The efficiency of microbes

In order to truly understand climate change, we have to understand the carbon cycle – which describes where that notorious element called ‘C’ is moving to. In order to understand that carbon cycle, we need to know who is using that carbon, when and how much. As it turns out, that’s a rather complicated knot to disentangle.

In a recent paper led by Anita Risch, we took a look at the role of some of the more elusive players in that whole business of carbon cycling: soil microbes. Soil microbes play a surprisingly big role in that story, but exactly hów big remains – as so often – hard to grasp. What is known is that soil microbial processes play an important role in the build-up and maintenance of the big chunk of carbon that’s stored in our soils. At the same time, however, soil microbes RELEASE a bunch of carbon into the air via a process called ‘heterotrophic respiration’, best understood as the breathing out we humans also do.

Grassland soils (here at an experimental site in South-Africa) store surprising amounts of carbon (All pictures by the NutNet-network)

From a climate change perspective, one would want microbes to store as much carbon in the soil, and to ‘breath out’ as little as possible. That balance can roughly be considered the ‘efficiency’ of the soil microbial respiration. In a recent paper, we set out to test what defines that efficiency.

A NutNet site in Bogong, Australia

For this assessment, we made use of a fantastic global experiment called NutNet, where scientists took natural grasslands and manipulated the amount of nutrients and herbivores. Then, the scientists from 23 grassland sites took a soil sample and sent it to the lab for a five-week laboratory experiment to assess microbial respiration.

So what did we find? Microbes – at least those in grassland soils – did not seem to care too much about nutrient addition and/or exclusion of herbivores. Indeed, both factors did not significantly affect their efficiency. What they did care about, however, was the local soil and microclimate conditions, which strongly affected that illustrious efficiency.

Herbivory in action in Kilpisjärvi (Finland)

So what to do with this information? Most importantly, perhaps, it explains why local studies across the globe have been finding such contradictory results on the matter. If it is indeed all local soil and microclimate conditions that decide how much microbes respire, it makes sense that each regional study will find a different effect of disturbance factors like nutrient addition or herbivory. A wise lesson again for us all: ecology can be darn complicated.

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