Water availability regulates tree mixture effects on total and heterotrophic soil respiration: A three‐year field experiment

Abstract Soil respiration is of critical importance to the energy balance and element cycling of terrestrial ecosystems. Biodiversity loss and changes in rainfall regimes are predicted to increase, and in so doing have the potential to affect soil C cycling. However, uncertainties remain concerning the effects of tree diversity on soil respiration, and how these effects might be modified by altered water availability. Here, we measured the total soil respiration (RS) and heterotrophic respiration (RH) rates during the 2017 to 2019 growing seasons in stands dominated by Populus tremuloides, Pinus banksiana, and their relatively even mixtures under reduced (−25% throughfall), ambient, and added (+25% throughfall) precipitation in a boreal forest of Central Canada. We found that the RS and RH were 28% and 26% greater, respectively, in the mixtures than expected from those of monocultures that received ambient water availability, while the RH/RS were unaffected by tree mixtures. On average, the addition of water increased Rs and RH by 19%, and 10%, respectively, with more pronounced increases of Rs in tree mixtures, which resulted in 55% higher Rs in mixtures relative to monocultures. In contrast, water reduction did not affect the Rs but decreased RH in mixtures. Additionally, the effects of mixtures on RH increased over the three-year duration of the experiments. Both the Rs and RH decreased, while the RH/Rs increased with higher soil fungal to bacterial ratios. Our results revealed that plant species mixtures and water availability interactively influenced soil respiration under natural conditions. We provided novel experimental insights, which indicated that the establishment of diverse forests can increase soil C cycling over time, particularly under increased water availability.