Spatial gradients in soil-carbon character of a coastal forested floodplain are associated with abiotic features, but not microbial communities

Abstract

<p><strong>Abstract.</strong> Coastal terrestrial-aquatic interfaces (TAIs) are dynamic zones of biogeochemical cycling influenced by salinity gradients. However, there is significant heterogeneity in salinity influences on TAI soil biogeochemical function. This heterogeneity is perhaps related to unrecognized mechanisms associated with carbon (C) chemistry and microbial communities. To investigate this potential, we evaluated hypotheses associated with salinity-associated shifts in organic C thermodynamics, biochemical transformations, and heteroatom content in a first-order coastal watershed in the Olympic Peninsula of Washington state, USA. In contrast to our hypotheses, thermodynamic favorability of water soluble organic compounds in shallow soils decreased with increasing salinity, as did the number of inferred biochemical transformations and total heteroatom content. These patterns indicate lower microbial activity at higher salinity that is potentially constrained by accumulation of less favorable organic C. Furthermore, organic compounds appeared to be primarily marine/algal-derived in forested floodplain soils with more lipid-like and protein-like compounds, relative to upland soils that had more lignin-, tannin-, and carbohydrate-like compounds. Based on a recent simulation-based study, we further hypothesized a relationship between microbial community assembly processes and C chemistry. Null modelling revealed strong influences of dispersal limitation over microbial composition, which may be due to limited hydrologic connectivity within the clay-rich soils. Dispersal limitation indicated stochastically assembled communities, which was further reflected in the lack of an association between community assembly processes and C chemistry. This suggests a disconnect between microbial community composition and C biogeochemistry, thereby indicating that the salinity-associated gradient in C chemistry was driven by a combination of spatially-structured inputs and salinity-associated metabolic responses of microbial communities that were independent of community composition. We propose that impacts of salinity on coastal soil biogeochemistry need to be understood in the context of C chemistry, hydrologic/depositional dynamics, and microbial physiology, while microbial composition may have less influence.</p>

Can switchgrass increase carbon accrual in marginal soils? The importance of site selection

“…To examine the distribution of soil organic C, we characterized the chemistry of hydrophilic organic compounds extractable with water, and hydrophobic compounds extractable with chloroform. The water‐extractable, or kinetic, pool contains more kinetically active C that reflects microbial metabolism while the hydrophobic pool is considered less bioavailable due to stronger associations with organic and mineral components of the soil (Bottos et al, 2018; Sengupta et al, 2019; Zhao et al, 2020). This method does not identify the chemistry of SOM that requires harsh extraction (e.g., high temp, extreme pH) or dissolution of minerals.…”

Interactive Effects of Salinity, Redox State, Soil Type, and Colloidal Size Fractionation on Greenhouse Gas Production in Coastal Wetland Soils

“…A third sample was collected at the center of a saline wetland (SW-C) from the floodplain of Beaver Creek (46.905938°, -123.978047°), a tidally influenced first-order tributary draining into Johns River, which flows into the Grays Harbor estuary in Washington state. Beaver Creek floodplain soils are characterized as hydric Ocosta silty clay loam (USDA Natural Resources Conservation Service n.d.) and soil texture is primarily silty clay, but ranges from sandy clay loam to clay (Sengupta et al, 2019). Groundwater in this floodplain is generally anaerobic and has salinities between 15-30 PSU during dry periods and 5-20 PSU during wet periods (Regier et al 2021).…”

Representing the function and sensitivity of coastal interfaces in Earth system models

“…distribution along the land-to-sea hydrologic gradient, as well as soil and sediment biogeochemistry and geomorphology in a bidirectional manner. For example, tidal exchange can both deposit marine-derived material onto terrestrial landscapes 25 and export terrigenous material to the sea 26,27 . Tidal influences on coastal ecosystems go beyond effects on salinity distributions to include effects on water velocity, flow direction, and flood frequency with consequences for carbon and nutrient exchanges in tidally affected freshwater wetlands 27,28 .…”

“…Gradients in microbial community composition from rivers to continental shelves are generally controlled by salinity and redox (spatially) and river discharge (temporally) with distinct assemblages present in tropical, temperate, and high-latitude settings 40,41 . The hydrologic and geochemical gradients that characterize soils of coastal landscapes, particularly salinity and dynamic redox conditions, exert a strong influence over soil microbial community composition and metabolic functioning 25 . It remains a challenge to differentiate the effects of inundation and water chemistry on microbially driven biogeochemical functions in soils.…”