Abstract:
Atmospheric nitrogen (N) and phosphorus (P) depositions are expected to increase in the tropics
as a consequence of increasing human activities in the next decades. Furthermore, a possible
shortened El Niño Southern Oscillation cycle might come along with more frequent calcium (Ca)
depositions on the eastern slope of the Ecuadorian Andes originating from Saharan dust. It is
crucial to understand the response of the old-growth montane forest in Ecuador to increased
nutrient deposition to predict the further development of this megadiverse ecosystem.
I studied experimental additions of N, P, N+P and Ca to the forest and an untreated
control, all in a fourfold replicated randomized block design. These experiments were conducted
in the framework of a collaborative research effort, the NUtrient Manipulation EXperiment
(NUMEX). I collected litter leachate, mineral soil solution (0.15 and 0.30 m depths), throughfall
and fine litterfall samples and determined N, P and Ca concentrations and fluxes. This approach
also allowed me to assess whether N, P and/or Ca are limiting nutrients for forest growth.
Furthermore, I evaluated the response of fine root biomass, leaf area index, leaf area and specific
leaf area, tree diameter growth and basal area increment contributed from a cooperating group in
the Ca applied and control treatments.
During the observation period of 16 months after the first fertilizer application, less than
10, 1 and 5% of the applied N, P and Ca, respectively, leached below the organic layer which
contained almost all roots but no significant leaching losses occurred to the deeper mineral soil.
Deposited N, P and Ca from the atmosphere in dry and wet form were, on balance, retained in the
canopy in the control treatment. Retention of N, P and Ca in the canopy in their respective
treatments was reduced resulting in higher concentrations and fluxes of N, P and Ca in
throughfall and litterfall. Up to 2.5% of the applied N and 2% of the applied P and Ca were
recycled to the soil with throughfall. Fluxes of N, P and Ca in throughfall+litterfall were higher in
the fertilized treatments than in the control; up to 20, 5 and 25% of the applied N, P and Ca,
respectively, were recycled to the soil with throughfall+litterfall.
In the Ca-applied plots, fine root biomass decreased significantly. Also the leaf area of the
four most common tree species tended to decrease and the specific leaf area increased
significantly in Graffenrieda emarginata Triana, the most common tree species in the study area.
These changes are known plant responses to reduced nutrient stress. Reduced aluminium (Al)
toxicity as an explanation of the Ca effect was unlikely, because of almost complete organocomplexation
of Al and molar Ca:Al concentration ratios in solution above the toxicity threshold.
The results suggest that N, P and Ca co-limit the forest ecosystem functioning in the
northern Andean montane forests in line with recent assumptions in which different ecosystem
compartments and even different phenological stages may show different nutrient limitations
(Kaspari et al. 2008). I conclude that (1) the expected elevated N and P deposition will be
retained in the ecosystem, at least in the short term and hence, quality of river water will not be
endangered and (2) increased Ca input will reduce nutrient stress of the forest.
