Variability in <sup>14</sup>C contents of soil organic matter at the plot and regional scale across climatic and geologic gradients

Abstract

Abstract. Soil organic matter (SOM) forms the largest terrestrial pool of carbon outside of sedimentary rocks. Radiocarbon is a powerful tool for assessing soil organic matter dynamics. However, due to the nature of the measurement, extensive 14C studies of soil systems remain relatively rare. In particular, information on the extent of spatial and temporal variability in 14C contents of soils is limited, yet this information is crucial for establishing the range of baseline properties and for detecting potential modifications to the SOM pool. This study describes a comprehensive approach to explore heterogeneity in bulk SOM 14C in Swiss forest soils that encompass diverse landscapes and climates. We examine spatial variability in soil organic carbon (SOC) 14C, SOC content and C : N ratios over both regional climatic and geologic gradients, on the watershed- and plot-scale and within soil profiles. Results reveal (1) a relatively uniform radiocarbon signal across climatic and geologic gradients in Swiss forest topsoils (0–5 cm, Δ14C = 130 ± 28.6, n = 12 sites), (2) similar radiocarbon trends with soil depth despite dissimilar environmental conditions, and (3) micro-topography dependent, plot-scale variability that is similar in magnitude to regional-scale variability (e.g., Gleysol, 0–5 cm, Δ14C 126 ± 35.2, n = 8 adjacent plots of 10 × 10 m). Statistical analyses have additionally shown that Δ14C signature in the topsoil is not significantly correlated to climatic parameters (precipitation, elevation, primary production) except mean annual temperature at 0–5 cm. These observations have important consequences for SOM carbon stability modelling assumptions, as well as for the understanding of controls on past and current soil carbon dynamics.

Comprehensive radiocarbon analysis of benzene polycarboxylic acids (BPCAs) derived from pyrogenic carbon in environmental samples

“…Radiocarbon analyses on soils have a great potential for the study of carbon turnover, yet time lags between assimilation and incorporation in soils as well as lateral and vertical spatial heterogeneity can challenge interpretations (Trumbore 2009; Van der Voort et al 2016). We therefore chose not to attempt source apportionment on the chernozem soil due to the absence of sufficient age constraints on charred soil organic matter (Carcaillet 2001).…”

Plant Wax n‐Alkane and n‐Alkanoic Acid Signatures Overprinted by Microbial Contributions and Old Carbon in Meromictic Lake Sediments

“…The present investigation employs 14 C primarily as a biological source indicator, shedding new light on the origin of individual n-alkanes and n-alkanoic acids in Lake Pavin. Lake ecosystems are diverse, and the same biomarker approach may not work in different settings, as evidenced by previous lacustrine CSRA investigations (Douglas et al, 2014;Uchikawa et al, 2008). Here we add detailed constraints on carbon cycling in a meromictic lake to better understand the dynamics pertinent to an ultraproductive, stratified lacustrine ecosystem.…”

“…Several studies based on CSRA have so far revealed complicated, site-specific patterns of OC cycling and transport across terrestrial and lacustrine reservoirs. Uchikawa et al (2008) found reasonably good age agreement between long-chain (C 27 -C 33 , odd-numbered chains) n-alkanes and higher plant macrofossils in a sediment core from a Hawaiian karstic pond, suggesting that these compounds could be suitable for independent age scale generation. In contrast, Douglas et al (2014) determined centennial-to millennial-scale age offsets between long-chain (C 26 -C 32 , even-numbered chains) n-alkanoic acids and terrigenous macrofossils preserved in a sediment core from Lake Chichancanab in Mexico, consistent with a source of aged OC from soil erosion.…”

“…The Lake Pavin CSRA results are in agreement with those of Douglas et al (2014), where a 350-1200 year age difference between codeposited plant waxes and lake sediments was observed and attributed to mobilization of old OC from catchment soils. Thus, the finding of Uchikawa et al (2008) that lacustrine sedimentary n-alkanes can provide an unbiased age scale appears valid for their specific study site but cannot be applied universally. For Lake Pavin, we suggest a centennial-scale temporal offset that in fact arises from the mixing of recent and refractory (millennial-scale) pools of soil OC.…”

Dynamics of deep soil carbon – insights from <sup>14</sup>C time series across a climatic gradient

“…Terrestrial carbon turnover time remains one of the largest uncertainties in climate model predictions (Carvalhais et al, 2014;He et al, 2016). At present, there is no consensus on the net effect that climate and land use change will have on SOM stocks (Crowther et al, 2016;Gosheva et al, 2017;Melillo et al, 2002;Schimel et al, 2001;Trumbore and Czimczik, 2008). Deep soil carbon is of particular interest because of its large stocks (Jobbagy and Jackson, 2000;Balesdent et al, 2018;Rumpel and Kögel-Knabner, 2011) and perceived stability.…”