The research of the project was done in two main parts. We first (i) determined the local adaptation of soil exoenzyme kinetic properties (Vmax, Km) and the causal biotic mechanisms across altitudinal gradients in the Alps (WP1 - survey). To do so, 160 soil samples previously collected between 2016 and 2020 from the long-term observatory ORCHAMP were gathered by JP along with the associated metadata (climate data, soil properties). Working on soil samples from the ORCHAMPS observatory allowed us to sample soils from many climates. Soils were sampled from 24 altitudinal gradients along the French Alps, ranging from 300m a.s.l to more than 3100 m a.s.l. The mean annual temperature of those soils ranged from -3.5 to 12.5°C, and soil pH ranged from 3.5 to 7.8 providing thus an extensive range of abiotic conditions to better test our first hypothesis. Hydrolytic soil exoenzyme activities of phosphatase, β-glucosidase, acetyl esterase and leucine-aminopeptidase have been measured at eight temperatures ranging from 1°C to 45 °C in Montpellier. We applied the Arrhenius laws and the recently proposed macromolecular rate theory (MMRT) to describe the temperature response of exoenzymes. We also measured the thermal stability of soil exoenzymes by conditioning soil samples at 60°C for 24h. Interestingly, our research suggests the presence of an evolutionary trade-off between enzyme thermal stability and enzymes thermal sensitivity. Using linear mixed model averaging, we found that maximum annual temperature strongly influenced the sensitivity of enzymes involved in C acquisition (β-glucosidase) but not the other enzymes involved in N and P. This may indicate that temperature is the limiting factor for C-acquiring enzymes in alpine ecosystems with a potentially strong impact under a warming climate. This is particularly true concerning the large amount of old SOC stored in cold areas such as mountain soils, potentially highly vulnerable to climate change. Results from this first research part have been put together in a publication (in review) and were also presented at two international conferences. Two students that JP supervised participated in this first research part (a master's student and one graduate student (in 2021).
In the second research part of this project, the main objective was to test the hypothesis of exoenzyme thermal adaptation under controlled environment conditions (Ecotron) while evaluating for potential changes in soil microbial community structure, microbial biomass, and carbon use efficiency. To enhance the generality of findings, we selected ecosystems that maximise differences between samples in terms of climate (warm vs cold-adapted) and soil pH. Three altitudinal gradients, which ranged from acidic to alkaline bedrocks, were selected. For each gradient, the lowest and highest elevation sites were chosen to sample cold versus warm-adapted soil microbial communities. Soils from the targeted sites were sampled during the summer of 2021 and were subjected to warming treatments at the microcosm platform at the Ecotron in Montpellier. Surprisingly we did not find any acclimation of soil enzymes thermal sensitivity to 100 days of warming. This was also the case for the CUE thermal sensitivity assayed using the isotope water labelling (18O) method. This was a significant result of the project, as it is currently expected that soil microorganisms will adapt quickly to climate warming. This could have substantial implications as non-adapted microorganisms could increase soil organic carbon losses from the soil as CO2. To confirm these results, we seized the opportunity to sample alpine soils from an undergoing long-term climate warming experiment. We showed that the thermal sensitivity did not change after 5 years of warming treatment, and this was associated with a loss in soil organic carbon content of 15%.
