Precipitation Intensity is the Primary Driver of Moss Crust-derived CO2

Ecosystem processes in arid regions are principally dependent on water, the most limiting factor determining the activity of desert organisms. Water availability is directly linked to precipitation and the majority of precipitation events occur as small (<5 mm) short-duration events. Therefore, the majority of arid ecosystems exhibit a pulse-dynamic response to precipitation, and soils which are almost continuously dry are sporadically interrupted by transitory periods of saturation following precipitation events. Individual precipitation events can provide brief pulses of resource availability for desert organisms. In such water-limited ecosystems, pulsed water inputs directly control soil CO₂ exchange through a series of soil drying and rewetting cycles.

Although re-wetting pulses generate a significant portion of the total annual CO₂ exchange between atmosphere and soil, there has been little recognition of the importance of photosynthetic and respiratory activities of biological soil crusts (biocrusts) in desert soil CO₂ exchange.

Therefore, researchers conducted field and laboratory experiments in the Gurbantunggut Desert of northwestern China to determine the extent to which precipitation intensity could influence soil CO₂ exchange of the desert ecosystem and the role played by moss crust in soil C balance during this process. In field experiments, net CO₂ exchange (NCE) was measured in moss crusted soil and in bareland once a month from March to November in 2013. In laboratory experiments, simulated precipitation treatments (0, 2, 5, 10 and 15 mm) were applied to moss crust, and NCE of moss crusted soil and its three flux components (crust photosynthesis, crust respiration, and subsoil respiration) were measured.

Temporal variation of NCE varied with soil moisture and temperature. Soil moisture alone can explain 71－74% of variation in NCE. Soil type (moss crusted soil or bareland) also had a significant effect on NCE (P<0.01), but this was dependent on soil moisture which is directly linked to precipitation pulse. The response of NCE to precipitation pulse in moss crust differed significantly from that of bareland. After a 2 mm precipitation pulse, the crust gross photosynthetic rate (GPc) was lower than the crust respiration rate (Rc), resulting in C efflux. When precipitation intensity was equal to or greater than 5 mm, GPc fully offset total respiration, resulting in an increase in C uptake. C gain was positively correlated with intensity of precipitation pulse. Regardless of different precipitation intensities, Rc was significantly higher than that of subsoil respiration. Thus, precipitation primarily drives moss crust-derived CO₂ exchange, which significantly influences the balance of soil-level CO₂ exchange in desert ecosystems.

Overall, this study demonstrates that in desert ecosystems, the regulation of atmospheric-soil C balance by moss crusts depends on the intensity of precipitation. The result was published in European Journal of Soil Biology in April 2015.