Priming effect on soil carbon decomposition by root exudate surrogates: A meta-analysis

While plant root exudates can greatly and rapidly influence soil organic carbon (C) decomposition via the priming effect, the general pattern of these priming effects remains unexplored. We conducted a meta-analysis with 104 publications to synthesize generalities of the priming effect responses to surrogates of root exudates such as simple sugars, low-molecular-weight organic acids, amino acids and phenolics, and explore their regulators. We found that in general addition of root exudate surrogates significantly stimulated microbial biomass C, enzyme activities, and soil C decomposition by on average 27%, 28% and 39%, respectively, i.e., a positive priming effect. We found distinct differences in the magnitude and regulators of the priming effects caused by root exudate surrogates. Amino acids induced the highest priming effect followed by simple sugars, low-molecular-weight organic acids and phenolics, likely because amino acids provided a source of nitrogen (N) that may have aided the priming effect through favoring microbial growth. The priming effect caused by simple sugars increased but then decreased with soil C to N ratio increasing, and the threshold was at a soil C to N ratio of ∼14. Another important regulator was the incubation temperature, whereas the low-molecular-weight organic acid-induced priming effect was positively correlated with the addition rate. These results emphasize the importance of the specificity of root exudate types with multiple mechanisms causing the priming effect. Future studies should conduct long-term experiments with continuous addition of mixed compounds to provide insights into how changes in root exudates modulate soil C decomposition via the priming effect.