Radiocarbon signatures across three decades reveal changing Soil Organic Carbon turnover times in a hurricane-impacted tropical forest

Abstract:

Tropical forests store a disproportionately large amount of the worlds’ soil organic carbon (C) and have amongst the highest net primary productivity rates globally. Large C stocks and high litter inputs coupled with fast decomposition rates mean that changing climate and increasing frequency of extreme events could result in globally significant fluxes to or from these C pools. By constraining a model with data sampled before and after three major hurricanes within thirty years, this study examines the cycling of C between soil pools and with depth, under conditions marked by sudden organic matter pulses, rapid shifts in forest structure, and fast decomposition rates.

We parameterized the SoilR model using soil and litter survey data from the Bisley Research Watersheds at the Luquillo Experimental Forest Long Term Ecological Research site in Puerto Rico. Soil chemistry and C isotopes, including radiocarbon, were measured from replicate soils taken from three depths between 0-60 cm at the same sites across a topographic gradient in 1988 and 2018. The radiocarbon data from each time point at these sites were used to constrain the SoilR model, resulting in a numerical estimate of the mean transit time and mean age of C in each pool.

The accumulation of C below the surface (10-35 cm) and the relatively enriched radiocarbon signatures in 2018 compared to 1988 all depths indicate increased incorporation of decadally cycling C at post-hurricanes. Mineral associated C at depth had the longest transit times overall, and the greatest decrease in mean pool C age post-hurricane, suggesting a relatively rapid movement of C from post-disturbance plant inputs to the mineral-associated pool. An improved understanding of the dynamics of future C cycling in tropical forests is key, given continuing increase in intensity and frequency of hurricanes into the future.