Emerging Patterns In The Isotopic Composition Of Soil CO2 Concentrations, Soil CO2 Production, And Soil-Atmosphere CO2 Exchange At The Watershed Scale: On The Intersection Between Hydrology And Biology In The Critical Zone

Abstract:

Stable isotopes are commonly used to understand how physical and biological processes mediate the exchange of carbon between terrestrial ecosystems and the atmosphere. Numerous studies have described fundamental relationships between environmental variables, the carbon isotopic composition (δ¹³C) of recently assimilated sugars in plants, litter, soil carbon, or recently respired CO₂. However, studies that examine the spatial variability of the ¹³C content of forest soils at the landscape scale are lacking. We report on measurements of the carbon isotopic composition of soil CO₂ concentrations (δ¹³C_C), soil CO₂ production (δ¹³C_P), and soil-atmosphere CO₂ exchange (δ¹³C_D) across a subalpine forest of the northern Rocky Mountains of Montana over two growing seasons. We evaluate the variability of these measurements across different landscape positions. Our analysis demonstrates that soil moisture and the lateral redistribution of soil water are strong predictors of the spatial variability of δ¹³C_C, δ¹³C_P, and δ¹³C_D at the watershed scale. We suggest that there are concomitant yet independent effects of soil water on physical (i.e., soil gas diffusivity) and biological (i.e., photosynthetic activity) processes that mediate the ¹³C composition of forest soils. We show systematic spatial variability in the δ¹³C of forest soils at the landscape scale that can be useful to accurately predict and model land-atmosphere CO₂ exchange over complex terrain.