Plant-soil feedbacks are important drivers of ecosystem dynamics, and have been hypothesized to affect woody encroachment in savannas. Woody encroachment is expected to increase savanna soil fertility through increased deposition of dead organic matter which, once decomposed, release immobilized nutrients, favoring further establishment and growth of woody individuals. In this context, we tested if litter deposition promotes forest seedling growth in dystrophic savanna soils, and if this was parallel by an increase in microbial activity.
MethodsIn a greenhouse, we planted woody seedlings of three forest species in savanna soils either mixed or not (control) with litter from closely related savanna or forest species (10 species in total). We evaluated woody seedlings growth as well as the response of the soil microbiota, in terms of activity and biomass, to litter addition.
ResultsWe found that litter addition had either none or negative effects on seedling growth rates, and that different seedling species responded differently to litter addition. However, we did found microbial activity to increase in response to litter addition , especially if phosphorus (P) and carbon (C) are also mixed in the soil.
ConclusionsLitter addition stimulated soil microbial activity. However, seedling response was not consistent with an effect associated with increased soil mineralization. Instead, plant responses to seedling addition was species-specific, depending both on litter and on seedling species, and their interactions, but was never positive. Moreover, while the effects of litter input in seedlings growth was potentially mediated by soil microorganism activity, the specific effect was more consistent with a negative or neutral response. Thus, our results did not support the hypothesis that litter deposition trigger positive feedbacks with woody encroachment.