Only microbes give rise to humus

Contribution of bacterial residues to soil fertility has so far been underestimated

Close-up of soil bacteria (electron microscope) © Burkhard Schmidt-Bridges, Institute of Materials Science / TU Dresden, Christian Schurig / UFZ
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Remains of dead bacteria are more important to soils worldwide than previously thought. Accordingly, most of the organic matter in the soil does not come directly from parts of plants, as previously thought, but was converted to 40 percent of microbes. This is shown by the study of an international research team, which has now been published in the specialist journal "Biogeochemistry" on 5 December, to coincide with the World Soil Day.

Organic soil constituents represent the largest proportion of terrestrial carbon in the biosphere. Therefore, they are not only of great importance for soil fertility and agricultural yields. They are also one of the key factors controlling the carbon dioxide concentration in the atmosphere. Depending on how this resource is managed, climate change can therefore be slowed down or accelerated. Until now, it has been thought that the organic components of the soil are mostly derived from decayed plant material, which is converted into humic matter. Scientists at the Helmholtz Center for Environmental Research (UFZ), together with colleagues from Tübingen, Hanover and Stockholm, have now been able to refute this thesis in a laboratory experiment and field trial.

Resistant film of organic molecules

For their study, the scientists had first in a incubation experiment model bacteria with the stable isotope 13C labeled and introduced into soil material. After 224 days of incubation, they determined the fate of the bacteria's carbon. "As a result, we found remains of bacterial cell walls throughout our soil samples. Such fragments have also been observed in other studies, but never further discussed and quantified, "explains Matthias Kästner from the UFZ. Apparently, peptides and proteins from the microbial cell ensure that a film of organic molecules forms on the mineral soil constituents. In this the carbon of the dead bacteria is stored.

When the fragments of dead bacterial cell walls dry out, they lose their rubbery properties and can become hard as glass, the researchers report. If the soil becomes moist again later, these organic residues can no longer absorb moisture - as a result, they can not easily be broken down by other bacteria. The scientists say this may explain why the normally readily degradable carbon compounds remain in the soil for so long. "This new approach explains many properties of organic soil constituents that used to appear contradictory, " says Kästner.

Soil formation on the Dammaglacier as a test case

Cell shells remain. Small-scale fragments of these shells (red) then represent the microparticulate matrix in soils and sediments. Burkhard Schmidt-Br cken, Institute of Materials Science / TU Dresden, Christian Schurig / UFZ

After the laboratory experiment, the review of the thesis followed in the field. In the summer of 2009, the researchers took soil samples in the run-up to the Dammagletscher in the Swiss canton of Uri. Over the past 150 years, this glacier has shrunk by about a kilometer in length. Back remained granite, which was slowly colonized by living things and covered with soil. Shrubs and later trees also followed on pioneer plants such as mosses and grasses. Not only for climatologists, but also for ecologists, the Damma Glacier has become an important outdoor laboratory in which various studies are conducted. display

The researchers examined samples from soil areas aged 0 to 120 years. Investigations by scanning electron microscopy showed that the mineral particles in the soil were covered with increasing age by an increasingly dense film of residues from bacterial cell walls. Thesis and laboratory results could be confirmed in the field. The overwhelming part of the entry of plant residues into fertile soils is thus quickly processed by microorganisms such as bacteria. This in turn leads to more organic matter in the soil. "This underlines the importance that bacteria play in organisms in every soil, " summarizes K stner. Even though most of the organic carbon is produced in the ecosystems of plants, it initially makes a detour via bacteria and fungi. Their residues finally make up the majority of organic soil components. (doi: 10.1007 / s10533-012-9791-3).

(Biogeochemistry, 05.12.2012 - NPO)