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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