Author Adam Pellegrini discusses the findings of his new Journal of Ecology article – “Frequent burning causes large losses of carbon from deep soil layers in a temperate savanna.” Find out more about how fire frequency can impact carbon storage and root biomass in deep soil layers.
Fires are dramatically changing in frequency and intensity, due to
shifting climate and altered land use across the globe. Changes in fire can
have dramatic effects, for instance altering plant cover and animal
populations. So when we think about fire effects on ecosystems, we often are
drawn to the striking changes aboveground. However, changes aboveground are
only part of the story. Fire also causes many transformations belowground.
However, the difficulty in measuring belowground processes and the slow processes
influencing soils has limited our understanding of belowground fire effects.
It has been repeatedly demonstrated that fire can cause losses of
soil carbon, through direct combustion and subsequent reduction in plant
biomass. However, those two mechanisms lead to a shift in soil carbon in the
upper soil layers. Consequently, most studies that evaluate the effect of fire
on soils only measure the topsoil, working under the assumption that changes in
the deeper soil layers are inconsequential.
In our study, we dug deeper to better understand how fire changes
soil throughout a one meter profile. Our study site was in a temperate oak
savanna in the Midwest region of the USA at a Long Term Ecological Research Network site, Cedar Creek Ecosystem Science
Reserve. Cedar Creek has an amazing burning experiment that
started in 1964, which has dramatically transformed the ecosystem. This
experiment has shown that a single factor (fire frequency) can transform this
system from forest to savanna to grassland, in only a few decades. The fire
treatments range from complete exclusion to frequent fire (burned 3 out of
every 4 years), with several other treatments in between. Across the fire
frequency gradient, we found that the change in deep soil (>20cm) carbon
roughly doubled the quantity of soil carbon lost due to frequent burning.
A survey of plant root biomass revealed that there tended to be an
increase in fine roots in the topsoil with more frequent burning, likely due to
the colonization of grasses. However, there was a large decline in root biomass
in deep soil. This suggests that changes in inputs within the deep layers are
behind shifts in carbon storage. Analyses of carbon isotopes revealed that the
relative balance between herbaceous and woody vegetation inputs to deep soil
carbon did not change, contrasting with the large shift in their relative
contributions in the topsoil. These two lines of evidence demonstrate the
importance of considering fire-driven changes in plant biomass allocation to
roots, and how this allocation changes with depth.
Our findings also have implications for conservation. We found that
the transition from a forest to a woody savanna resulted in relatively little
change in soil carbon. However, soil carbon declined with more frequent
burning, causing the landscape to lose more plant biomass. This result suggests
that savannas can be maintained with periodic burning without losing much carbon
belowground — demonstrating that the conservation of savanna ecosystems need
not be at odds with carbon storage.
Like many ecological studies, our results raise more questions. It
is difficult to reconcile the contrasting sensitivity of deep soil carbon to
fire across ecosystems — while this oak savanna was very sensitive other
systems, such as certain tropical savannas, are not. Moreover, although
our findings suggest shifts in root biomass inputs are behind changes in deep
soil carbon, this mechanism still needs to be directly tested.
- Adam
Pellegrini, Department of Plant Sciences, University of Cambridge, UK
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