Field collection of samples:
Samples were collected in the Summer (6/26/2017), Fall (10/13/2017),
and Spring (3/18/2018) from two different vegetation communities in
the Jornada LTER, Chihuahuan Desert, New Mexico. One represented a
perennial grass community with black grama (Bouteloua eriopoda), as
the dominant plant and the other represented a shrub dominated
vegetation community with tarbush Flourensia
cernua. Samples were collected from three different
locations of each vegetation type (T_West, T_East, T_Taylor, G_Ibpe,
G_Beetle, G_Ranch). At each location light algal, dark algal, and
cyanolichen crust samples were collected based on the biocrust
community classification scheme (Pietrasiak, 2014). Samples were
collected using sterile sampling techniques within a 20
m2 area to obtain 200-250 g composite soil
samples for each crust type. The G_Ibpe site had only light algal and
dark algal crusts. A total of 51 samples were collected at each time
point from the 6 locations representing all three crust types. Samples
were placed in a cooler with ice packs and transported back to NMSU
for further analysis.
Laboratory Analysis:
All crust samples were homogenized by gently breaking the crust
aggregates inside the bag using finger force and were stored in the
cold room at 4°C for a maximum of two weeks until extraction. For soil
extractions, 85 g of a well-mixed crust sample was weighed and placed
into an extraction tray measuring 30 cm x 22 cm (modified after
Whitehead & Hemming, 1965). Crusts were placed on top of two
layers of large Kimwipes© on the strainer compartment of the tray. The
bottom compartment was filled with enough sterile distilled (DI) water
to wet and saturate the crust from below. Samples were soaked for 16
hours overnight. After this incubation, all liquid from the bottom of
the tray was strained through a 10-micron mesh filter and collected
into a separate beaker. The tray was then washed with sterile DI water
to ensure maximum microfauna removal from the tray to the mesh. Since
the majority of protozoans are larger than 10 microns, they were
isolated on the mesh filter, rinsed with sterile DI water, and
collected in a separate sterile beaker.
Microfauna observed were grouped into phyla or, if possible by class,
which included the following taxa: Nematoda, Tardigrada, Rotifera,
Amoebozoa, Colpodea, Heterotchrichea, Oligohymenophora, and
Mastigophora. Bacillariophyceae were also abundant and included in the
assessment even though they are not considered microfauna. Counts of
protozoa and other microfauna for each sample were completed within
the same day of sieving to preempt changes due to reproduction or
death. Each washed sample yielded approximately 50 mL of protozoa
solution. A well-mixed 20 mL subsample of the original wash was
observed with a Carl Zeiss Axiovert 100 inverted scope at 400 X total
magnification. Prior to observation, samples were mixed by pouring the
liquid back and forth four times into a separate, sterile beaker. The
subsample was poured into a watch glass and direct counts were
measured by tally marking each time an organism was observed.
Soil Analyses:
Phospholipid fatty acid analysis (PLFA) and soil chemical analyses
were performed at Ward Laboratories, Inc. PLFA provides a snapshot of
microbial community structure and abundance at the time of sampling,
and accounts for the biomass of various functional groups of interest
such as protozoa, different bacterial groups including actinomycetes,
gram positive and negative bacteria, and rhizobia, as well as fungal
groups such as arbuscular mycorrhizal and saprophytic fungi, etc. The
soil chemical analyses included soil pH, lime content, soluble salts,
organic matter content, and concentration of nitrogen, calcium,
sulfur, phosphorous, potassium, magnesium, manganese, boron, copper,
and iron.
