Experimental and sampling design
Biological soil crusts, and related environmental data and
samples, were collected at four sites in the Chihuahuan Desert
with varying vegetation and soils. At each of the four sites in
the study, 2 intersecting 30m transects were laid out in an X
shape crossing at the 15m mark. From these transects, intact,
replicate samples of 5 types of biological soil crusts were
collected (n = 25 per type, total of 125 per site). Collections
occurred between May and July of 2020. Soil metagenomic analyses
were performed using subsamples of each crust (n=5 per type, 25
per site), and the data from these analyses are stored at the NCBI
Sequence Read Archive under NCBI BioProject #PRJNA748083. Further
information about these analyses, including the environmental
context of the original samples, can also be found in the related
EDI dataset knb-lter-jrn.210549001. After being returned to the
lab, the intact crust samples were wetted, incubated and observed
using the following methods.
Incubations and photosynthesis measurements
Intact, dry crust samples were wet to field capacity, cut into
1.6x1.7cm square and incubated under low light (~90 umol/m2/s) for
allotted periods of time before photosynthetic measurements were
collected at 30min, 2hr, 6hr, 12hr, or 24hrs since initial
re-wetting..Samples with longer incubation periods were maintained
in a wet state by rewetted every 6 hours and 30min before ending
the incubation. Photosynthetic measurements were carried out with
5 individual crust samples per sampling site (4), incubation
period (5), and crust type (5) for a total of 500 photosynthetic
response trials. Sample data for one trial, JER DA two rep5, was
accidentally deleted at some point after data collection. During
each set of photosynthetic measurements, each sample was weighed,
placed into LI-6400XT photosynthesis analyzer leaf chamber and
subjected to a 12-point light regime (PAR levels of 0, 25, 50,
100, 150, 300, 500, 750, 1000, 1250, 1600, 2000 umol/m2/s). After
the incubations ended, samples were dried at 105˚C for 48hr and
reweighed for dry weight. Samples were processed July-August 2020
in the apartment of Mikaela Hoellrich, due to COVID-19 lab
restrictions.
Photosynthesis data processing
Photosynthetic response curve points were extracted from the raw
LiCor outputs, which are attached to this dataset as Excel files
for each site, using an R script. The raw LiCor files contain a
multitude of other variables collected during the chamber
measurements, and a csv file describing these is attached to this
data package (RawLicor_variable_key.csv). Once the response points
were extracted, further photosynthetic data were calculated from
each curve, including data on maximum carbon fixation, net
fixation rate, respiration rate, quantum yield, and saturating
light intensity, using tidyverse.
