Extreme heat-stress events rapidly alter soil microbial carbon use efficiency and nutrient cycling

Extreme weather events, such as heatwaves, can disrupt biogeochemical cycling at a regional and global scale, leading to devastating environmental and socioeconomic impacts across the agrifood system. Previous studies exploring extreme heat-stress events rarely exceed 35 °C or have replicated global surface air temperature models where soils are subjected to mean annual temperature (MAT) ± 1.5 to 5 °C. However, this overlooks the rapid, short-term temperature extremes where soil surfaces heated by solar irradiation can reach >40 °C, exceeding the microbial thermal optima.

This study replicated heatwave conditions recorded in July 2025 in North Wales, UK, where bare, unshaded soil surface temperature reached up to 59.7 °C. Using ¹⁴C-radioisotope tracing, we explored the impact of varying duration (15-minutes to 7-days) of extreme heat-stress and thermal diffusion within the upper soil profile (0-5 cm) on microbial carbon (C) cycling, carbon use efficiency (CUE), and biogeochemistry.

We found that ¹⁴C-glucose mineralisation rapidly increased 1.5- to 2-fold from 37 ± 1 % in the control (20 °C) to 44-77 % in soils subjected to 59.7 °C for >1-hour, with a noticeable lag-phase in C cycling occurring in the first 8-hours following ¹⁴C-glucose addition. This subsequently reduced microbial CUE at a rate of 0.01 units min^-1 from 0.62 ± 0.01 (control) to 0.19 ± 0.01 after 2-hours exposure to extreme heat, after which no further decline occurred. Soil pH and extractable ammonium increased with heat exposure due to nitrification inhibition, with microbial biomass C decreasing stepwise (from 2.36 ± 0.15 to 0.63 ± 0.06 g C kg^-1) with increasing heat-stress duration. Notably, after a 14-day recovery period these trends still occurred, indicating that the critical temporal threshold reached (>1-hour) has a legacy effect on microbial activity and soil nutrient cycling, with implications for soil C sequestration.