Experimental and Sampling Design
A 45m x 60m gypsum sand sheet area was sampled in three different
months of three successive years at White Sands National Park, New
Mexico, USA. Sampling occurred in July of 2020 (summer), in
September 2021 (fall), and in March 2022 (winter). Sampling time
was selected to represent different seasonal time points with
different precipitation conditions. Summer 2020 was sampled before
the start of the monsoon rains, fall 2021 after the start of the
monsoon rains, and winter 2022 subject to the less substantial
winter precipitation.
Sampling biocrust samples were defined as soil surface aggregates
approximately (5cm x 5cm x 1cm) that stay independently intact
when collected with visible biofilaments. Samples were collected
using a pallet knife, and upon collection were gently wrapped in
paper towels and placed in dry paper cups for laboratory storage
before measurements were taken (no longer than 3 months at room
temperature. The sampling design for 2020 differed slightly from
2021 and 2022 in order to make assessments with higher replication
and to understand if there was a significant change in carbon flux
after 24hr. In 2020, 5 biocrust types (light cyanobacterial, dark
cyanobacterial, Peltula sp. cyanolichen, Clavascidium sp.
chlorolichen, and moss dominated crust (moss spp. in Pottiaceae)
were collected for 5 incubation times at a replication of 5 per
type and incubation time (125 total samples). Biocrust sampling
was done along two 30m transects crossing in an "X" at
15m, systematically collecting one representative specimen of each
biocrust for each incubation condition alongside a 1.6m x 10.55m
area of each transect line. For 2021 and 2022 we again established
two intercrossing 30m transects similar to 2020 but in a different
quadrant of the sand sheet area. One of each replicate for each
incubation condition was collected from a 1m x 15m area and
replication was increased to 10 per type and investigated 6
incubation times (300 total samples).
Incubations and photosynthesis measurements
Carbon exchange measurements were performed under controlled
laboratory conditions. Biocrusts were rehydrated to full
saturation (the amount of water held by the soil without
overflowing, characterized by water glistening at the surface of
the sample) with reverse osmosis purified water. Samples were then
cut into 1.7 cm x 1.6 cm rectangles with a tin clay cutter and
light incubated at room temperature (PPFD 60-80 µmol m−2·s−1) for
30 min, 2 hr, 6 hr, 12 hr, 24 hr, or 36 hr. Biocrust samples were
not reused for multiple time intervals. Samples were
systematically measured in a pattern of light cyanobacterial, dark
cyanobacterial, cyanolichen, chlorolichen, and moss dominated at
each time interval from 30min, 2hr, 6hr, 12hr, 24hr, 36hr to
minimize potential confounding effects storage time and instrument
variability. This pattern was followed for each replicate until
all measurements were taken. For 2020, samples were rewet to
saturation every 6 hr as well as 30min before taking carbon
reading. When inserting into the machine samples were no longer
glistening at the surface, however, in 2020 it was noticed that
for a small number of samples with higher clay content, water
inhibition was occurring in the first few data points. For this
reason, the last watering event for the 2021 and 2022 series was
changed from half an hour to two hours before taking measurements.
Samples were subjected to a twelve-point light regime (PPFD: 0,
25, 50, 100, 150, 300, 500, 750, 1000, 1250, 1600, 2000 µmol
m−2·s−1) with a LI-6400XT photosynthesis system. The CO2 mixer was
set to 400 μmol CO2 m−2·s−1, flow was set to 300 μmol CO2 m−2·s−1,
and carbon fixation was determined under ambient temperature
generated in the light chamber (~26.26±2.8°C). After the
incubations ended, samples were dried at 105˚C for 48hr and
reweighed for dry weight.
Photosynthesis data processing
Photosynthetic response curve points were extracted from the raw
LiCor output files, which are attached to this dataset as Excel
files for each year (2021 & 2022, see note below), using an R
script. The raw LiCor files contain a multitude of other variables
collected during the chamber measurements. 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.
References
A related incubation experiment with crusts from White Sands and
three other locations was completed in 2020. Data for this
experiment is in EDI data package knb-lter-jrn.210549002, and it
includes the raw LiCor data for the 2020 response curves included
in the present dataset.
