Digging into the ecology of the microbial dynamics during soil drying and rewetting – an experimental and computational investigation taking soil depth and land-use into account

Popular Abstract

Blandine Lyonnard

How microbes in different soils react to drought and rainfall?

Drought periods followed by rainfall events induce large release of CO2 from the soil to the atmosphere. This phenomenon, known as the “Birch effect” can contribute to global warming. With climate change, there will be more and more cycles of drought followed by rainfall events. Therefore, there will be more Birch effect happening. But why is this Birch effect happening?


This is because of the microbes in the soil, the bacteria and fungi. When there is a drought, a lot of microbes die. Some other enter dormancy, which means they switch off to survive the drought. Some can remain active by producing substances that help them cope with the drought, such as osmolytes. When the dried soil is rewetted by a rainfall event, the microbes which survived and produced substances can release those substances. But many other microbes die. This is because of the change from dry to wet, which is too abrupt for some microbes to be able to release the substances on time. The other microbes which entered dormancy during the drought can reactivate, they can switch on again. On top of all that, when a dried soil is rewetted, water disrupts soil aggregates. The disruption of soil aggregates releases molecules that were trapped inside the aggregates.

The molecules that are released from the aggregates after wetting dried soil, plus the dead microbes during drought and rewetting, and the substances that some microbes released when the soil became wet again are all food sources that the active microbes can use. They use those resources to grow and multiply, to move around. Meanwhile, they respire. It is there respiration which releases CO2 to the atmosphere and creates the Birch effect.

Communities of microbes do not all recover the same way...
After drought and rainfall, some communities of microbes recover growth quicker than other. If the drought is harsher, more intense, or longer, the community takes more time to recover. If however, the community is used to cycles of drought and rainfall, they recover faster. In this study, I wanted to investigate what is happening at different depths in soil, and how everything is impacted by the type of soil, such as pasture soil, or arable soil. I collected soil samples at 2 depths in pastures and adjacent arable fields. I subjected those soil samples to drought and rewetting in the lab. Then, I measured microbial activity along time after rewetting to know how the communities of microbes were recovering from rewetting dried soil.

The communities did not recover the same way in pasture soils and arable soils. They recovered quicker in arable soils than pastures. This can be due to the fact that arable soils tend to undergo drying and rewetting cycles more than pasture soils. Indeed, when there is no plant cover, the soil is directly in contact with the air and the sun and can dry more easily, without roots to retain water. When there are huge crops, they can retain a lot of water with their roots, but also suck up a lot of water, inducing more changes in moisture content in the soil. The communities of microbes in arable soils are potentially more used to drying and rewetting and therefore recovered quicker in my study.

I also observed a small effect of depth. In pasture soils, the communities recovered quicker in the shallow soil layers than in the deep soil layers. This is probably because there were less microbes in the deeper layers. With a smaller community, it takes more time to recover.
In the arable soils, the microbes grew more in the deeper layers. Imagining there was less microbes in depth like in pasture soils, the new resources made available by rewetting dried soils were proportionally larger in depth than in the shallow layers.

Another finding was that among microbes, bacteria and fungi did not react the same way to rewetting dried soils and showed a potential interaction. Bacteria grew first, then fungi did, and then bacteria grew a second time. This could indicate that among the new available resource, bacteria used the easily degradable one and fungi the more recalcitrant one (difficult to degrade). While degrading the recalcitrant resource, fungi provided more easily degradable resource to bacteria which could grow again.

All together those results indicated that complex ecological mechanisms take place behind the Birch effect. I used a model to analyse those results and identify the ecological mechanisms driving them. The model is called EcoSMMARTS. It represents microbial dynamics after rewetting dried soil and reproduces the communities recovering quickly and the communities recovering less quick according to the history of moisture variations. I did not manage to exactly reproduce the results from the lab with the parameters I used in the model. However, I saw that the model can be capable of reproducing those results with some minor modifications and the use of different parameters.



Master’s deree project in Biology 45 credits 2021
Department of Biology, Lund University

Supervisors: Johannes Rousk & Albert Brangarí
Microbial Ecology, Department of Biology, Lund University

• author: Lyonnard, Blandine
• supervisor: Johannes Rousk; Albert Brangarí
• organization: Degree Projects in Biology
• course: BION02 20201
• year: 2021
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9043528
• date added to LUP: 2021-05-03 12:00:21
• date last changed: 2021-05-03 12:00:21