Temporal dynamics of dryland soil CO2 efflux using high-frequency measurements: Patterns and dominant drivers among biocrust types, vegetation and bare soil

Abstract

Soil respiration is an important component of the carbon (C) cycle and a major contributor to total ecosystem C efflux. Knowledge of the factors that drive soil respiration in drylands is limited, despite these regions represent more than 40% of the Earth land’s surface. In these environments, biocrusts play an important role in CO2 exchange towards and from soils. However, the temporal dynamics and main drivers of CO2 efflux in biocrusts compared to other representative dryland covers such as bare soil and soil under perennial grasses, has not been fully investigated. In this study, we measured the soil CO2 molar fraction (χc) at 2 and 5 cm depths in representative surface covers (cyanobacteria and lichen dominated biocrusts, soil under the alpha grass Macrochloa tenacissima and bare soil) from a semiarid area in SE Spain (Tabernas desert, Almeria) using small solid-state CO2 sensors, during one hydrological year. We determined the CO2 efflux (Fs) from the 0 to 5 cm soil profile using the gradient method. Our results show that soil χc and Fs were low in all surface covers (on average, 464 ppm at 5 cm) during dry soil periods (soil moisture <0.05 m3 m−3). χc and subsequent Fs rapidly increased after rainfall, and showed the highest values in the soil under grass (M. tenacissima) and lichen biocrusts. Time series analysis of Fs allowed identifying periodic patterns in Fs strongly related to moisture and temperature periodicities. Moisture was the main driver for Fs on timescales of weeks and months, while temperature was the main driver on daily scales. Moisture exerted a greater influence on Fs in lichen and soil under grass, while temperature had a greater effect on Fs in cyanobacteria and bare soil. Estimated annual CO2 efflux was 633 g CO2 m−2 y-1 in the soil under M. tenacissima, 450 g CO2 m−2 y-1 in the lichen biocrust, 268 g CO2 m−2 y-1 in the cyanobacterial biocrust and 188 g CO2 m−2 y-1 in the bare soil. On the whole, we demonstrate the suitability of automated Fs measurements for characterising rapid changes in C efflux from dryland surfaces due to changing environmental conditions, which can help improve C predictions in drylands under current climate change.