Can current moisture responses predict soil CO<sub>2</sub> efflux under altered precipitation regimes? A synthesis of manipulation experiments

Abstract

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 current 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 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 variables) 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 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 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 potential 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.

Shrubland primary production and soil respiration diverge along European climate gradient

“…A recent meta-analysis investigated the effect of soil moisture on Rs suggesting that site-specific soil types and plant communities drive observed Rs responses, making predictions difficult8. Equations used for modelling the effects of soil moisture on Rs in the meta-analysis follow the general concept that a reduction in soil moisture reduces Rs8. However, if soil moisture is naturally so high that it limits Rs due to low soil aeration at the hydric UK site, a reduction in soil moisture will stimulate Rs2021.…”

“…Belowground autotrophic and heterotrophic processes can result in C loss from terrestrial systems and feed back to the atmosphere. Rates of above and belowground C turnover have been linked to soil hydraulic properties389 but the effect of environmental change on the magnitude of change in above and belowground C pools remains to be identified. Our data offer empirical evidence for the effects of environmental change on above and belowground ecosystem C pools across a range of European shrublands (Table 1, Supplementary Table S1).…”

Untangling crop management and environmental influences on wheat yield variability in Bangladesh: An application of non-parametric approaches

“…Examples include Thierfelder et al (2013), who used CART to simplify information from complex multi-environment, multi-treatment "step" experiments investigating the entry points for conservation agriculture in southern Africa's smallholder maize farming systems. Vicca et al (2014) also employed recursive partitioning to analyze data from 38 manipulative studies examining soil CO 2 efflux responses to fluctuating soil moisture and temperature regimes. Their results indicated that most variation resulted from the time intervals between CO 2 efflux measurements, rather than researcher imposed treatments, indicating that experimental error and artifacts can have a pronounced effect in commonly used experimental designs and sampling procedures.…”

Remotely-sensed detection of effects of extreme droughts on gross primary production

“…Severe and recurrent droughts can reduce the terrestrial carbon sink2, eliciting a positive feedback on climate change. Ecosystem responses to drought, however, are highly variable in time and space345. Our knowledge of these responses is still very limited, in part because research on the effects of extreme droughts began only relatively recently and because extreme events occur only rarely in nature.…”