Thermal adaptation of soil microbial functional responses: Insights from a geothermal gradient in Aotearoa New Zealand

Abstract

Natural soil temperature gradients provide an excellent proxy to study how soil microbial communities, and their associated activities, will adapt to global warming. The rate at which soil microbes adapt to warming has important implications for biogeochemical cycling and predictions of soil carbon loss. In Aotearoa New Zealand, we take advantage of a decades long geothermal gradient, ranging from 15-42°C mean annual soil temperature, to investigate thermal adaptation of soil microbial respiration (with unlimited substrate), bacterial growth, and extracellular enzyme activities (β-glucosidase, acid phosphatase, and N-acetyl-β-ᴅ-glucosaminidase). We sampled soils from across this gradient and constructed temperature response curves for each soil sample in the lab by incubating them at six or more different temperatures. Using these temperature response curves, we then estimated rates of thermal adaptation for each microbial function and compared how different microbial processes adapt differentially. Despite major changes in microbial community diversity and composition along the gradient, we found only modest shifts in thermal adaptation of the microbial functional responses. The temperature response curves for soil microbial respiration and bacterial growth increased at a rate of approximately 0.2°C per 1°C increase in mean annual temperature. In contrast, we did not find a significant relationship between mean soil temperature and the temperature response of extracellular enzyme activity, suggesting that temperature responses are highly conserved across a variety of soil microbial functions. Nonetheless, it is important to consider how these changes in microbial rates may affect predictions of soil carbon loss with global warming.