Limitation by multiple nutrients revealed by a long-term, large-scale nutrient augmentation experiment in lowland Panama

Background/Question/Methods

Nutrient limitation of ecological or ecosystem-level processes can be determined directly via manipulation of nutrient elements.  Partly owing to a variety of logistical challenges (e.g., environmental spatial heterogeneity, long-lived trees, diverse species composition), this straight-forward experiment has been attempted in relatively few intact, old-growth tropical forests.  We initiated a nutrient-addition press-experiment in 1998 in lowland moist forest of the Barro Colorado Nature Monument (Gigante Peninsula), Republic of Panama.  The experiment comprises factorial NPK-additions and a separate micronutrient treatment, each replicated four times.  Each fertilized and control plot is a 0.16-ha square within which we have been assessing components of net primary productivity (e.g., fine-root dynamics, litterfall, trunk diameter growth) and other ecosystem processes (e.g., decomposition), several of which have been monitored for over a decade.   

Results/Conclusions

At least one monitored ecological or ecosystem-level process has responded to each of the three augmented macronutrients or to the addition of micronutrients.  For example, the addition of K was associated with decreases in stand-level fine-root biomass that appeared after 2 yr and persisted for at least a decade.  The addition of P was initially (after 4 yr) associated with a significant increase in the reproductive fraction of litter and later (after 10 yr) with a marginally significant increase across all litter fractions.  The addition of K and N together was associated with significant increases in growth rates of stems 1- to 10-cm diameter at breast height.  One or more micronutrients were associated with enhanced leaf-litter decomposition, K enhanced cellulose decomposition, and P enhanced both.  Our long-term, large-scale nutrient augmentation experiment suggests that key ecological and ecosystem-level processes in late-secondary to old-growth tropical forests on highly weathered, but relatively fertile mainland soils may nevertheless be limited by multiple elements.