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Abstract

As a key component of the carbon cycle, soil CO2 efflux (SCE) is being increasingly
studied to improve our mechanistic understanding of this important carbon flux. Predicting ecosystem responses to climate change often depends on extrapolation of cur-5 rent relationships between ecosystem processes and their climatic drivers to conditions not yet experienced by the ecosystem. This raises the question to what extent these relationships remain unaltered beyond the current climatic window for which observations
are available to constrain the relationships. Here, we evaluate whether current
responses of SCE to fluctuations in soil temperature and soil water content can be
10 used to predict SCE under altered rainfall patterns. Of the 58 experiments for which
we gathered SCE data, 20 were discarded because either too few data were available,
or inconsistencies precluded their incorporation in the analyses. The 38 remaining
experiments were used to test the hypothesis that a model parameterized with
data from the control plots (using soil temperature and water content as predictor vari15
ables) could adequately predict SCE measured in the manipulated treatment. Only for
seven of these 38 experiments, this hypothesis was rejected. Importantly, these were
the experiments with the most reliable datasets, i.e., those providing high-frequency
measurements of SCE. Accordingly, regression tree analysis demonstrated that measurement
frequency was crucial; our hypothesis could be rejected only for experiments
20 with measurement intervals of less than 11 days, and was not rejected for any of the
24 experiments with larger measurement intervals. This highlights the importance of
high-frequency measurements when studying effects of altered precipitation on SCE,
probably because infrequent measurement schemes have insufficient capacity to detect
shifts in the climate-dependencies of SCE. We strongly recommend that future
25 experiments focus more strongly on establishing response functions across a broader
range of precipitation regimes and soil moisture conditions. Such experiments should
make accurate measurements of water availability, they require high-frequency SCE
measurements and they should consider both instantaneous responses and the po-tential legacy effects of climate extremes. This is important, because we demonstrated
that at least for some ecosystems, current moisture responses cannot be extrapolated
to predict SCE under altered rainfall.