Macroclimate is typically considered the predominant driver of litter decomposition. However, cumulative monthly or yearly precipitation typically fails to predict decomposition in drylands. In these systems, the windows of opportunity for decomposer activity may instead depend on the precipitation size and frequency, as well as on local factors affecting litter desiccation, such as soil-litter mixing. We used two complementary microcosm experiments to disentangle the relative importance of cumulative precipitation, pulse size, pulse frequency, and soil-litter mixing on litter decomposition. In the first experiment, we evaluated the relative importance of cumulative precipitation, pulse frequency, and soil-litter mixing on microbial decomposition, in a greenhouse incubation. In the second experiment, we investigated how pulse size altered the dynamics of litter moisture and microbial respiration, in a controlled environment chamber.
In the first experiment, decomposition saturated with increasing cumulative precipitation when pulses were large and infrequent. Greater precipitation pulse frequency thus increased decomposition, suggesting that periods of microbial activity depend more on precipitation frequency than quantity. These results suggest that above a pulse size threshold, litter moisture no longer increases and/or microbial activity is no longer limited by water availability. The existence of such a threshold was confirmed in the second experiment, where both litter moisture and microbial respiration saturated with increasing pulse size. Additionally, soil-litter mixing consistently increased decomposition in the greenhouse experiment, with greatest relative increase (+194%) under the lowest cumulative precipitation. Collectively, our results indicate that for a given cumulative precipitation, litter microbial decomposition can vary in response to pulse size, pulse frequency, and soil-litter-mixing. These results highlight the need to incorporate soil-litter mixing as key driver of decomposition in drylands, and to consider precipitation at finer temporal scale than yearly or monthly cumulative precipitation. This challenge is particularly important in the context of climate change that not only affects the cumulative precipitation, but also the size and frequency of precipitation events.