Landscape-scale variation in nitrogen cycling across the Luquillo Experimental Forest, Puerto Rico

Background/Question/Methods

Many lowland tropical forests exhibit characteristics of nitrogen (N) richness – high rates of nitrification, mineralization and denitrification, high riverine losses of inorganic N, and enriched soil d¹⁵N. Nevertheless, tropical forests exhibit biogeochemical heterogeneity unparalleled at higher latitudes, and the drivers of this diversity remains poorly understood. We explored the relative importance of three potential drivers of N cycling in the Luquillo Experimental Forest of Puerto Rico: topography, parent material and forest community. We selected sixteen sub-watersheds in a full factorial combination of two parent materials (volcaniclastic and quartz diorite) and two forest communities (Colorado and Tabonuco). In each sub-watershed we sampled ridge, slope and valley positions. Within this framework, we measured soil C, N, and d¹⁵N. In the eight sites dominated by Tabonuco forests, we measured foliar d¹⁵N and N:P, KCl-extractable nitrate, ammonium, net nitrification and mineralization. In addition, we used acetylene block soil incubations to measure N₂O and N₂ production. We hypothesized that N cycling would differ across the landscape as a result of variation in soil texture (controlled by parent material) and topography (which influences soil saturation and oxygen availability), and that the different metrics of N availability would co-vary.

Results/Conclusions

Contrary to our hypothesis, we found that the interpretation of landscape scale variation in N cycling was dependent on the metric used. Soil C:N was higher, and d¹⁵N was lower, in Colorado compared with Tabonuco forests (n=16, p<0.002). Since forest type and elevation are correlated across this landscape, we tested this relationship in eight mid-elevation sites where that correlation breaks down, and found these community-driven differences persisted (n=8, p<0.006). Within Tabonuco forests, net mineralization (p=0.03) and soil d¹⁵N (p=0.0003) were higher on clay-rich volcaniclastic-derived soils than on sandier quartz diorite-derived soils, regardless of topographic position. In contrast, N gas emissions (N₂O+N₂) were marginally higher on quartz diorite-derived soils (p=0.085), and N₂ emissions were marginally higher in valleys (p=0.079), but N₂O emissions did not vary across the two parent materials (p>0.1). Topography, not parent material, best explained the variance in foliar N:P of Tabonuco (Dacryodes excelsa) trees, which was lowest in valleys (p=0.014). We conclude that N cycling varies across this landscape, but that different processes are influenced by different landscape-scale parameters. This highlights the challenge of scaling plot-scale metrics of N cycling up to a landscape-level description of ecosystem N status.