title: Impact of elevated N input on soil N cycling and losses in old-growth lowland and montane forests in Panama.
abstract: Nitrogen deposition is projected to increase rapidly in tropical ecosystems, but changes in soil-N-cycling processes in tropical ecosystems under elevated N input are less well understood. We used N-addition experiments to achieve N-enriched conditions in mixed-species, lowland and montane forests in Panama. Our objectives were to (1) assess changes in soil mineral N production (gross rates of N mineralization and nitrification) and retention (microbial immobilization and rapid reactions to organic N) during 1- and 9-yr N additions in the lowland forest and during 1-yr N addition in the montane forest and (2) relate these changes to N leaching and N-oxide emissions. In the old-growth lowland forest located on an Inceptisol, with high base saturation and net primary production not limited by N, there was no immediate effect of first-year N addition on gross rates of mineral-N production and N-oxide emissions. Changes in soil-N processes were only apparent in chronic (9 yr) N-addition plots: gross N mineralization and nitrification rates, NO3- leaching, and N-oxide emissions increased, while microbial biomass and NH4+ immobilization rates decreased compared to the control. Increased mineral-N production under chronic N addition was paralleled by increased substrate quality (e.g., reduced C:N ratios of litterfall), while the decrease in microbial biomass was possibly due to an increase in soil acidity. An increase in N losses was reflected in the increase in 15N signatures of litterfall under chronic N addition. In contrast, the old-growth montane forest located on an Andisol, with low base saturation and aboveground net primary production limited by N, reacted to first-year N addition with increases in gross rates of mineral-N production, microbial biomass, NO3- leaching, and N-oxide emissions compared to the control. The increased N-oxide emissions were attributed to increased nitrification activity in the organic layer, and the high NO3- availability combined with the high rainfall on this sandy loam soil facilitated the instantaneous increase in NO3-leaching. These results suggest that soil type, presence of an organic layer, changes in soil-N cycling, and hydrological properties are more important indicators than vegetation as an N sink on how tropical forests respond to elevated N input.
authors.affiliation: Buesgen Institute, Soil Science of Tropical and Subtropical Ecosystems, Georg-August University of Goettingen, Buesgenweg 2, Goettingen 37077, Germany. mcorre@gwdg.de
authors.forename: Marife D
authors.initials: MD
authors.lastname: Corre
authors.forename: Edzo
authors.initials: E
authors.lastname: Veldkamp
authors.forename: Julia
authors.initials: J
authors.lastname: Arnold
authors.forename: S Joseph
authors.initials: SJ
authors.lastname: Wright
mesh_terms: D000531:Altitude; D017753:Ecosystem; D009566:Nitrates / Q000737:chemistry; D009584:Nitrogen / Q000737:chemistry* / Q000378:metabolism*; D009587:Nitrogen Isotopes; D010176:Panama; D000644:Quaternary Ammonium Compounds / Q000737:chemistry; D012987:Soil / Q000032:analysis*; D014197:Trees; D014329:Tropical Climate
chemical_list: D009566:Nitrates; D009587:Nitrogen Isotopes; D000644:Quaternary Ammonium Compounds; D012987:Soil; D009584:Nitrogen