Organic matter mineralization is among the most fundamental of ecosystem processes. Such mineralization occurs via extracellular enzyme activity (EEA), a collective term for the action of a wide variety of enzymes that cleave, hydrolyze or otherwise degrade organic molecules in litter and soils. Given the central roles of such enzymes in carbon and nutrient cycling, resolution of the major controls over EEA can be important to understanding ecosystem responses to a changing environment. Past work shows that both biotic and abiotic drivers can alter EEA, and that, for example, the activity of nutrient-liberating enzymes may be modulated by the availability of that nutrient in the local environment. As a result, EEA can help drive plant-soil feedbacks that either magnify or reduce ecosystem responses to a given exogenous driver. Here, we used an in situ manipulation of both rainfall and litterfall in a lowland tropical forest to explore how changes in climate and organic matter inputs to soils would alter EEA for a suite of enzymes key to C, N and P cycling.
Results/Conclusions
The experiment was carried out in a forest located in the NW portion of the Osa Peninsula in Costa Rica. The site receives > 5,000 mm of rain each year, in a strongly seasonal pattern. Manipulation plots included control, no litter, double litter, and two levels of rainfall removal: 25% and 50%, respectively, below ambient. Measurements of EEA for three C-degrading enzymes (xylosidase, β-1,4-glucosidase and cellobiohydrolase), two involved in N cycling (β-1,4-N-acetylglucosaminidase and leucine aminopeptidase), and one in P mineralization (acid phosphatase) showed that all six enzyme activities: 1) increased between the late dry and early wet season, and then stayed relatively constant throughout the rainy season; and 2) increased substantially in double-litter plots. Interestingly, despite the seasonal trends, rainfall exclusion had no significant effects on EEA, nor did litter removal, despite substantial losses in total organic matter in such plots. Finally, we found that in litter addition plots, EEA increased much more than did microbial biomass, suggesting a higher specific activity in organic-matter rich plots. Data from other work in these plots show a concomitant shift in microbial community structure, which may partially account for the changes in both EEA and its specific activity.
