The pervasive and multifaceted influence of biocrusts on water in the world's drylands

Abstract

The capture and use of water are critically important in drylands, which collectively constitute Earth's largest biome. Drylands will likely experience lower and more unreliable rainfall as climatic conditions change over the next century. Dryland soils support a rich community of microphytic organisms (biocrusts), which are critically important because they regulate the delivery and retention of water. Yet despite their hydrological significance, a global synthesis of their effects on hydrology is lacking. We synthesized 2,997 observations from 109 publications to explore how biocrusts affected five hydrological processes (times to ponding and runoff, early [sorptivity] and final [infiltration] stages of water flow into soil, and the rate or volume of runoff) and two hydrological outcomes (moisture storage, sediment production). We found that increasing biocrust cover reduced the time for water to pond on the surface (−40%) and commence runoff (−33%), and reduced infiltration (−34%) and sediment production (−68%). Greater biocrust cover had no significant effect on sorptivity or runoff rate/amount, but increased moisture storage (+14%). Infiltration declined most (−56%) at fine scales, and moisture storage was greatest (+36%) at large scales. Effects of biocrust type (cyanobacteria, lichen, moss, mixed), soil texture (sand, loam, clay), and climatic zone (arid, semiarid, dry subhumid) were nuanced. Our synthesis provides novel insights into the magnitude, processes, and contexts of biocrust effects in drylands. This information is critical to improve our capacity to manage dwindling dryland water supplies as Earth becomes hotter and drier.

Biogeography of global drylands

“…Cyanobacteria are also an important feature in arid and semiarid regions of North America, southern Africa, Eastern Asia and Australia (Figs 6b–d, S5, S6). Major functional roles played by cyanobacteria in such regions are nitrogen fixation, runoff modulation and soil stabilization by creating an extracellular matrix (Büdel et al ., 2016; Eldridge et al ., 2020).…”

“…They dominate biocrusts in semiarid drylands of western North America, Portugal, Spain, China, Argentina, southern Africa and Australia (Figs 6, S6), and are particularly diverse and abundant in gypsum soils (Bowker et al ., 2017). Lichens are important contributors to carbon fixation, sediment trapping and microbial activity regulation in these areas (Bowker et al ., 2017; Eldridge et al ., 2020).…”

Global diversity and ecological drivers of lichenised soil fungi

“…Plant cover also plays roles in sustaining soil stability and fertility (Maestre, 2003; O’Bryan et al ., 2009; Bowker et al ., 2016) and, therefore, has positive effects on the development of lichenised symbiosis. Conversely, an extensive cover of lichen‐forming biocrusts may benefit adjacent vascular plants by increasing the availability of runoff water and by reducing evaporation (Eldridge et al ., 2020). Consequently, the net effect of competition and facilitation is the co‐occurrence of lichen symbiosis and vascular plants.…”

Preventing rangeland degradation: a shared problem for Australia and China

“…Cryptogam crust response to woody plant invasion and herbivore grazing is poorly understood (Soliveres and Eldridge 2020), but the role of crusts in regulating the delivery and retention of water for plant growth have recently been highlighted by meta-analysis. (Eldridge et al 2020).…”