Respiratory thermal response of wood decay basidiomycetes

Terrestrial ecosystems absorb over one-quarter of anthropogenic carbon dioxide (CO2) released into the atmosphere each year. Heterotrophic soil microbial respiration, associated with the decomposition of organic matter, contributes to approximately half of the CO2 released from the terrestrial biosphere to the atmosphere per year. However, with increasing global temperatures, there is the potential for soil microbial respiration to increase, resulting in a substantial release of CO2 into the atmosphere, therefore contributing extensively to the positive land C-climate feedback and accelerating climate change. In the short-term (days-years), heterotrophic soil microbial respiration is strongly and positively related to temperature. In the long-term (years-decades), however, the positive response of soil microbial respiration to warming declines, which could be caused by direct (acclimation, evolution and species sorting) or indirect (e.g. substrate availability, moisture) effects of warming. The primary focus on whole soil microbial community responses has made it difficult to identify potential mechanisms involved in controlling long-term warming responses, thus the response of respiration to warming in the long term remains controversial. To address this uncertainty, it is necessary to study individual species and gradually more diverse and complex decomposer communities. To this end, the respiratory thermal response of wood decay basidiomycetes, the dominant decomposers of wood, were investigated. Respiration rates of individual species and two- and three-species assemblages of basidiomycetes and semi-natural wood decay communities, decomposing beech wood (Fagus sylvatica), were measured during a 90-day cooling approach. In addition, a warming approach was applied to the two- and three-species assemblages and semi-natural wood decay communities. The direction of any thermal response (decreased temperature sensitivity of respiration (compensatory), increased temperature sensitivity of respiration (enhancing), and no change in the temperature sensitivity of respiration (no response)) was determined. To increase our understanding of the respiratory thermal responses, the growth response of the basidiomycete species to temperature was also measured. Following cooling, individual species of basidiomycetes showed an overall enhancing response, with no compensatory responses identified. Two-species assemblages and the three-species assemblage showed no thermal responses overall to cooling, but with some evidence of compensatory and enhancing responses. Semi-natural wood decomposing communities showed no thermal response overall to cooling, with more enhancing than compensatory responses detected. With warming, two-species assemblages showed no thermal response overall, with more compensatory than enhancing responses detected, whereas the three-species assemblage that was dominated by one species towards the end of incubation demonstrated a compensatory response overall. The compensatory responses from the two- and three-species assemblages were likely caused by exceeding the optimum temperature for growth for some species or by the differences in the progression of the species interactions. Therefore, there was little evidence of compensatory responses that would decrease the temperature sensitivity of respiration. Semi-natural wood decay communities showed no thermal response overall to warming, but some enhancing responses were identified. The findings showed that the temperature sensitivity of wood decomposition was increased when basidiomycetes were grown alone, however, this was reduced during competitive interspecific interactions between basidiomycetes and species in wood decomposing communities. With increasing global temperatures, individual species, growing alone during the early colonisation and decomposition of wood and in decay columns in stable wood communities, may increase their respiration, but simple communities of interacting basidiomycetes and more diverse wood decomposing communities in natural systems, will more often cause no change in the temperature sensitivity of respiration overall. However, the species present in communities will most likely determine the direction and strength of respiratory responses to temperature, and thus the overall temperature sensitivity of respiration of wood decay communities. With limited evidence for compensatory responses and more evidence of enhancing responses detected, it is, thus, considered unlikely that the temperature sensitivity of wood decomposition will decline as global temperatures rise. Therefore, there remains the potential for a positive feedback to climate change through increased wood decomposition with warming.NER