Researchers are studying the effects of climate change on the degradation of organic matter in the soil. First results underline the importance of the thermal insulation provided by an intact layer of snow on the dynamics of soil microorganisms.
You’d think that during the winter, all life trapped beneath a layer of snow would shut down and wait for warmer days. Think again! A thick uncompressed snow cover can act as an insulating blanket, keeping the underlying turf warm and the resident microorganisms happy - and active. Just how active they are, what they are doing, and how they would respond to a shift towards a warmer, drier climate are questions that Hermine Durand, a Master’s student from the Ecole Normale Supérieure in Paris, and Konstantin Gavazov, a PhD candidate from the Ecological Systems Laboratory (ECOS) are currently addressing.
“We already know that lignin and cellulose, the hard to digest components of organic matter found in soils, are decomposed during the winter months,” explains Gavazov. But due to the difficulty of studying the behavior of plants and microbial communities under a snow cover, only very little research has focused on carefully characterizing soil respiration under these conditions.
Soil respiration involves the degradation of organic matter present in the soil and the release of CO2 into the atmosphere. “It is important to distinguish between autotrophic and heterotrophic respiration,” explains Professor Alexandre Buttler, head of the ECOS lab. During the summer months, the majority of the released CO2 is autotrophic, stemming from a plant’s own metabolism. But during the winter months, heterotrophic respiration - the decomposition of organic matter in the soil by microorganisms, and the subsequent release of CO2 into the atmosphere - takes over.
Simulating climate change in the field
How would a change in weather patterns towards a warmer and drier climate affect soil respiration? Durand came to ENAC from the ENS in Paris to join forces with Gavazov to investigate this question as part of her Master’s thesis. To do so, they latched on to a field campaign set up by Gavazov, that studies the effects of climate change on plant life by means of an ingenious experimental setup. By transplanting turf from three different pasture types at an altitude of 1400 meters above sea level to lower altitudes of 1000 m, and 600 m, the effect of warmer, drier weather could be simulated in situ, in the field, and the responses by the three different pasture types can be compared.
“What we are seeing is that, under a deep, intact snow cover, the soil respiration is up to 10 times more active than in the absence of snow”, explains Hermine Durand. While on average, it may be warmer at lower altitudes, lack of the thermal insulation provided by the snowpack means that the soil is more directly subjected to atmospheric temperature fluctuations, potentially leading to more frequent freezing. And when the soil freezes, microbial activity appears to drop drastically.
To track the respiration rate of each turf transplant, they traveled to the field sites twice a month to measure the CO2 released into the atmosphere. To be able to access the ground even below a layer of snow, they covered a patch of each transplant with a PVC chamber about the size of a coke can, to which a Li-COR, an instrument capable of measuring the flux of CO2, could be connected via a long tube. Their preliminary results clearly confirm the hypothesis that, thanks to the insulating effect of snow, soil respiration is enhanced by a snow cover, as long as the snow did not fall on already frozen soil.
Back in the lab
In parallel to the field experiment, Durand and Gavazov set up a series of laboratory experiments to find out how a change in climate would affect the microbial fauna present in the soil and its respiratory dynamics.
Durand extracted soil samples from each transplant, some of which were buried beneath up to a meter and a half of snow to measure their respiration in the lab at a controlled temperature and humidity. Her preliminary findings corroborate the results obtained in the field. Soil samples from unwooded pastures seems to respire more than those from densely wooded pastures, and samples extracted at higher altitudes showed more respiratory activity than those extracted farther down. A warm, insulating blanket of snow, it seems, goes a long way in keeping soil microorganisms happy.
Lacking a snow cover, soil respiration may drop to low levels, as microorganisms resident in the soil wait for spring. In a marathon 48 hour experiment, Durand showed that, by “simulating the onset of spring” watering the samples with water enriched with sugars and acids to simulate the easily available nutrients from springtime’s carbon rich root exudate, the CO2 flux emitted from the soil increased four to six-fold, and that for all samples, regardless of their pasture type and altitude. Could this input in easily available carbon somehow jumpstart the activity of the soil microbes, enabling them to degrade carbon from less accessible sources, such as cellulose and lignin? An upcoming series of experiments will attempt to address this question.
So, what would happen in the absence of the winterly snow cover? “We are still trying to find out how this would affect the carbon pool and its interactions with the atmosphere,” says Gavazov. But according to him, the absence of snow could lead to the accumulation of organic material on the topsoil, increasing the risk of wildfires. And by keeping the sunlight from reaching the surface of the soil, the growth of saplings in the spring could be limited.