Responses of nitrous oxide fluxes and soil nitrogen cycling to nutrient additions in montane forests along an elevation gradient in southern Ecuador

• Published in: Biogeochemistry 2013-03, Vol.112 (1-3), p.625-636
• Main Authors: Martinson, Guntars O, Corre, Marife D, Veldkamp, Edzo
• Format: Article
• Language: English
• Subjects: altitude; Biogeosciences; Earth and Environmental Science; Earth Sciences; Earth, ocean, space; Ecological function; Ecosystems; Emissions; Environmental Chemistry; Exact sciences and technology; fertilizer application; Forest soils; gas emissions; Geochemistry; Histosols; Life Sciences; Montane forests; Mountain forests; Mountain regions; Nitrification; Nitrogen; Nitrogen cycle; Nitrous oxide; nutrient availability; Nutrient cycles; Nutrient status; nutrients; old-growth forests; Organic foods; Organic matter; Organic soils; phosphorus; rain forests; Rainforests; Soil and rock geochemistry; Soil ecology; Soil moisture; soil nutrient dynamics; Soil nutrients; Soil water; Soils; Surficial geology; temperature; Tropical forests; Tropical soils; tropics; Urea
• ISSN: 0168-2563; 1573-515X
• DOI: 10.1007/s10533-012-9753-9

Tropical montane forests are commonly limited by N or co-limited by N and P. Projected increases in N deposition in tropical montane regions are thought to be insufficient for vegetation demand and are not therefore expected to affect soil N availability and N₂O emissions. We established a factorial N- and P-addition experiment (i.e., N, P, N + P, and control) across an elevation gradient of montane forests in Ecuador to test these hypotheses: (1) moderate rates of N and P additions are able to stimulate soil-N cycling rates and N₂O fluxes, and (2) the magnitude and timing of soil N₂O-flux responses depend on the initial nutrient status of the forest soils. Moderate rates of nutrients were added: 50 kg N ha⁻¹ year⁻¹ (in the form of urea) and 10 kg P ha⁻¹ year⁻¹ (in the form of NaH₂PO ₄ . 2H₂O) split in two equal applications. We tested the hypotheses by measuring changes in net rates of soil–N cycling and N₂O fluxes during the first 2 years (2008–2009) of nutrient manipulation in an old-growth premontane forest at 1,000 m, growing on a Cambisol soil with no organic layer, in an old-growth lower montane forest at 2,000 m, growing on a Cambisol soil with an organic layer, and an old-growth upper montane rainforest at 3,000 m, growing on a Histosol soil with a thick organic layer. Among the control plots, net nitrification rates were largest at the 1,000-m site whereas net nitrification was not detectable at the 2,000- and 3,000-m sites. The already large net nitrification at the 1,000-m site was not affected by nutrient additions, but net nitrification became detectable at the 2,000- and 3000-m sites after the second year of N and N + P additions. N₂O emissions increased rapidly following N and N + P additions at the 1,000-m site whereas only smaller increases occurred at the 2,000- and 3,000-m sites during the second year of N and N + P additions. Addition of P alone had no effect on net rates of soil N cycling and N₂O fluxes at any elevation. Our results showed that the initial soil N status, which may also be influenced by presence or absence of organic layer, soil moisture and temperature as encompassed by the elevation gradient, is a good indicator of how soil N cycling and N₂O fluxes may respond to future increases in nutrient additions.