Tropical Andean Forests Are Highly Susceptible to Nutrient Inputs—Rapid Effects of Experimental N and P Addition to an Ecuadorian Montane Forest

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.

Performance of Seedlings of a Shade-Tolerant Tropical Tree Species after Moderate Addition of N and P

“…In southern Ecuador, nutrient deposition has been suggested to largely origin from extensive biomass burning occurring in the Amazon basin leading to increasing depositions of >10 kg N ha −1 in the last decade (Fabian et al, 2005;Wilcke et al, 2013). The continued nutrient addition in this experiment had already shown effects on various belowground to aboveground compartments in the experiment's first years: Homeier et al (2012) found in the common tree species a positive growth response after N (two out of four species), P (two out of four species) and N+P addition (three out of four species) and an increase of leaf litter production after N and NP addition suggesting that at 2000 m. a.s.l. N and P might be co-limiting.…”

“…Phosphorus (P) and nitrogen (N) are supposed to limit stand productivity in the majority of terrestrial ecosystems as well as in tropical forests (Tanner et al, 1998;Elser et al, 2007;Vitousek et al, 2010;Harpole et al, 2011;Homeier et al, 2012). Anthropogenic influences have modified natural nutrient availability worldwide by pollutants released to the atmosphere and the resulting increased deposition to soils.…”

“…Anthropogenic influences have modified natural nutrient availability worldwide by pollutants released to the atmosphere and the resulting increased deposition to soils. It is expected that increasing nutrient deposition has the potential to alter the dynamics of tropical forest ecosystems causing structural and compositional changes of the plant communities in response to modified availability of limiting elements (e.g., Lewis and Tanner, 2000;Homeier et al, 2012).…”

Litter fall production and decomposition in a fragment of secondary Atlantic Forest of São Paulo, sp, southeastern Brazil

“…Xu et al (2013) showed that total N and C:N ratios in the mineral soil increased with increased litter fall inputs. Homeier et al (2012) found moderate nutrient inputs in a tropical montane forest in southern Ecuador. The authors suggested that whether this trend carries on, these parts of tropical…”

Soil N2O fluxes along an elevation gradient of tropical montane forests under experimental nitrogen and phosphorus addition

“…In contrast, Wang et al (2014) showed that the interaction between N and P addition could result in increased N 2 O emissions in a tropical lowland forest in Southern China. Phosphorus is known to limit plant growth in TMFs (Tanner et al, 1998;Homeier et al, 2012), but the effect of elevated P deposition on soil N cycling in these areas has been little studied.…”