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Fluxes and fates of nitrogen in soils of old-growth tropical montane forests with elevated nitrogen input

Edzo Veldkamp1, Angelica Baldos1, Marife D. Corre1
1 Soil Science of Tropical and Subtropical Ecosystems, University of Göttingen

O 10.6 in Links between the N cycle and other elements

17.07.2014, 16:10-16:30, H20

Nitrogen (N) retention in tropical forest soils and how its mechanisms change with increase in N input are of current interest because of their link to soil carbon dynamics and the presently increasing N deposition in tropical regions. We conducted an in-situ 15N pulse chase study to: (1) assess the net fluxes and fates of mineral N (NH4+ and NO3-) in tropical montane forest soils with low N availability, and (2) determine the effects of four years of low N addition on the fluxes and fates of mineral N in these forest soils. Our study sites were located in Ecuadorian Andes at 1000 m and 3000 m elevations, where control (no manipulation) and N-addition (50 kg urea-N ha-1 year-1) treatments, with four replicate plots (20 m x 20 m each), have been established since 2008. From November 2010 to October 2011, we traced the fates of NH4+ and NO3- in different soil N pools (NH4+, NO3-, extractable organic N, microbes, fine roots and soil organic N), using one-time application of either 15NH4+ or 15NO3- at very low amounts but 99 % 15N enriched. Both 15N tracers showed similar dynamics: a redistribution phase (i.e. short-term fate) characterized by large fluxes and fast transfers of 15N among N pools, and an equilibrium phase (i.e. long-term fate) depicted by small fluxes and stable transfers of 15N among N pools. The short- and long-term fates of either 15N tracer also showed similar mechanisms of paths and fluxes among of different N pools in both forest sites, signifying the lack of preferential retention for either NH4+ or NO3-. In control plots of both forest sites, the short-term fate signified a complete recovery of both 15N tracers with the largest sink in the soil organic N pool (52-62 % of the added 15N), which was mainly contributed by microbial N turnover and fine-root related N release. The long-term fate showed low 15N flux rates among N pools but a consistent flux into the slowly cycling soil organic N pool, suggesting that internal soil N cycling might not be largely dependent on the recycling of recently incorporated N in the soil organic matter. The low N availability in these control forest soils might have induced the efficient and equal retention of both NH4+ and NO3- relative to their availability. Four years of low N addition resulted in reduced N retention with long-term fate in soil organic N pool accounting only 30-38 % of the added 15N tracers. This was because the movement of N out of microbial and fine-root N pools did not lead to its accumulation in soil organic N. These results robustly show that these Andean montane forests are highly sensitive to changes in N deposition at a rate expected to occur at these sites. Therefore, greater attention should be paid to the biological implications of increased N deposition with the consequent decrease in N retention, and thus possibly also carbon, in these forests.



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last modified 2014-06-19