Controlling photosynthesis, carbon dioxide (CO2) is one of the most important compounds affecting plant growth. The continuous, high-precision measurement of changes in atmospheric CO2 concentrations started in 1958 and showed a continuously increasing concentration since then, from 317 ppm in 1958 to 397 ppm in September 2015. The environmental impacts of increased greenhouse gases’ concentrations are widely studied and CO2 is by far the most common greenhouse gas studied. The effects of higher atmospheric CO2 concentration on plant physiology and growth have been challenged on almost all biomes. But what is the situation in soils?
Carbon dioxide concentration in soil
Aeration is a critical physical factor influencing root growth by changing oxidation-reduction balance of soil constituents but also directly affecting root metabolism. Decreasing oxygen (O2) and increasing carbon dioxide concentrations in the soil environment are caused by soil microbial, faunal and plant root respiration and restricted aeration in the soil compared to above-ground. As a consequence, high CO2 concentrations in soil are generally associated with low O2 concentration (hypoxia). An average value for CO2 concentration in lighter soil is 0.25–0.3 % and 1 % or more in wet soils that are rich in humus at 15 cm depth. Above the soil, the average CO2 concentration is 0.038 %.
Emissions of carbon dioxide from soil have increased in the last 50 years, without any clear explanation (Bond-Lamberty and Thomson 2010) but, to my knowledge, no data are available about the climate change effect on CO2 concentration in soils. Particular temporal or spatial conditions such as waterlogging or soil compaction can lead to high CO2 accumulation in soil. This kind of particular situation also appears in winter under the snow cover. Increasing frequency of rain-on-snow events and higher winter time temperature fluctuations due to climate change lead to repeated freeze-thaw events and ground ice encasement. Ice layers are impervious to gases, restrict the soil-atmosphere gas exchange and thus lead to hypoxia and high CO2 accumulation in the subnivean environment.
Effect on soil and plants
Impacts of elevated soil carbon dioxide concentration on plants and soil microbes are little studied. Only indirect effects of elevated CO2 concentration have been measured on soil respiration or microbial community composition through changes in plant processes. As photosynthesis does not take place below-ground, CO2 is not essential for any plant metabolic activity. Physiological effects of high CO2 concentration on root growth in waterlogged soils have been reviewed (Greenway et al. 2006). However, the question of the effect on plant growth is tricky as high CO2 is commonly associated with hypoxia. So the real question turns to be: which is most damaging factor to roots, high CO2 or hypoxia?
The majority of the studies dealing with soil aeration in stressful situations focused on perennial grasses and crop species due to the risk of economical impacts of yield losses. Globally, results suggest a high species-specificity in CO2 tolerance. Most of the studies dealing with winter ice encasement also concern crops and perennial grasses. Castonguay et al. (2009) showed that low oxygen concentration was more damaging than high carbon dioxide alone, but combination of high CO2 and low O2 concentrations seemed to be more damaging than low O2 alone to perennial grasses.
Ice encasement in boreal forest is studied in an Academy-funded project “Winter in changing climate: effects of snow conditions on plants, soil and their interactions in the boreal forest”, ongoing until 2017 in Luke, Rovaniemi. Ice encasement was artificially created in a coniferous forest in Rovaniemi area. Hypoxia and accumulation of CO2 were measured also in control plots (ambient conditions) but the major differences with ice encasement was a higher maximum CO2 concentration (7.3%, more than 3 times higher than in AMB plots) and, more importantly, the duration of high CO2 concentration (>4% CO2), which was 45 days in ice encased plots compared to 0 days in control plots (Martz et al., unpublished). The effects of changing overwintering conditions on conifer seedlings were monitored and results after one year of snow manipulation showed deleterious effects of IE on winter survival and growth. The relative damaging effect of hypoxia, carbon dioxide accumulation, or their combination in soil remains to be found but our results show that increased frequency of warm spells in winter could influence forest regeneration with important implications for boreal forest ecology and economy.
Many environmental conditions or anthropogenic activity can affect soil gas composition. All gases, not only O2, have the potential to impact soil processes; their relative impact is little studied and should deserve more attention, in particular their effects on the microbiome composition and activity.
Bond-Lamberty, B., and Thomson, A. 2010. Temperature-associated increases in the global soil respiration record. Nature 464: 579-582.
Castonguay, Y., Thibault, G., Rochette, P., Bertrand, A., Rochefort, S., and Dionne, J. 2009. Physiological responses of annual bluegrass and creeping Bentgrass to contrasted levels of O2 and CO2 at low Temperatures. Crop Science 49: 671-689.
Greenway, H., Armstrong, W., and Colmer, T.D. 2006. Conditions Leading to High CO2 (>5 kPa) in Waterlogged–Flooded Soils and Possible Effects on Root Growth and Metabolism. Annals of Botany 98: 9-32.