warmed_biocrusts_nine_years

<b>Abstract.</b> Biological soil crusts (biocrusts) are predicted to be
sensitive to the increased temperature and altered precipitation associated
with climate change. We assessed the effects of these factors on soil carbon dioxide
(CO₂) balance in biocrusted soils using a sequence of manipulations
over a nine-year period. We warmed biocrusted soils by 2 and, later, by 4 °C to
better capture updated forecasts of future temperature at a site on the
Colorado Plateau, USA. We also watered soils to alter monsoon-season precipitation
amount and frequency, and had plots that received both warming and altered
precipitation treatments. Within treatment plots, we used 20 automated flux
chambers to monitor net soil exchange (NSE) of CO₂ hourly, first in
2006-2007 and then again in 2013-2014, for a total of 39 months. Net CO₂
efflux from biocrusted soils in the warming treatment increased a year after
the experiment began (2006-2007). However, after 9 years and even greater
warming (4 °C), results were more mixed, with a reversal of the increase in
2013 (i.e., controls showed higher net CO₂ efflux than treatment
plots) and with similarly high rates in all treatments during 2014, a wet year.
Over the longer-term, we saw evidence of reduced photosynthetic capacity of the
biocrusts in response to both the temperature and altered precipitation treatments.
Patterns in biocrusted soil CO₂ exchange under experimentally
altered climate suggest that (1) warming stimulation of CO₂ efflux
was diminished later in the experiment, even in the face of greater warming and
(2) treatment effects on CO₂ flux patterns were likely driven by changes
in biocrust species composition and by changes in root respiration due to vascular
plant responses.<b><br>
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