34-43% of the stream DIC is from soil respiration. (Table 6) indicate that the abundance of phytoplankton in the surface water during stratification affords the mesozooplankton the chance to avoid organic detritus and to selectively feed on living plankton (Marty and Planas 2008).…”

Forest type effects on the retention of radiocesium in organic layers of forest ecosystems affected by the Fukushima nuclear accident

“…Deciduous leaf fall is an annual occurrence and the decomposition of broad leaves in such forests has been shown to be rapid1920, whereas the mean leaf longevity of evergreen coniferous trees is longer than a year21 and the decomposition of needle-like leaves appears to be much slower compared to broad leaves2223. The rate of decomposition of litter materials in organic layers is also influenced by climatological factors such as temperature and precipitation.…”