Geomorphology and canopy chemistry influence soil nitrogen availability on variable time scales in a lowland tropical forest

Background/Question/Methods

Lowland tropical forests are broadly considered to be phosphorus (P) rather than nitrogen (N) limited systems. However, recent work suggests that patterns in P and N cycling and limitation vary substantially with state factor heterogeneity. For example, complex terrain can influence the distribution of essential elements due to variation in physical and chemical weathering and trees can display differential nutrient limitation across a wide phylogenetic spectrum.

Here, we test how topography and canopy chemistry influence the availability and cycling of soil N across the geomorphically diverse Osa Peninsula, Costa Rica. We hypothesized N availability would be higher on geomorphically stable terraces than slopes. By contrast, we expected N availability would be similar between narrow, knife-edged ridges and slopes. We measured metrics of short- (NH₄⁺, NO₃^-, net mineralization, net nitrification) and long-term (δ¹⁵N) soil N availability across catenas with either terraces or knife-edge ridges. Within a given landscape position, we expected canopy and soil N to be correlated. We identified ¼ha circular plots on terraces with either high or low canopy N concentrations (as determined from hyperspectral data collected by the Carnegie Airborne Observatory). We measured the same soil parameters in triplicate cores (0-10cm) from the center of each plot. 

Results/Conclusions

Surface soil d¹⁵N on terraces was significantly higher than on adjacent slopes (5.2±0.2‰ and 4.3±0.2‰, p=0.05). Soil d¹⁵N on knife-edge ridges was similar to adjacent slopes (4.3±0.3‰ vs. 4.1±0.3‰, p=0.54). We found no significant effects of topographic position or ridge type on short-term metrics of N availability. Within a single topographic setting (terraces), soils under high N canopies had higher inorganic N concentrations and net process rates ([NO3-N]: 1.1 μg kg^-1, [NH4-N]: 1.0 μg kg^-1, net nit: 1.0 μg kg^-1 d^-1) compared with those under canopies with low N ([NO3-N]: 0.2, μg kg^-1, [NH4-N]: 0.8 μg kg^-1, net nit: 0.6 μg kg^-1 p<0.1 in all cases). Soil δ¹⁵N concentrations did not vary among the canopy plots. Overall, our findings suggests that inorganic forms of nitrogen reflect feedbacks between canopy tree species and the soils beneath them, whereas δ¹⁵N is controlled more by long-term geomorphic processes that determine soil residence times.