Spatial distribution of pores helps determine where carbon is stored in
the soil

Date:
April 21, 2022
Source:
Helmholtz Centre for Environmental Research - UFZ
Summary:
Soils store more carbon than all the vegetation on the Earth's
surface.

However, there are still many unanswered questions about precisely
which processes favor accumulation in the soil. Soil scientists
have now developed a new method to show where and under what
conditions carbon is stored f in the soil. It turns out, it is
primarily the network of soil pores that controls the spatial
distribution of carbon.



FULL STORY ========================================================================== Soils store more carbon than all the vegetation on the Earth's surface.

However, there are still many unanswered questions about precisely
which processes favour accumulation in the soil. Under the leadership
of the Helmholtz Centre for Environmental Research (UFZ), a team of
soil scientists have developed a new method to show where and under
what conditions carbon is stored f in the soil. As they write in Nature Communications, it is primarily the network of soil pores that controls
the spatial distribution of carbon.


==========================================================================
In the public debate on climate protection, the importance of soil is
often forgotten. However, soils store considerably more carbon globally
than forests or the atmosphere. The long-term storage of carbon can be
quite complex. On one hand, it depends on how much atmospheric carbon
enters the soil through root growth, various mixing processes (e.g. soil cultivation or the activity of earthworms), and the seepage of dissolved organic compounds. On the other hand, it depends on whether the existing
carbon in the soil can be stabilised or is decomposed by bacteria and
fungi. Which process is more efficient -- storage or decomposition --
is determined primarily by the structure of the soil (e.g. the size of
the network of pores that help transport air, water, and nutrients).

"The carbon stored in plant residues and humus is not decomposed if
bacteria or fungal hyphae are larger than the pores in the soil where
it is stored," says Dr. Steffen Schlu"ter, UFZ soil physicist and lead
author of the study. What's more: If the pores are permanently filled
with water and thus without oxygen supply (e.g. in intact peat soils),
bacteria find it more difficult to use the carbon. "One of the decisive
factors for where carbon is stored in the soil is thus the spatial
distribution of the pores," says Schlu"ter. It had previously not been
possible to study the distribution pattern of the organic carbon within
the millimetre and micrometre sized pores.

But the scientists at the UFZ have now managed to do this. With their new method, they can precisely localise the carbon in the soil. It is based
on the staining of the organic compounds with osmium tetroxide, which
sorbs onto the carbon-containing double bonds and is then visualised
using X-ray computed tomography (CT). By scanning the soil sample
before and after staining, the researchers can infer the distribution
of the carbon from the differences in the images. Until now, this was
possible only with the help of elaborate synchrotron CT methods. However, because there are only two particle accelerators of this kind in Germany, access is severely limited. In contrast, X-ray CT is more widespread at
soil science institutes in Germany. The new approach thus facilitates
research. "You can't normally look inside the soil.

But this methodological innovation allows us to draw conclusions about
where and how well carbon is enriched in soil depending on the pore system
and organic material such as roots and litter," says Prof. Hans-Jo"rg
Vogel, head of the Department of Soil System Science at the UFZ. This
provides important information about processes in the soil and thus also
about the consequences they have for the stabilisation and decomposition
of carbon in soil.

As an example, the soil scientists tested their methodology at three
sites with different soil types and different moisture regimes:
a Chernozem site with low annual precipitation at the UFZ research
station in Bad Lauchsta"dt, a fine- textured Luvisol site with seasonal
water logging in the foothills of the Alps, and a permanently wet
Gleysol site influenced by groundwater near Giessen. The result: in
the immediate vicinity of the pores (i.e. in a margin of 50 to 100 micrometres), the concentration of carbon is lower than in the rest
of the soil. This is mainly because microbial activity decreases with increasing distance from the pores. "This pattern has been seen at all
three sites regardless of the moisture regime. The proximity to the
pore system thus favours the decomposition of organic matter, and the
distance to these pores promotes the stabilisation of carbon in the
topsoil -- the layer that is particularly important for agriculture,"
says Schlu"ter. In contrast, there are differences in the carbon content
around organic material such as plant residues. In dry Chernozem soil,
the carbon decreases with distance to the plant residues. The bacteria
and fungi are attached to the plant residues so they can easily get at
the carbon and metabolise it. The decomposition products then accumulate
around the plant residues. In the Gleysol, the opposite was true. The scientists found no enrichment of products around the plant residues.

One explanation is that dissolved decomposition products are more easily transported over longer distances under the wet conditions. "The moisture regime strongly influences the stabilisation patterns of carbon in the
soil because it controls the extent of carbon relocation from the plant residues into the surrounding soil," says Schlu"ter.

The new methodological approach now opens up interesting research
perspectives such as how the carbon is distributed in the subsoil
(i.e. the deeper soil layers up to one metre). So far, there is only circumstantial evidence that the distribution patterns of carbon in the
subsoil are different from those in the topsoil. That's because the latter
is not influenced by tillage and is not so intensely mixed by burrowing animals. Carbon can reach deeper layers only through accumulation at the
few roots or the diffusion of dissolved organic substances. "It would
be exciting to find out more about these processes.

Because of climate change and the resulting frequent droughts in the
topsoil, plants are increasingly lacking water. "The importance of the
subsoil for plant growth is thus increasing," says Vogel. With the new
method, the researchers hope to better understand the processes of carbon storage in deeper layers as well as the causes of the differences in the
carbon balance between different forms of land management (e.g. grassland
use and arable farming).


========================================================================== Story Source: Materials provided by Helmholtz_Centre_for_Environmental_Research_-_UFZ. Note: Content may be
edited for style and length.


========================================================================== Journal Reference:
1. Steffen Schlu"ter, Frederic Leuther, Lukas Albrecht, Carmen
Hoeschen,
Ru"diger Kilian, Ronny Surey, Robert Mikutta, Klaus Kaiser,
Carsten W.

Mueller, Hans-Jo"rg Vogel. Microscale carbon distribution
around pores and particulate organic matter varies with soil
moisture regime. Nature Communications, 2022; 13 (1) DOI:
10.1038/s41467-022-29605-w ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/04/220421094101.htm

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