Shrubification of herbaceous peatlands modulates root exudates, increasing rhizosphere soil CO2 emissions while decreasing CH4 emissions

Abstract

As shrubs continue to expand in peatlands due to climate change, it is critical to examine the mechanisms underlying carbon dioxide (CO₂) and methane (CH₄) emissions. While the decrease in water levels during shrub encroachment into peatlands is known to increase soil carbon emissions, the role of the genetic potential of microbial metabolic processes mediated by root exudates in affecting rhizosphere soil carbon emissions is less well understood. Here, we conducted in situ field monitoring of shrub and herb peatlands, combined with laboratory incubation experiments involving the addition of root exudates. Using metagenomics and metabolomics technologies, we aimed to elucidate the microbiological mechanisms behind changes in carbon emissions. This study found that the rhizosphere soil under shrubs had a higher CO₂ emission rate with greater genetic potential for CO₂ production (19.36%), but exhibited a lower CH₄ emission rate conferring a lower genetic potential for CH₄ production (52.30%) than that under herbs. These differences were attributed to the distinct ways in which the root exudates of shrubs and herbs influence the structure and function of the microbial community, thereby favoring CO₂ and CH₄ emissions differently. Specifically, the relatively higher amounts of sugars and amino acids in shrub root exudates stimulate the genetic potential for cellulose and hemicellulose decomposition, leading to a 13.23% increase in CO₂ emissions. In contrast, the relatively higher amounts of lipids in herb root exudates promote the genetic potential for the acetic acid-type methanogenic process, resulting in a 33.50% increase in CH₄ emissions. Shrubification increased the genetic potential for labile carbon decomposition while decreasing the genetic potential for chitin and lignin decomposition, possibly promoting recalcitrant carbon conservation in the rhizosphere soil. Altogether, shrubification of herbaceous peatlands increases rhizosphere soil carbon emissions by regulating root exudates. It is recommended that appropriate ecosystem management measures be implemented to control shrub expansion, thereby optimizing carbon emissions from peatlands.