Abstract
Tropical regions are facing increasing atmospheric inputs of nutrients, which will have unknown consequences for the structure and functioning of these systems. Here, we show that Neotropical montane rainforests respond rapidly to moderate additions of N (50 kg ha−1 yr−1) and P (10 kg ha−1 yr−1). Monitoring of nutrient fluxes demonstrated that the majority of added nutrients remained in the system, in either soil or vegetation. N and P additions led to not only an increase in foliar N and P concentrations, but also altered soil microbial biomass, standing fine root biomass, stem growth, and litterfall. The different effects suggest that trees are primarily limited by P, whereas some processes—notably aboveground productivity—are limited by both N and P. Highly variable and partly contrasting responses of different tree species suggest marked changes in species composition and diversity of these forests by nutrient inputs in the long term. The unexpectedly fast response of the ecosystem to moderate nutrient additions suggests high vulnerability of tropical montane forests to the expected increase in nutrient inputs.
Highlights
Since the 1950/60s, anthropogenic changes to the cycling of the key nutrients nitrogen (N) and phosphorus (P) have dramatically altered the structure and functioning of many ecosystems in the world’s industrialized regions [1,2,3,4,5,6]
Higher P inputs are mostly due to the deposition of dust [4,21,22,23,24], but sources of N can be varied, including oxidised and reduced N compounds emitted with farming, livestock breeding and the combustion of fossil fuels, and N released through biomass burning with the conversion of tropical forests [1,25,26]
Soil nutrient pools and soil biological activity Nutrient addition did not result in a significant increase of the organic layer N pool while the P pool increased after combined addition of N and P (Fig. 1)
Summary
Since the 1950/60s, anthropogenic changes to the cycling of the key nutrients nitrogen (N) and phosphorus (P) have dramatically altered the structure and functioning of many ecosystems in the world’s industrialized regions [1,2,3,4,5,6]. Elevated N and P inputs affect virtually all components and processes of terrestrial and aquatic ecosystems, including plant growth, plant longevity and stress tolerance, plant community composition and diversity, biotic interactions (plant-plant, plant-fungus, plant-animal), the composition and activity of heterotrophic communities, and the storage and cycling of carbon, nutrients and water [7,8,9]. In the past 30 years, research has focused on the structural and functional responses of temperate and boreal forests to atmospheric N inputs [15,16,17,18] because the bulk of fertilizer use worldwide was in the industrialized nations of the northern hemisphere These regions had pronounced gaseous NOx emissions originating from the combustion of fossil fuels and NH3 emissions from animal production [1]. Higher P inputs are mostly due to the deposition of dust [4,21,22,23,24], but sources of N can be varied, including oxidised and reduced N compounds emitted with farming, livestock breeding and the combustion of fossil fuels, and N released through biomass burning with the conversion of tropical forests [1,25,26]
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