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

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.

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