Resource stoichiometry mediates soil C loss and nutrient transformations in forest soils

Root exudation is increasingly being recognized as an important driver of ecosystem processes; however, few studies have examined the degree to which variations in exudate stoichiometry and soil resources affect microbial controls of nutrient availability and decomposition. We added root exudate mimics of varying chemical quality to soils collected from two adjacent forest stands (one a ∼70 year-old spruce plantation, the other a ∼200 year-old spruce-fir forest) that differ strongly in N availability. The exudate treatments were added for 50 consecutive days, and included water (control), C alone, N alone, and three combinations of C and N that varied stoichiometrically (i.e., C:N ratio of 10, 50 and 100). Exudate additions containing little or no N promoted the greatest losses of soil C in two soils, with the greatest losses occurring in the moderately labile (i.e., acid-extractable) fraction of the low N plantation soils. However, despite the uniformity of priming effects between sites (∼7% loss of soil C for both), there was little congruence in exudate-induced effects on microbial biomass and activity. In the plantation soils, exudates generally increased microbial biomass (especially fungi), accelerated N cycling and increased lignin-degrading enzyme activities relative to controls. In contrast, exudate additions to spruce-fir soils mostly decreased microbial biomass, decelerated N cycling, and had variable impacts on lignin-degrading enzyme activities (decreased phenol oxidase but increased peroxidase). Collectively, this study suggests that while root exudates with low C and N have the potential to accelerate soil C losses by stimulating microbes to mine N from soil organic matter, the consequences of exudate inputs on nutrient fluxes are less predictable, and may hinge on the recalcitrance of (soil organic matter) SOM, N availability and microbial communities.