Geological and biological influence on soil and foliar δ15N in the Luquillo Mountains of Puerto Rico

Background/Question/Methods

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.

Results/Conclusions

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.