Carbon sink or carbon source?
Barron-Gafford et al. (2011) sought out to answer this question by measuring carbon dioxide fluxes in three different microhabitats (mesquites, bunchgrasses, and intercanopy soils) in Arizona (US), using solid state CO2 sensors. The study site is an area that is experiencing “woody encroachment”, where woody plants (e.g. mesquites) start to invade native grasslands, which can result in altered seasonal and annual carbon fluxes in the region.
Continuous measurements used to estimate soil respiration in multiple microhabitats allowed the authors to gain a better understanding of the effects of soil moisture and temperature on soil respiration and how it changes over an annual time scale.
Soils below mesquite vegetation were found to contribute significantly more to the ecosystem-level soil respiration than grassland soils due to their ability to respond quickly to rain events, and because of their higher average baseline rates. Bunchgrasses showed a shorter pulse in soil respiration after a rain event.
These ecological shifts from semiarid grasslands (e.g. bunchgrasses) to shrublands (e.g. mesquites) increases the amount of carbon dioxide lost from the soil in these areas, which means that they are acting as carbon sources. The authors highlight the difficulties in accurately estimating soil respiration in vegetative cover transition areas, and note that above and belowground flux measurements in vegetation-types will be necessary in future to validate these ecosystem-level CO2 flux estimates.
Barron-Gafford et al. (2011), “The relative controls of temperature, soil moisture, and plant functional group on soil CO2 efflux at diel, seasonal, and annual scales”