Drylands are dominated by pulses of precipitation and the ability to effectively utilize precipitation when it is available is essential to the survival of desert organisms. This is especially true of soil surface communities known as biological soil crusts (biocrusts), which occur on the top few centimeters of soil in drylands worldwide. These diminutive but ubiquitous communities rely on soil moisture at the uppermost surface layer to photosynthesize, store carbon, and cycle nutrients. Changes in precipitation frequency and amounts have been shown to impact both the physiology and biogeochemistry of biocrust organisms, however, the degree of response over a range of precipitation change is unclear. Here, we administered a gradient of watering amounts and frequencies, from small frequent water events to large infrequent events, to field-collected moss and darkly-pigmented cyanobacterial crusts in a greenhouse setting over a four-month period. We measured metrics of biocrust physiological response, including photosynthesis, respiration, gross primary production, net soil CO2 exchange, and chlorophyll a content, along with the nutrient dynamics in the mineral soil beneath the biocrust.
At the end of four months, we observed few differences in biocrust chlorophyll content or photosynthetic rates, however, non-photosynthetic quenching (NPQ) increased in mosses that were given large, infrequent watering events. Similarly, nutrients within the soil remained relatively similar across watering treatments, with the exception of nitrogen, which was released in greater concentrations during the larger watering events. Taken together, our results suggest that biocrusts can take advantage of a wide variety of precipitation amounts and frequencies and that additional factors beyond precipitation amount, such as evaporative demand or duration of wetness, may be better indicators of the conditions under which biocrusts experience stress and mortality with altered precipitation patterns.