Soil surface disturbance alters cyanobacterial biocrusts and soil properties in dry grassland and shrubland ecosystems

Abstract

Biological soil crusts (biocrusts) dominate soil surfaces in drylands, providing services that include soil stabilization and carbon uptake. In this study, we investigated the direct and biocrust-mediated effects of anthropogenic disturbances in two dryland ecosystems. We applied low intensity soil surface disturbance (twice-yearly footfalls) in grassland and shrubland ecosystems in northern Chihuahuan Desert, USA. After five years of disturbance, biocrust photosynthetic capacity (chlorophyll a) declined by 44%. Declines were largest in interspaces between grassland plants. Levels of scytonemin, a biocrust sunscreen pigment, were 38% greater in shrubland than grassland and 44% greater under grass canopy than in interspaces, but decreased only 5% with disturbance. Disturbance reduced soil surface stability 2 times more in the grassland than shrubland. Disturbance effects on other hydrologic and physical properties were indirectly mediated by the photosynthetic capacity of biocrusts. Disturbance indirectly increased infiltration depth and shallow (2–3 cm) soil moisture in the grassland but reduced surface moisture (<1 cm) in the shrubland. Biocrusts were more sensitive to low intensity soil disturbance in a grassland than shrubland ecosystem. While biocrusts mediated the effects of soil disturbance on dryland soil hydrological and physical properties, the nature of their influence differed between ecosystem types.