The effect of repeated hurricanes on the age of organic carbon in humid tropical forest soil.

Increasing hurricane frequency and intensity with climate change is likely to affect soil organic carbon (C) stocks in tropical forests. We examined the cycling of C between soil pools and with depth at the Luquillo Experimental Forest in Puerto Rico in soils over a 30-year period that spanned repeated hurricanes. We used a nonlinear matrix model of soil C pools and fluxes ("soilR") and constrained the parameters with soil and litter survey data. Soil chemistry and stable and radiocarbon isotopes were measured from three soil depths across a topographic gradient in 1988 and 2018. Our results suggest that pulses and subsequent reduction of inputs caused by severe hurricanes in 1989, 1998, and two in 2017 led to faster mean transit times of soil C in 0-10 cm and 35-60 cm depths relative to a modeled control soil with constant inputs over the 30-year period. Between 1988 and 2018, the occluded C stock increased and δ13C in all pools decreased, while changes in particulate and mineral-associated C were undetectable. The differences between 1988 and 2018 suggest that hurricane disturbance results in a dilution of the occluded light C pool with an influx of young, debris-deposited C, and possible microbial scavenging of old and young C in the particulate and mineral-associated pools. These effects led to a younger total soil C pool with faster mean transit times. Our results suggest that the increasing frequency of intense hurricanes will speed up rates of C cycling in tropical forests, making soil C more sensitive to future tropical forest stressors.

High methane concentrations in tidal salt marsh soils: Where does the methane go?

Tidal salt marshes produce and emit CH4 . Therefore, it is critical to understand the biogeochemical controls that regulate CH4 spatial and temporal dynamics in wetlands. The prevailing paradigm assumes that acetoclastic methanogenesis is the dominant pathway for CH4 production, and higher salinity concentrations inhibit CH4 production in salt marshes. Recent evidence shows that CH4 is produced within salt marshes via methylotrophic methanogenesis, a process not inhibited by sulfate reduction. To further explore this conundrum, we performed measurements of soil-atmosphere CH4 and CO2 fluxes coupled with depth profiles of soil CH4 and CO2 pore water gas concentrations, stable and radioisotopes, pore water chemistry, and microbial community composition to assess CH4 production and fate within a temperate tidal salt marsh. We found unexpectedly high CH4 concentrations up to 145,000 µmol mol-1 positively correlated with S2- (salinity range: 6.6-14.5 ppt). Despite large CH4 production within the soil, soil-atmosphere CH4 fluxes were low but with higher emissions and extreme variability during plant senescence (84.3 ± 684.4 nmol m-2 s-1 ). CH4 and CO2 within the soil pore water were produced from young carbon, with most Δ14 C-CH4 and Δ14 C-CO2 values at or above modern. We found evidence that CH4 within soils was produced by methylotrophic and hydrogenotrophic methanogenesis. Several pathways exist after CH4 is produced, including diffusion into the atmosphere, CH4 oxidation, and lateral export to adjacent tidal creeks; the latter being the most likely dominant flux. Our findings demonstrate that CH4 production and fluxes are biogeochemically heterogeneous, with multiple processes and pathways that can co-occur and vary in importance over the year. This study highlights the potential for high CH4 production, the need to understand the underlying biogeochemical controls, and the challenges of evaluating CH4 budgets and blue carbon in salt marshes.

