Catarina S. C. Martins, Loïc Nazaries, Manuel Delgado-Baquerizo, Catriona A. Macdonald, Ian C. Anderson, Sarah E. Hobbie, Rodney T. Venterea, Peter B. Reich, Brajesh K. Singh
Will future forests cool or warm the planet? Forest ecosystems influence climate because they are capable of consuming both CH4 and CO2 emissions and emit N2O (all greenhouse gases) – but it is not yet clear how a changing climate will itself alter the way tomorrow’s forest slow down or accelerate that change.
The main cause of Earth’s temperature increase in recent decades is human activity, the majority originating from industrial and agricultural activities such as burning of fossil fuels, clearing land, livestock cultivation and use of fertilisers in agriculture. Together these activities have raised atmospheric carbon dioxide levels (CO2) by 40%, methane (CH4) by 150% and nitrous oxide (N2O) by 20% since the industrial revolution. In the process, climate has changed globally, further influencing greenhouse gas exchanges between forests and atmosphere, but in ways we don’t yet understand. To remedy this we measured CO2, CH4 and N2O fluxes after 5 years of experimental warming (+3.4°C), and 2 years of ≈45% summer rainfall reduction, in two forest sites in a boreal-temperate transition zone under closed and open canopy conditions in Minnesota, USA. We also measured microbial gene abundances and soil physicochemical properties to identify the drivers of greenhouse gas emissions. Additionally, we focused on better understanding how environmental conditions such as soil temperature and moisture influence soil greenhouse gas emissions, in contrast to the effects of the soil microbial community.
We found that CO2 exchange was largely driven by temperature and moisture, whereas CH4 exchange was controlled by both soil texture and CH4-consuming soil microorganisms. Nitrous oxide emissions were regulated strictly biologically: by N2O-producing soil microorganisms. Interestingly, we also found that warming did not alter CO2 and CH4 emissions after 5 years of manipulation, while N2O emissions were greater with warming under an open canopy. This study shows that future climate conditions may impact greenhouse gas emissions less than initially expected in these ecosystems, and that both physical and biological soil attributes contribute to measured greenhouse gas emissions from forest soils.
Image: Temperate-boreal forest sites showing warming plots in (A) open and (B) closed canopy, (C) water removal apparatus and (D-E) gas chamber deployed during sampling for GHG collection. Photographs by Catarina S. C. Martins.