Climate control on terrestrial biospheric carbon turnover

Abstract

Terrestrial vegetation and soils hold three times more carbon than the atmosphere. Much debate concerns how anthropogenic activity will perturb these surface reservoirs, potentially exacerbating ongoing changes to the climate system. Uncertainties specifically persist in extrapolating point-source observations to ecosystem-scale budgets and fluxes, which require consideration of vertical and lateral processes on multiple temporal and spatial scales. To explore controls on organic carbon (OC) turnover at the river basin scale, we present radiocarbon (¹⁴C) ages on two groups of molecular tracers of plant-derived carbon-leaf-wax lipids and lignin phenols-from a globally distributed suite of rivers. We find significant negative relationships between the ¹⁴C age of these biomarkers and mean annual temperature and precipitation. Moreover, riverine biospheric-carbon ages scale proportionally with basin-wide soil carbon turnover times and soil ¹⁴C ages, implicating OC cycling within soils as a primary control on exported biomarker ages and revealing a broad distribution of soil OC reactivities. The ubiquitous occurrence of a long-lived soil OC pool suggests soil OC is globally vulnerable to perturbations by future temperature and precipitation increase. Scaling of riverine biospheric-carbon ages with soil OC turnover shows the former can constrain the sensitivity of carbon dynamics to environmental controls on broad spatial scales. Extracting this information from fluvially dominated sedimentary sequences may inform past variations in soil OC turnover in response to anthropogenic and/or climate perturbations. In turn, monitoring riverine OC composition may help detect future climate-change-induced perturbations of soil OC turnover and stocks.

Keywords: carbon cycle; carbon turnover times; fluvial carbon; plant biomarkers; radiocarbon.

Fig. 1.

Riverine biomarker ¹⁴C ages. The catchment areas of all rivers analyzed in this study are color coded by (A) plant-wax fatty-acid and (B) lignin-phenol ¹⁴C ages (SI Appendix, Table S1). Rivers with catchment areas smaller than 30,000 km² are shown as colored circles for clarity. The legend above (A) applies to both panels. (C) Biomarker ages as a function of the absolute latitude at the river mouth, showing both fatty acids (black circles) and lignin phenols (white squares).

Fig. 2.

Multivariate statistical analysis. (A) Matrix of Pearson correlation coefficients (r values) between environmental control variables (x-axis) and POC and biomarker ¹⁴C and δ¹³C responses (y-axis) (SI Appendix, Table S4). Box sizes and colors correspond to the strength of the correlation (sizes: magnitude only; colors: magnitude and sign). Correlations that are significant at the P = 0.05 level are outlined with a thick, black border. “Sample type” refers to the following: suspended sediment, bank/bedload sediment, or shelf-deposit sediment. (B) RDA triplot showing the RDA1 and RDA2 canonical axes (SI Appendix, Table S5); labels show the percent of total sample variance explained by each axis. Environmental control variable loadings are plotted as gray arrows, POC and biomarker ¹⁴C and δ¹³C response variable loadings are plotted as red arrows, and individual sample scores are plotted as black circles. Environmental and response variable loadings are scaled for visual clarity. Numbers and roman numerals correspond to control and response variables, respectively, as listed in (A). Only control variables that are statistically significantly correlated with at least one response variable are included in the analysis (SI Appendix, Table S4). TSS, total suspended sediment; POC, particulate organic carbon; CV, coefficient of variation; cont. perm., continuous permafrost cover; discont. perm., discontinuous permafrost cover; MAP, mean annual precipitation; NPP, net primary production.

Fig. 3.

Relationships between τ_soil, soil mean carbon age, and biomarker ¹⁴C age. (A and B) Plant-wax fatty-acid and (C and D) lignin-phenol ¹⁴C ages as a function of weighted-catchment τ_soil (Left, ref. 5), and soil mean carbon age (0 to 100 cm, Right, ref. 34). Solid and dashed black lines are reduced major-axis regression lines; reported values are the corresponding reduced major-axis regression slopes and r² values (Materials and Methods). Uncertainty (±1σ) is always smaller than marker points.

Fig. 4.

Environmental controls on τ_soil and biomarker ¹⁴C ages. Logarithmic plant-wax fatty-acid ¹⁴C ages (black circles), lignin-phenol ¹⁴C ages (white squares), and catchment τ_soil [gray triangles (5)] as functions of (A) MAT and (B) logarithmic MAP (SI Appendix, Tables S1 and S3). Solid black, dashed black, and gray lines are fatty-acid–, lignin-phenol–, and τ_soil-reduced major-axis regression lines. Relationship slopes and r² values are as follows: (A) fatty-acid ¹⁴C ages: slope = −0.036 ± 0.004, r² = 0.62; lignin-phenol ¹⁴C ages: slope = −0.036 ± 0.006, r² = 0.67; and τ_soil: slope = −0.030 ± 0.002, r² = 0.83. (B) Fatty-acid ¹⁴C ages: slope = −1.46 ± 0.17, r² = 0.55; lignin-phenol ¹⁴C ages: slope = −1.44 ± 0.30, r² = 0.56; and τ_soil: slope = −1.13 ± 0.11, r² = 0.72. Uncertainty (±1σ) is always smaller than marker points.