Abstract
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 more15N enriched than are most temperate forests. In order to test this hypothesis, we compared the nitrogen stable isotopic composition of tree leaves and soils from a variety of tropical and temperate forests. Foliar δ15N values from tropical forests averaged 6.5‰ higher than from temperate forests. Within the tropics, ecosystems with relatively low N availability (montane forests, forests on sandy soils) were significantly more depleted in15N than other tropical forests. The average δ15N 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.
Highlights
A number of lines of evidence suggest that N in most tropical forests is relatively more available than is N in most temperate forests
The average concentration of nitrogen in leaves from tropical forests was 1.9-t-0.8%o (n = 78), which was not significantly different from the 1.64-0.5%o (n = 28) average value found for temperate forests
The results clearly showed that the 15N values of tree leaves and soil organic matter were significantly higher in tropical forests than in temperate forests
Summary
A number of lines of evidence suggest that N in most tropical forests is relatively more available than is N in most temperate forests. Comparable data on rates of N mineralization and leaching losses are sparser, but they generally show greater rates of N cycling and loss in many lowland tropical forests (Vitousek & Denslow 1986: Lewis 1986; Matson et al 1987; Neill et al 1995). Overall, these observations suggest that N functions as an excess nutrient in most tropical forests, but not in the majority of temperate forests. The major exceptions are forests dominated by symbiotic N fixing trees (usually monocultures), and forests that receive substantial anthropogenic N deposition (Binkley et al 1992; Aber et al 1995; Berendse et at. 1993)
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