Impact of elevated N input on ecosystem processes in old-growth lowland and montane forests in Panama

Eingeladen durch Prof. Matzner.

Nitrogen deposition is projected to increase rapidly in tropical ecosystems, but few studies address how such environmental change affects soil N cycling, retention, trace gas fluxes, leaching losses and net primary production in tropical forests. We used N addition experiments to achieve N-enriched conditions in mixed-species, lowland and montane forests in Panama. In the lowland site, control and N-addition plots were laid out in a stratified random design with four replicates. N addition started in June 1998. Just outside these long-term manipulation plots, we established four additional plots in 2006 to represent the first-year N-addition treatment. In the montane site, control and N-addition plots were set up in a paired-plots design with four replicates. N addition started in February 2006. At both sites, each treatment plot was 40x40 m and plots were separated by at least 40 m. The N-addition plots received 125 kg urea-N/ha.yr split in four equal applications. 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 after chronic (9-11 yr) N addition: 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 increase in soil acidity. Increase in N losses was reflected in the increase in 15N signatures of litterfall under chronic N addition. Soil CO2 and CH4 fluxes were not affected by 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. This contradicts the current assumption that N-limited tropical montane forests will respond to N additions with only small and delayed increases in soil N-oxide missions. We attribute this fast and large response of N-oxide emissions to the presence of an organic layer (a characteristic feature of this forest type) in which nitrification increased substantially during the first year N addition. The high NO3- availability combined with the high rainfall on this sandy loam soil facilitated the instantaneous increase in NO3- leaching. Soil CO2 efflux was reduced after 2-3-yr N addition compared to the control. This reduction was caused by a decrease in soil CO2 efflux during the high stem-growth period of the year, suggesting a shift in carbon partitioning from below- to aboveground in the N-addition plots in which stem diameter growth was promoted. Soil CH4 fluxes were not affected during 3-yr N addition. 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 N sink on how tropical forests respond to elevated N input.

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