We explored the influence of three independent state factors (topography, soil parent material, and plant community composition) on two different proxies of ecosystem N richness: bulk soil and foliar δ15N in the Luquillo Mountains of eastern Puerto Rico. In July 2010, we sampled soils (to a depth of 80cm) from sixteen watersheds with a full factorial combination with four replicates of two soil parent materials (quartz diorite and basaltic-andesite volcaniclastics), two forest types (Tabonuco and Colorado) and three topographic positions (ridge, slope, and valley). Three replicated pits were excavated in each site, and composited by depth for a total of 144 soil samples. In June 2011, we returned to the Tabonuco Forest sites and sampled full sun leaves from two tree species: a late-successional canopy-dominant species, Dacryodes excelsa Vahl (49 individuals); and an early-successional species, Cecropia schreberiana (86 individuals). Soil samples were air dried, foliar samples were dried at 65°C, and both were finely ground before combustion and analysis of C, N, δ15N and δ13C by CF-IRMS. We used non-parametric multivariate regression trees to partition variance in our dependent variables among the three state factors.
Bulk soil δ15N ranged from ~1 to 10 ‰ across this landscape, with the least variation in the shallowest depth interval. Forest type explained 37% of the variance in soil δ15N for the 0-20cm soil, and the two forest types were significantly different. Although forest type is correlated with elevation in this landscape, a subset of sites for which elevation and forest type are not correlated also showed this significant difference. Neither bedrock, nor topographic position, nor precipitation significantly influenced soil δ15N across this landscape. Foliar δ15N varied from -5.41 to 6.78‰. Species differences alone explained 42% of the variance in foliar δ15N across this landscape. There were no significant differences based on elevation, bedrock, precipitation or topography for the foliar δ15N of these two species. However, for the difference between soil δ15N and foliar δ15N, D. excelsa (the late-successional canopy-dominant species), showed greater differences in δ15N signatures in valleys compared to slopes and ridges. Latitudinal differences in δ15N have been inferred to inform our understanding of temperate vs. tropical N cycling. However these data suggest both biologic and geologic factors control tropical soil and foliar δ15N at much finer spatial scales. Documentation of these influences may help refine our understanding of spatial heterogeneity in tropical forest N cycling.
Lu H.S., Mage S., Porder S. (2012): Geological and biological influence on soil and foliar δ15N in the Luquillo Mountains of Puerto Rico. ESA 2012.
This Paper/Book acknowledges NSF CZO grant support.