- A Novel Approach to Cultivate Biocrusts for Restoration and Experimentation
Biocrusts (also known as biological soil crust, cryptogamic crusts, or cyptobiotic soils) are an aridland community of soil dwelling organisms composed primarily of cyanobacteria, mosses, and lichens that provide valuable ecosystem functions, including: soil stabilization, nitrogen fixation, and increased runoff infiltration in some systems (Evans and Ehleringer 1993, Mazor et al. 1996, Eldridge et al. 2010). These functions are lost following soil disturbance, and natural recovery is often slow (Belnap and Warren 2002). Through assisted recovery of extirpated or depauperate biocrust communities we may complement traditional means of post-disturbance mitigation, such as vascular plant seeding, to restore a greater degree of ecosystem functionality (Bowker 2007).
Techniques to cultivate and reestablish biocrusts are in an early stage. While there are methods to grow restoration relevant quantities of biocrusts, they are specific to the cyanobacterial component, relying upon a liquid culture not suitable for other crust constituents, such as mosses or lichens (Chen et al. 2006). Here we describe a novel system for rapidly cultivating all key functional groups of a biocrust community on a soil substrate, and outline its use for development of restoration materials or experimentation.
Biocrust physiology is unlike that of vascular plants, and their peculiarities must be taken into account for successful cultivation. Water is readily absorbed and lost through surface tissues due to a property shared across taxa, poikylohydry, which allows biocrusts to desiccate under conditions of low humidity, and enter a state of temporary metabolic dormancy (Potts 1999, Proctor et al. 2007). Thus, water limitation is the primary determinant of growth. In some hyperarid regions, recovery from disturbance may take centuries (Belnap and Warren 2002). Growth rates may be substantially faster if water and nutrients are not limiting (Chen et al. 2006, Xu et al. 2008).
Biocrusts are not adapted to prolonged hydration, and chronically wet soils may favor non-target organisms, such as opportunistic fungi and green-algae (Pointing and Belnap 2012). To suppress these weedy species, we advise enforced periods of drying. Brief periods of wetting may favor respiration over photosynthesis, however, and can stress biocrust organisms (Coe et al. 2012). Rapid desiccation is also a source of stress (Barker et al. 2005). Therefore, enforced drying must occur gradually and hydration should last several hours, especially if source tissues had stressful site histories (K. Coe, unpub. data).
Periodic fertilization may accelerate rates of biocrust growth, particularly in nutrient poor substrates. Taxa specific fertilizing media are available, such as BG-11 for some cyanobacteria, and Knop or Hoaglands solution for bryophytes (Chen et al. 2006, Xu et al., 2008). It is not advised to add these solutions to watering reservoirs as this may promote development of opportunistic green algae. Rather, topical application of liquid media with a hand-pump sprayer is an effective tool for nutrient delivery.
Effective propagation techniques should exploit the natural reproductive tendencies of biocrust. Many biocrust organisms, including cyanobacteria, are only capable of vegetative reproduction. Generating reproductive structures can be costly in arid environments, and many biocrust organisms capable of sex preferentially reproduce vegetatively (Bowker et al. 2000). From a restoration perspective, this is advantageous. While most vascular plants seeded for aridland restoration face a severe seedling establishment bottleneck phase, vegetative biocrust tissues may be simply crumbled, or transplanted intact, onto a cultivation substrate or target site where they may generate a new biocrust (Belnap 1993, James et al. 2011).
Our soil-based cultivation system utilizes the general wicking properties of sand (other soils have also proven compatible), a common biocrust substrate, to hydrate biocrust tissues. Timers control pumps and water is transported at the desired frequency and duration from reservoirs through a drip irrigation manifold to nested basins where biocrusts are grown (Figure 1A, 1B, and 1C). As the height of the water in the lower basin reaches that of the upper basin, it begins to wick through holes in the upper basin and into the substrate. A temporary water film forms on the substrate surface and biocrust propagules are hydrated and remain so as long as the...