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

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 (chi(c)) 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 (F-s) from the 0 to 5 cm soil profile using the gradient method. Our results show that soil chi(c) and F-s were low in all surface covers (on average, 464 ppm at 5 cm) during dry soil periods (soil moisture <0.05 m(3) m(-3)). chi(c) and subsequent F-s rapidly increased after rainfall, and showed the highest values in the soil under grass (M. tenacissima) and lichen biocrusts. Time series analysis of F-s allowed identifying periodic patterns in F-s strongly related to moisture and temperature periodicities. Moisture was the main driver for F-s on timescales of weeks and months, while temperature was the main driver on daily scales. Moisture exerted a greater influence on F-s in lichen and soil under grass, while temperature had a greater effect on F-s 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 F-s 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.

Automated summary

Despite covering over 40% of the Earth's land surface, factors driving soil respiration in drylands are still poorly understood. This study found that CO2 efflux in soil under grass and lichen biocrusts was significantly higher compared to other surface covers such as bare soil and cyanobacterial biocrusts.