Response of element cycling and budgets to nutrient additions in a tropical montane forest of Ecuador

The tropical montane forests in southern Ecuador are subject to rising nitrogen (N), low phosphorus (P), and episodic calcium (Ca) deposition. To investigate the response of the vegetation, soil organic layer and mineral soil to 0.3 m depth to increased nutrient inputs, we initiated in 2008 an interdisciplinary Nutrient Manipulation Experiment (NUMEX) at 2000 m a.s.l. We have applied N as urea at 50 kg ha−1 year−1, P as NaH2PO4 at 10 kg ha−1 year−1, combined N and P at 50 + 10 kg ha−1 year−1, and Ca (as CaCl2), at 10 kg ha−1 year−1. From 2008 to 2012, we set up annual budgets by calculating net fluxes of N, P, Ca and Na for the canopy, the organic layer and the mineral soil and determined δ15N values in the foliage of the four most abundant tree species, litterfall and organic layer. The addition of P and N + P increased P leaching from the canopy, suggesting a reduced retention of deposited P by canopy organisms. All added nutrients were largely retained in the soil organic layer and tightly cycled between the organic layer and the vegetation via litterfall and throughfall. The small leaching losses of N, P, Ca and Na from the organic layer were retained in the upper mineral soil. The retention of the added nutrients in the ecosystem indicated a strong nutrient demand. Nevertheless, the 15N enrichment in the organic layer was an early indicator of beginning N losses from the ecosystem by leaching and volatilization, which could not yet be detected by our flux-based budgeting approach.