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.
