Moisture-driven divergence in mineral-associated soil carbon persistence.

Mineral stabilization of soil organic matter is an important regulator of the global carbon (C) cycle. However, the vulnerability of mineral-stabilized organic matter (OM) to climate change is currently unknown. We examined soil profiles from 34 sites across the conterminous USA to investigate how the abundance and persistence of mineral-associated organic C varied with climate at the continental scale. Using a novel combination of radiocarbon and molecular composition measurements, we show that the relationship between the abundance and persistence of mineral-associated organic matter (MAOM) appears to be driven by moisture availability. In wetter climates where precipitation exceeds evapotranspiration, excess moisture leads to deeper and more prolonged periods of wetness, creating conditions which favor greater root abundance and also allow for greater diffusion and interaction of inputs with MAOM. In these humid soils, mineral-associated soil organic C concentration and persistence are strongly linked, whereas this relationship is absent in drier climates. In arid soils, root abundance is lower, and interaction of inputs with mineral surfaces is limited by shallower and briefer periods of moisture, resulting in a disconnect between concentration and persistence. Data suggest a tipping point in the cycling of mineral-associated C at a climate threshold where precipitation equals evaporation. As climate patterns shift, our findings emphasize that divergence in the mechanisms of OM persistence associated with historical climate legacies need to be considered in process-based models.

Climate Effects on Subsoil Carbon Loss Mediated by Soil Chemistry.

Subsoils store at least 50% of soil organic carbon (SOC) globally, but climate change may accelerate subsoil SOC (SOCsub) decomposition and amplify SOC-climate feedbacks. The climate sensitivity of SOCsub decomposition varies across systems, but we lack the mechanistic links needed to predict system-specific SOCsub vulnerability as a function of measurable properties at larger scales. Here, we show that soil chemical properties exert significant control over SOCsub decomposition under elevated temperature and moisture in subsoils collected across terrestrial National Ecological Observatory Network sites. Compared to a suite of soil and site-level variables, a divalent base cation-to-reactive metal gradient, linked to dominant mechanisms of SOCsub mineral protection, was the best predictor of the climate sensitivity of SOC decomposition. The response was "U"-shaped, showing higher sensitivity to temperature and moisture when either extractable base cations or reactive metals were highest. However, SOCsub in base cation-dominated subsoils was more sensitive to moisture than temperature, with the opposite relationship demonstrated in reactive metal-dominated subsoils. These observations highlight the importance of system-specific mechanisms of mineral stabilization in the prediction of SOCsub vulnerability to climate drivers. Our observations also form the basis for a spatially explicit, scalable, and mechanistically grounded tool for improved prediction of SOCsub response to climate change.

Soil organic carbon pool and chemical composition under different types of land use in wetland: Implication for carbon sequestration in wetlands.

This study was conducted to understand how different wetland vegetation-land use types influenced the storage and stability of soil organic carbon (SOC) in surface soils. We determined the concentration and chemical composition of SOC in both density (including light fraction organic carbon (LFOC) and heavy fraction organic carbon (HFOC)) and particle size fractions (including <2 µm, 2-63 µm, 63-200 µm and 200-2000 µm) in four wetland land use types covered with different vegetation: lake-sedge, reed, willow and poplar wetlands. Results showed that the concentrations and stock of SOC and LFOC in willow and poplar wetlands were significantly higher than those in lake-sedge and reed. However, a higher proportion of alkyl-C and a lower proportion of O-alkyl-C were observed in lake-sedge and reed wetlands than in willow and poplar, suggesting that accumulated C in willow and poplar wetlands was less stable than that in lake-sedge and reed. For all particle-size fractions except the silt (2-63 µm), the SOC concentrations were highest in willow and lowest in reed wetland surface soils, while their alkyl-C/O-alkyl-C (A/O-A) and hydrophobic-C/hydrophilic-C ratios progressively decreased from lake-sedge and reed wetland surface soils to poplar and willow surface soils. Moreover, the ratios of A/O-A and hydrophobic-C/hydrophilic-C in surface soils generally decreased with increasing concentrations of SOC in particle-size fractions, with these stability indexes being lowest in the largest particle-size fraction. These results indicate that the wetland vegetation-land use types that could incorporate more C into finer particle-size fractions had a greater potential for sequestering more stable C in such wetland ecosystems. Different wetland vegetation-land use types resulted in significant changes in the concentration and chemical structure of SOC, which could affect soil C sequestration and dynamics, C cycling in wetland ecosystems. Although both willow and poplar forests could increase SOC stock, the stability of SOC in willow wetland was higher. Therefore, on balance (stock and stability) the land use of wetland for willow forest could be a more promising way for enhancing soil C sequestration in wetlands.

Effects of season and interval of prescribed burns on pyrogenic carbon in ponderosa pine stands in the southern Blue Mountains, Oregon, USA.

In ponderosa pine (Pinus ponderosa) forests of the western United States, prescribed burns are used to reduce fuel loads and restore historical fire regimes. The season of and interval between burns can have complex consequences for the ecosystem, including the production of pyrogenic carbon (PyC). PyC plays a crucial role in soil carbon cycling, displaying turnover times that are orders of magnitude longer than unburned organic matter. This work investigated how the season of and interval between prescribed burns affect soil organic matter, including the formation and retention of PyC, in a ponderosa pine forest of eastern Oregon. In 1997 a prescribed burn study was implemented in Malheur National Forest to examine the ecological effects of burning at 5 and 15-year intervals in either the spring or fall. In October 2015, both O-horizon and mineral soil (0-15 cm) samples were collected and analyzed for PyC concentration, content, and structure using the benzene polycarboxylic acid (BPCA) method. O-horizon depth, carbon and nitrogen concentration and content, pH, and bulk density were also measured. Plots burned in the spring and fall had lower C and N stocks in the O-horizon compared to the unburned controls due to a reduction in O-horizon depth; however, we did not observe any differences in O-horizon concentration of C or N. Moreover, the concentration and stock of C and N in the mineral soil of plots burned in the spring or fall was the same as or only very slightly different from the unburned controls, suggesting that the prescribed burns on these sites have not adversely affected SOM quantity. Compared to unburned controls, we estimate that fall burns increased the mean PyC concentration of the mineral soil by 8.42 g BPCA/kg C. We did not detect a difference in mean PyC concentration of the mineral soil between the spring burns and the unburned controls; however, the spring burn plots did contain a number of isolated pockets with very high concentrations of PyC, suggesting a patchier burn pattern for these plots. In general, there was no detectable difference in any of the response variables when comparing the various prescribed burn treatments to one another. The disturbance caused by the reintroduction of fire to this ecosystem may have obscured subtle differences caused by the different seasons and intervals of burn that could become more apparent over time.