Las marismas salinas producen y emiten CH4 . Por lo tanto, es esencial comprender los controles biogeoquímicos que regulan la dinámica espacial y temporal del CH4 en estos humedales. El paradigma predominante asume que la metanogénesis acetoclástica es la vía dominante para la producción de CH4 y que altas concentraciones de salinidad inhiben la producción de CH4 en estos ecosistemas. Hay evidencia que el CH4 se produce las marismas salinas a través de la metanogénesis metilotrófica, un proceso no inhibido por la reducción del sulfato. Para explorar esta paradoja, realizamos mediciones de los flujos de CH4 y CO2 del suelo a la atmósfera junto con perfiles de concentraciones de CH4 y CO2 en el suelo, isótopos estables y radioisótopos, química del agua y composición de la comunidad microbiana para evaluar la producción y el destino del CH4 en una marisma salina templada. Encontramos concentraciones de CH4 sorprendentemente altas de hasta 145,000 µmol mol−1 correlacionadas positivamente con S2− (rango de salinidad: 6.6 a 14.5 ppt). A pesar de la gran producción de CH4 en el suelo, los flujos de CH4 del suelo a la atmósfera fueron bajos, pero con mayores emisiones y variabilidad extrema durante la época de senescencia de las plantas (84.3 ± 684.4 nmol m−2 s−1 ). El CH4 y el CO2 en el suelo se produjeron a partir de carbono joven, con la mayoría de los valores Δ14 C-CH4 y Δ14 C-CO2 en o por encima de valores modernos. Encontramos evidencia de que el CH4 en los suelos fue producido por metanogénesis metilotrófica e hidrogenotrófica. Existen varias vías que el CH4 producido sigue, incluida la difusión hacia la atmósfera, la oxidación del CH4 y la exportación lateral a arroyos adyacentes a la marisma; siendo este último el flujo dominante más probable. Nuestros hallazgos demuestran que la producción y los flujos de CH4 son biogeoquímicamente heterogéneos, con múltiples procesos y vías que pueden coexistir y variar en importancia a lo largo del año. Este estudio destaca el potencial de alta producción de CH4 , la necesidad de comprender los controles biogeoquímicos de la producción de CH4 y los retos que existen para evaluar las reservas de CH4 y el carbono azul en marismas salinas.

Optimization of the LLNL/CAMS gas-accepting ion source and 1 MV compact AMS for natural abundance radiocarbon analysis of CO2.

The Lawrence Livermore National Laboratory - Center for Accelerator Mass Spectrometry (LLNL/CAMS) 1 MV AMS system was converted from a biomedical AMS instrument to a natural abundance 14C spectrometer. The system is equipped with a gas-accepting hybrid ion source capable of measuring both solid (graphite) and gaseous (CO2) samples. Here we describe a series of experiments intended to establish and optimize 14CO2 measurement capabilities at natural abundance levels. A maximum instantaneous ionization efficiency of 8 % was achieved with 3 % CO2 in helium at a flow rate of approximately 220 µL/min (3.5 µg C/min). For modern materials (e.g., OX I) we measured an average of 240 ± 50 14C counts/µg C, equivalent to a total system efficiency of approximately 3 %. Experimental CO2 samples with F14C values ranging from 0.20 to 1.05 measured as both graphite and directly as CO2 gas produced equivalent values with an average offset of < 2σ.

Conversion of the LLNL/CAMS 1 MV biological AMS system to a semi-automated natural abundance 14C spectrometer: system optimization and performance evaluation.

The Lawrence Livermore National Laboratory - Center for Accelerator Mass Spectrometry compact 1 MV biomedical accelerator mass spectrometer was repurposed and optimized for the semi-automated radiocarbon measurement of natural abundance environmental samples. Substantial efforts were made to greatly improve instrument precision and develop semi-automation capabilities for unattended operation. Here we present results from 15 months of routine system operation and evaluate the system performance based on 30 sample wheels measured with directly comparable operating conditions over 7 months from August 2019 to March 2020. Unattended operation was enabled through software that tracks specific error conditions and can initiate a complete instrument shutdown when specific criteria were met. The average measurement precision was found to be 2.7 ± 0.7 ‰ based on repeated measurements of OX I standards. Accuracy was assessed with measurements of standard materials with known 14C-content, spanning 0.5 to 1.5 modern, and by comparison to split samples measured with the 10 MV FN AMS system. We also assessed sample size and age limitations using 14C-free materials, finding that we can routinely analyze samples as small as 300 µg C and less than 33000 years without the need for size-specific correction protocols.