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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, NO 3 − leaching, and N-oxide emissions increased, while microbial biomass and NH 4 + 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 15 N 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, NO 3 − 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 NO 3 − availability combined with the high rainfall on this sandy loam soil facilitated the instantaneous increase in NO 3 − 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.

This study focuses on the microbial N cycle in the acid soil of a beech forest that falls in the upper range of the N saturation continuum. Our objectives were: (1) to quantify microbial N cycling under long-term N-saturated and limed conditions and (2) to determine the factors controlling the differences in microbial N cycling. Our study site has a long history of high N deposition: $\\geq\\!25\\>kg\\>N\\!\\cdot\\!ha^{-1}\\!\\cdot\\!yr^{-1}$ since measurements began in 1971. This was further enhanced by 11 yr (1983-1993) of fertilization (140 kg ammonium sulfate-N·ha-1·yr-1) to create an N saturation plot. Another plot was limed with 30 Mg/ha dolomitic limestone in 1982. In 1999-2000, gross rates of microbial N cycling were measured using15N pool dilution techniques. Despite the chronic high N deposition, the control plot showed a tightly coupled microbial N cycle; NH4 +and NO3 -immobilization rates were comparable to gross N mineralization and nitrification rates, respectively. These were supported by low levels of $NH_4{^+},\\>NO_3{^-}$, and dissolved organic N (DON) in percolate. Liming increased gross N mineralization and nitrification rates but did not cause similar increases in microbial biomass or NH4 +, and NO3 -immobilization rates. In addition, NO3 -immobilization rates were somewhat less than gross nitrification rates; relatively high levels of NO3 -and DON in percolate were also observed. The N-saturated plot suggested an uncoupled microbial N cycle; NH4 +immobilization rates were lower than gross N mineralization rates, and NO3 -immobilization rates were somewhat less than gross nitrification rates. These were corroborated by high levels of $NH_4{^+},\\>NO_3{^-}$, and DON in percolate. The reduced NH4 +and NO3 -immobilization rates in the N-saturated plot could be attributed to the measured decreases in microbial biomass, and the low microbial biomass was likely due to decreases in the supply of labile C. Our study demonstrates that while hydrological N input/output budgets can indicate whether or not a forest ecosystem is in a state of N saturation, the microbial N cycle can provide quantitative information on key processes that govern N losses.

An ongoing roof experiment, where N and acid inputs were reduced to the recommended critical load levels, has been conducted since 1991 in an N-saturated spruce stand in Soiling, Germany. Our study was aimed at (1) quantifying the changes in gross rates of microbial N cycling under ambient and reduced N conditions, and (2) relating the soil N dynamics to the changes in N leaching and N status of trees. Two roofs were used, one to achieve \"ambient\" and the other reduced (\"clean rain\") inputs, with a roofless plot as a control for possible roof effects. In 2001, the ambient roof and ambient no-roof plots showed an apparent decrease in gross N mineralization rates and significantly lower microbial NH4 +immobilization rates and turnover rates of NH4 +and microbial N pools. The microbial NO3 -immobilization rates and NO3 -pool turnover rates were lower than the microbial NH4 +immobilization rates and NH4 +pool turnover rates, showing that less NO3 -cycled through microorganisms than NH4 +. There was also low abiotic NO3 -immobilization. High NO3 -input from throughfall and low microbial turnover rates of the NO3 -pool, combined with low abiotic NO3 -retention, may have contributed to the high NO3 -leaching losses in these ambient plots. The clean rain plot showed a slight increase in gross N mineralization rates and significantly higher microbial NH4 +immobilization rates and turnover rates of NH4 +and microbial N pools. Neither nitrification nor soil NO3 -was detectable. There was an increase in abiotic NO3 -immobilization. Foliar N concentration had decreased but was still adequate. An efficient cycling of NH4 +through microorganisms, combined with the high abiotic NO3 -immobilization, indicated efficient mineral N retention in the clean rain plot. These results indicated that long-term reduction of throughfall N and acid inputs had induced high but tightly coupled microbial NH4 +cycling and an increase in abiotic NO3 -retention, which contributed to the reversal of N saturation.