Ecosystem CO2 exchange
During the peak of the growing season (August 2018), we used a Li-Cor
7500 open-path infrared gas analyzer (IRGA; Li-Cor, Lincoln, Nebraska,
USA) to measure changes in CO2 concentration and air
temperature in the headspace of a chamber placed over the plots. We
placed the IRGA inside a 1m3 cube that was made from a
polyvinyl chloride (PVC) frame and transparent 6-mil plastic sheeting
(Polar Plastics Inc., Oakdale, MN, USA), with internal fans to ensure
adequate mixing of air, sealed to the ground with two 1.27 cm diameter ×
3.04 m length steel chains wrapped around the base of the chamber. Each
measurement cycle began by lowering the chamber, sealing it, and once a
consistent rate of CO2 exchange was achieved (typically
less than a minute), we began logging a two minute flux measurement in
the light at a sampling frequency of 0.5 Hz. At time of flux
measurement, we also measured photosynthetically active radiation (PAR)
using a MQ-100 Apogee PAR meter (Apogee Instruments, Logan, UT, USA)
that was mounted to the top of the IRGA just below upper surface of the
chamber. Full light measurements were recorded in a minimum of 900 µmol
m-2 s-1 between the hours of 0930
and 1530. For each full light measurement, we calculated net ecosystem
exchange (NEE; µmol m s) using Eqn 1 to quantify the maximum continuous
exchange of CO2 between the atmosphere, vegetation and
soil.
NEE = (ρ*V *(dC/dt)/A) (1)
In Eqn 1, ρ is the air density (mol air m-3),
defined as P/RT , where P is the average pressure (Pa),R is the ideal gas constant (8.314 J mol-1 air
K-1), and T (K) is the mean temperature.V is the chamber volume (1 m3), dC/dt is
the slope of the chamber CO2 concentration against time
(µmol CO2 mol air s), and A is the surface area
of the ground (1 m2) within the chamber.
In order to calculate standardized NEE values to a fixed light level
(PAR = 800 µmol m-2 s-1) and to
partition flux measurements between ER and GPP, we used garden shade
cloth (Agfabric, Wellco Industries Inc., Corona, CA, USA) to reduce
light penetrating our chamber in order to create an NEE light response
curve (Lasslop et al. 2010; Williams et al. 2014). Each shade cloth
reduced light availability by 50% within chamber. We measured
continuous CO2 exchange over two minute intervals with a
single layer of garden shade cloth (~50% of ambient
light), as well as two layers of shade cloth (~25% of
ambient light). Finally, we used a 100% light blocking tarp to measure
carbon flux at 0 µmol m-2 s-1 PAR
(i.e. Ecosystem Respiration, Re). From the four light
measurements (Full sun, half-light, quarter-light, and no light), we fit
both hyperbolic and linear functions to predict NEE at a standardized
light of PAR = 800 µmol m-2 s-1(NEE800). We compared r-squared values for these two
fits and found that the hyperbolic function always fit better. Negative
NEE values indicate fluxes from the atmosphere to the ecosystem,i.e. net carbon uptake, and positive values indicate net fluxes
to the atmosphere from the ecosystem, i.e. net carbon emissions.
NEE values, standardized by light, were used to compare NEE fluxes
across subplots and plots. Next, we calculated GPP (Lasslop et al. 2010)
by subtracting Re from the standardized NEE values (Eqn
2).
Gross Primary Productivity (GPP) = NEE800 –
Re (2)
To account for differences in plant biomass among treatments, we also
calculated NEE, Re, and GPP that were standardized per
gram of total plant biomass (i.e., sum of above and below-ground
biomass). In early August 2018, we estimated above- and below-ground
plant biomass in all subplots. To estimate aboveground plant biomass we
clipped vegetation in two 0.1 × 1m strips, dried and weighed this
vegetation to the nearest 0.001 g; to estimate root biomass, we took 3
soil cores of 5 cm diameter and 30 cm depth, sieved all roots, washed
and dried them, and then weighed them to the nearest 0.001 g. By
standardizing carbon flux measurements to units of plant biomass, we
were able to parse differences in carbon fluxes (NEE, GPP, and
Re) between different subplots and plots caused due to
differences in plant biomass from those driven by other factors that
mediate carbon fluxes (e.g., plant trait shifts, plant community
composition, microbial community changes, etc.). Mass-specific NEE
(NEEmsp) was calculated as in Eqn 3.
NEEmsp = NEE800 / Total plant biomass
(g-1 m-2) (3)