Rates and Mechanisms of CO2 Consumption in Mojave Desert Soils
Abstract
Carbon cycling in dryland ecosystems is an important component of global C fluxes yet significant uncertainty remains regarding the primary controls on CO2 exchange in these ecosystems. While arid soils contain relatively low levels of organic C, they can contain large amounts of inorganic C in the form of pedogenic carbonate. Although a net loss or uptake of soil inorganic C requires a significant alkalinity source or sink, the short-timescale dynamics of inorganic C speciation and its impact on CO2 fluxes remain poorly understood. Here we explore the mechanisms and implications of unexpected sinks of soil CO2 observed in a long-term study of a climosequence in the Mojave Desert, which we hypothesize are driven by inorganic C dynamics.
We report two years of in-situ observations of meterological and soil conditions (CO2 concentration, water content, and temperature to 125cm depth) from soils along an elevation gradient. Data were collected from both inter- and under-canopy soils, capturing prolonged dry periods and relatively wet winter/summer months. In general, soil CO2 dynamics are well-described using a CO2 production-diffusion model driven by temperature and water-content dependent respiration. However, there are a number of intervals when the soils become CO2 sinks, which cannot be explained by organic C processes. At the most arid site, negative surface fluxes on the order of -0.1mmol/m2/s regularly occur at night during the dry season in inter-canopy soils. Following the first rain after the dry season, the CO2 concentration at 5cm depth decreased by 100ppm (-0.2mmol/m2/s surface flux). At higher elevations, episodic CO2 consumption events occur deeper in the soil profile (50-125cm depth), often accompanied by falling temperatures. Possible mechanisms for these CO2 sinks include (1) thermal impacts on soil water CO2 solubility and (2) carbonate dissolution and reprecipitation reactions. To interpret these observations, we develop a hydrogeochemical model of carbon cycling in arid soils that specifically aims to capture the couplings between the soil carbonate system and soil organic C cycle, both of which are tightly tied to temperature and water availability. We explore the relative importance of inorganic C dynamics to the overall C cycle and how these processes may respond to climatic perturbations.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.B21K2365M
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCES;
- 0428 Carbon cycling;
- BIOGEOSCIENCES;
- 0486 Soils/pedology;
- BIOGEOSCIENCES;
- 1622 Earth system modeling;
- GLOBAL CHANGE