Several lines of evidence suggest that nitrogen in most tropical forests is relatively more available than N in most temperate forests, and even that it may function as an excess nutrient in many tropical forests. If this is correct, tropical forests should have more open N cycles than temperate forests, with both inputs and outputs of N large relative to N cycling within systems. Consequent differences in both the magnitude and the pathways of N loss imply that tropical forests should in general be more 15N enriched than are most temperate forests. In order to test this hypothesis, we compared the nitrogen stable isotopic composition 15 of tree leaves and soils from a variety of tropical and temperate forests. Foliar 8 N values from tropical forests averaged 6.5%o higher than from temperate forests. Within the tropics, ecosystems with relatively low N availability (montane forests, forests on sandy soils) were significantly more depleted in 15N than other tropical forests. The average 315N values for tropical forest soils, either for surface or for depth samples, were almost 8% higher than temperate forest soils. These results provide another line of evidence that N is relatively abundant in many tropical forest ecosystems. Table 1. 315N (%) values of plant species. %N is the nitrogen concentration (%). Species Site Region Country 15N % N Ref

Abstract. Methane (CH4) fluxes between soils and the atmosphere were measured in two tropical forest-to-pasture chronosequences in the state of Rond6nia, Brazil. Forest soils always consumed atmospheric CH 4 with maximum uptake rates in the dry season. Pasture soils consumed atmospheric CH 4 during the dry season, but at lower rates than those in the forests. When soil moisture increased in the pasture soils, they became a source of CH 4 to the atmosphere.

Several lines of evidence suggest that nitrogen in most tropical forests is relatively more available than N in most temperate forests, and even that it may function as an excess nutrient in many tropical forests. If this is correct, tropical forests should have more open N cycles than temperate forests, with both inputs and outputs of N large relative to N cycling within systems. Consequent differences in both the magnitude and the pathways of N loss imply that tropical forests should in general be more 15N enriched than are most temperate forests. In order to test this hypothesis, we compared the nitrogen stable isotopic composition 15 of tree leaves and soils from a variety of tropical and temperate forests. Foliar 8 N values from tropical forests averaged 6.5%o higher than from temperate forests. Within the tropics, ecosystems with relatively low N availability (montane forests, forests on sandy soils) were significantly more depleted in 15N than other tropical forests. The average 315N values for tropical forest soils, either for surface or for depth samples, were almost 8% higher than temperate forest soils. These results provide another line of evidence that N is relatively abundant in many tropical forest ecosystems. Table 1. 315N (%) values of plant species. %N is the nitrogen concentration (%). Species Site Region Country 15N % N Ref

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Although vast areas in tropical regions have weathered soils with low potassium (K) levels, little is known about the effects of K supply on the photosynthetic physiology of trees. This study assessed the effects of K and sodium (Na) supply on the diffusional and biochemical limitations to photosynthesis in Eucalyptus grandis leaves. A field experiment comparing treatments receiving K (+K) or Na (+Na) with a control treatment (C) was set up in a K-deficient soil. The net CO2 assimilation rates were twice as high in +K and 1.6 times higher in +Na than in the C as a result of lower stomatal and mesophyll resistance to CO2 diffusion and higher photosynthetic capacity. The starch content was higher and soluble sugar was lower in +K than in C and +Na, suggesting that K starvation disturbed carbon storage and transport. The specific leaf area, leaf thickness, parenchyma thickness, stomatal size and intercellular air spaces increased in +K and +Na compared to C. Nitrogen and chlorophyll concentrations were also higher in +K and +Na than in C. These results suggest a strong relationship between the K and Na supply to E. grandis trees and the functional and structural limitations to CO2 assimilation rates.

We undertook this study to increase our understanding of how forest clearing for pasture in the Brazilian Amazon is affecting N20 fluxes from tropical soils. More specifically, we wanted to do four things: (1) develop an understanding of the dynamics of N20 fluxes during pasture establishment in the Amazon; (2) evaluate the finding that intact forests have 34,179

The isotopic signatures of 15N and 18O in N2O emitted from tropical soils vary both spatially and temporally, leading to large uncertainty in the overall tropical source signature and thereby limiting the utility of isotopes in constraining the global N2O budget. Determining the reasons for spatial and temporal variations in isotope signatures requires that we know the isotope enrichment factors for nitrification and denitrification, the two processes that produce N2O in soils. We have devised a method for measuring these enrichment factors using soil incubation experiments and report results from this method for three rain forest soils collected in the Brazilian Amazon: soil with differing sand and clay content from the Tapajos National Forest (TNF) near Santarém, Pará, and Nova Vida Farm, Rondônia. The 15N enrichment factors for nitrification and denitrification differ with soil texture and site: -111 per thousand +/- 12 per thousand and -31 per thousand +/- 11 per thousand for a clay-rich Oxisol (TNF), -102 per thousand +/- 5 per thousand and -45 per thousand +/- 5 per thousand for a sandier Ultisol (TNF), and -10.4 per thousand +/- 3.5 per thousand (enrichment factor for denitrification) for another Ultisol (Nova Vida) soil, respectively. We also show that the isotopomer site preference (delta15Nalpha - delta15Nbeta, where alpha indicates the central nitrogen atom and beta the terminal nitrogen atom in N2O) may allow differentiation between processes of production and consumption of N2O and can potentially be used to determine the contributions of nitrification and denitrification. The site preferences for nitrification and denitrification from the TNF-Ultisol incubated soils are: 4.2 per thousand +/- 8.4 per thousand and 31.6 per thousand +/- 8.1 per thousand, respectively. Thus, nitrifying and denitrifying bacteria populations under the conditions of our study exhibit significantly different 15N site preference fingerprints. Our data set strongly suggests that N2O isotopomers can be used in concert with traditional N2O stable isotope measurements as constraints to differentiate microbial N2O processes in soil and will contribute to interpretations of the isotopic site preference N2O values found in the free troposphere.