Microbiotic Crusts: Their Nature and Establishment in Israel and New Mexico
Investigators (most current known information)
Microbiotic crusts were long regarded as important components in desert ecosystem. Providing organic carbon and nitrogen and affecting the hydrological behavior of the surface, crust play an important role in the desert food chain. The crust may thus be responsible for water redistribution and affect plant germination and establishment. Above all, the crust may play a cardinal role in surface stabilization and especially in dune stabilization.
Although drawing attention for many years, information concerning some aspects of the crust is largely unavailable. The crust impact upon the hydrological behavior of the surface is unclear, owing to contradictional published data. Whereas some researchers claimed that the crust promote runoff, other reports stress the fact that the crust either do not change the rainfall-runoff relationship or may retard runoff. Little is also known concerning the crust's moisture sources, and its effect on crust growth. Information concerning the effect of eolian input on crust distribution was only seldomly addressed. Only meager quantitative data are available concerning the crust regeneration time.
Microclimatological measurements were carried out at the habitats of two distinct crust types defined in three sites within the northern Chihuahuan Desert: the Jornada Experimental Range, The White Sands National Monument and the Sevilleta wild life refuge. Surface and subsurface moisture and temperature were measured. Specially designed gypsum blocks served to evaluate the source of the wet-dry cycles noted following winter rainstorms. The sensors allowed for reliable measurements of the water content and duration, facilitating a study concerning the possible relationships between the crust moisture regime and the crust biomass. Rain precipitation and possible dew condensation were also measured. At the Jornada Experimental Range, eolian input was monitored via erosion pins, and the relationships between eolian input and crust cover was studied.
All crust types in each site were defined, and their biomass was measured. In addition, four pairs of runoff plots were established in all three sites. Each pair consisted a scalped plot and a plot having an intact microbiotic crust. The scalped plots served for chlorophyll measurements in order to evaluate the crust regeneration time. The crust role in runoff and sediment production was evaluated when compared to the scalped plots, which represented the parent material.
Wet-dry cycles were not noted following summer rains. The cycles were however common following winter rains. Moisture transfer at the upper surface was monitored, indicating vapor flow. The wet dry cycles explained in part the long moistness duration following winter rainstorms, being twice as long as those following summer rainstorms. The wet-dry cycles may also explain the extended periods of time during which gypsiferous sand was wet, especially following winter storms. Our data indicate easy transfer of vapor flow in the gypsiferous sand.
A high correlation between moisture duration and crust biomass was found. The results indicated that moisture duration may be largely responsible for crust biomass. Consequently, the data pointed out that crust biomass may thus serve as an indicator for surface moisture duration, supporting previous findings from the Negev Desert. Likewise, high correlation was found between eolian input and crust cover, pointing out the fact that crust cover may serve as an indicator for incremental quantification of eolian input.
The microbiotic crust impact upon runoff relationships was complex. Whereas no runoff was monitored in the White Sands regardless of the crust presence, microbiotic crusts on sand at the Sevilleta promoted runoff yield and therefore sediment yield, whereas microbiotic crusts on loess at the Jornada retarded runoff. The differential impact was explained by crust properties and especially by the parent material. Whereas no water redistribution due to runoff will occur at the White Sands, the crust presence may partly explain the high sediments load of the Rio Puerco which drain the research site at the Sevilleta. Owing to its high sediment load and consequently its poor water quality, Rio Puerco, which is drained to the Rio Grande, is a focus of extensive research. This river is responsible for a substantial decline of the water quality of Rio Grande, the principal water source of New Mexico and the northern Chihuahuan Desert.
Please see project 99R-09 for a complete list of outcomes for this and related projects.