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. In the literature, it is frequently assumed
that tropical montane forests are N-limited, while tropical lowland forests are P-limited. In a low-level N
and P addition experiment, we determined the short-term response of N and P cycles in a north Andean
montane forest on Palaeozoic shists and metasandstones at an elevation of 2100m a.s.l. to increased N
and P inputs. We evaluated experimental N, P and N+ P additions (50 kg ha−1 yr−1 of N, 10 kg ha−1 yr−1
of P and 50 kg + 10 kg ha−1 yr−1 of N and P, respectively) and an untreated control in a fourfold replicated
randomized block design. We collected litter leachate, mineral soil solution (0.15 and 0.30m depths),
throughfall and litterfall before the treatment began (August 2007) until 16 months after the first nutrient
application (April 2009). Less than 10 and 1% of the applied N and P, respectively, leached below the
organic layer which contained almost all roots and no significant leaching losses of N and P occurred
to below 0.15m mineral soil depth. Deposited N and P from the atmosphere in dry and wet form were
retained in the canopy of the control treatment using a canopy budget model. Nitrogen and P retention
by the canopy were reduced and N and P fluxes in throughfall and litterfall increased in their respective
treatments. The increase in N and P fluxes in throughfall after fertilization was equivalent to 2.5% of the
applied N and 2% of the applied P. The fluxes of N and P in litterfall were up to 15% and 3%, respectively,
higher in the N and N+ P than in the control treatments. We conclude that the expected elevated N and P
deposition in the tropics will be retained in the ecosystem, at least in the short term and hence, N and P
concentrations in stream water will not increase. Our results suggest that in the studied tropical montane
forest ecosystem on Palaeozoic bedrock, N and P are co-limiting the growth of organisms in the canopy
and organic layer.
