Posted by Design Studio
29 October 2024
Challenge 2: Soil Carbon
Introduction
Rewetting by rain of a dry soil is a traumatic event for soil microorganisms, but also an opportunity to use resources released when soil moisture increases. It is a traumatic event because microbes need to rapidly adjust their osmotic potential to the new conditions, and need to transition from dormant to active state, resuming growth as quickly as possible to outcompete other microbes. At the same time, the resources made available as soon as soil moisture increases, thereby improving hydraulic connectivity in the pore space, allow microbes that survived the previous dry period to thrive. The combined effects of this stress response and growth recovery on the whole microbial community metabolism has so far not been studied—a gap we fill with this contribution.
Growth recovery and changes in functional group dominance after rewetting
Just after rewetting, microbial respiration is very high (the so-called ‘Birch effect’) as microbes consume carbon to resume their activity and dispose of excess osmolytes. Slowly, growth also resumes, so that microbial community carbon use efficiency (CUE, defined as the ratio of microbial growth to carbon uptake) increases up to a peak in the first days after rewetting. Microbes with higher CUE before the soil drying achieved higher values of the CUE peak. Because fungi are more resistant to dry conditions, they are the first to respond at rewetting, but later on bacteria take larger role, resulting in decreasing fungal to bacterial dominance (ratio of fungal growth over bacterial growth) through time.
Shifting microbial life history strategies after rewetting
Microbes shift their resource investment from stress tolerance to growth and resource acquisition after rewetting. Higher soil pH and carbon availability—by providing a more suitable environment for microbial recovery—accelerated this transition from stress tolerance to growth. We conclude that microbial CUE recovery depends on the metabolic capacity of the community before the soil drying, whereas life history strategies shift faster in a less stressful soil environment.
Data
The database developed in this study contains more than 120 time series of microbial growth and respiration rates, CUE, fungal to bacterial dominance, and life history strategies after rewetting. The data is freely available: https://bolin.su.se/data/li-2024-microbial-growth-respiration-1.
After rewetting a dry soil, microbial carbon use efficiency (ratio of growth over carbon uptake) increases, while fungal to bacterial dominance (ratio of fungal growth over bacterial growth) decreases, as illustrated by the schematic on the left. At the same time, microbial communities transition from stress tolerance (S strategy) to growth (Y strategy) and resource acquisition (A strategy), as shown in the Y-A-S life history strategy triangle on the right (figure from Li et al. 2024, Global Change Biology).
Read the publication in full https://doi.org/10.1111/gcb.17522
