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Some like it dry, some like it wet: Soil microbes under climate change

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Around a billion bacteria, fungi and other microorganisms can be found in just one teaspoon-full of forest soil. “We know very little about many of these microorganisms, but they play an important role in global material cycles, and are involved in soil formation and supplying nutrients to plants,” says Martin Hartmann, a microbiologist at WSL. What we do not know is how these tiny organisms and the microbial processes asso­ciated with them respond to warmer and drier climates. This is, however, crucial to understand since climate models predict that dry and hot periods will become more frequent.


In Pfynwald, a semi-arid pine forest in Canton Valais, WSL researchers have been irrigating different experimental plots since 2003. There they are trying to find out whether drought is responsible for the extensive pine dieback in this
region. For this experiment, the Research Group at WSL headed by Beat Frey have analyzed the differences in the composition of the soil microbes in dry and irrigated plots.

Living with restrictions

To identify the species in both the irri­gated and dry soils, the researchers investigated the genetic composition of the microorganisms in the cor­responding soil samples. The com­position of the microbes differed greatly: the organisms living in the moist soils release a lot of CO2 due to their rapid growth and higher nutrient demand. The species in the dry soils produce, in contrast, little CO2 because they grow slowly and are less demanding.

Trees growing on dry sites filter less CO2 out of the air, which means less carbon is deposited in the soil. Through their reduced respiration, however, the drought-specialized microorganisms keep the quantity of carbon stored in the soil at a stable level. “This is a valuable insight as forest soils are important carbon reservoirs,” says Martin, and adds: “We are not sure, however, whether the microbes adapted to these dry conditions can, with their reduced activities, supply trees with enough nutrients like nitrogen and phos­phorous to sustain them in the long term.” (Stephanie Schnydrig, Diagonal 2/17)