COS 188-1 - Limited and complex climate legacies affect respiration responses to moisture at global scales

Abstract: With climate change, many regions are expecting increased drought frequency and severity, which can affect key carbon (C) fluxes such as soil respiration. Most predictive models assume that microbial communities instantaneously acclimate to new moisture conditions, but in previous work, we demonstrated that soil respiration can be constrained by historical climate. However, different ecosystems can have distinct responses to drought, and determining why those differences exist is important for improving forward projections. Here, we tested two hypotheses regarding how soil respiration responds to moisture: (1) the climate history hypothesis, in which respiration moisture responses depend on long-term precipitation history; and (2) the drought legacy hypothesis, which suggests that the effects of contemporary drought on the respiration-moisture relationship depend on historical drought.

To test these ideas, we used intact soil cores from a global network of drought manipulations (DroughtNet) with mean annual precipitation (MAP) ranging from 155 to 2234mm and mean annual temperature (MAT) from 2.2 to 18.9°C. All sites implemented both a rainfall reduction treatment (Drought) and a control treatment (Ambient) for 4-6 years. We assayed the soil respiration response to moisture over 2 weeks, and the average rate of respiration was calculated per g of soil organic matter. We extracted three parameters from the respiration-moisture curves: slope (moisture sensitivity), minimum, and maximum. To test the climate history and drought legacy hypotheses, we examined these parameters as a function of rainfall treatment, MAT, MAP, and the MAP coefficient of variation (MAP_CV).

Maximum respiration rates were affected by contemporary drought (P=0.02), with 17% lower maxima in the Drought vs. Ambient treatment on average. Drought did not alter minimum rates of respiration or moisture sensitivity. When considering climate history effects in the Ambient treatment, there were two trends: minimum respiration varied with the three-way interaction of MAP*MAP_CV*MAT (r²=0.87, P</em>=0.09) and maximum respiration varied with the MAP*MAT interaction (r²=0.91, P</em>=0.09). In the Drought treatment, minimum respiration depended on the MAP_CV*MAT interaction (r²=0.89, P</em>=0.02), but maximum respiration was not associated with climate history. The respiration moisture sensitivity was unrelated to any climate parameters in either treatment.

We found limited support for both the climate history and drought legacy hypotheses, but climate dependencies were complex. In addition, contemporary drought effects were limited to the maximum rate of respiration, which suggests that some aspects of the response are more drought-resistant than others. Additional sites and environmental parameters will be analyzed to better examine these ideas.