Multiple indices of nutrient limitation in a topographically dissected wet tropical forest

Background/Question/Methods

Tropical forests display remarkable biogeochemical heterogeneity from local to regional scales, and this variation can contribute to complex patterns of probable nutrient limitation. For instance, across topographically dissected wet lowland forests in southwest Costa Rica, multiple indices of nitrogen (N) availability decline significantly as one moves from flat ridgetops to steeper slopes. That decline is likely driven by high rates of erosive N loss on steeper portions of the landscape, which in turn prevent N accumulation within the local plant-soil system. Here, we assessed whether soil microorganisms living in these distinct geomorphic habitats exhibit differences in relative nutrient limitation, reflective of gradients in N availability. We expected a shift from a dominance of phosphorus (P) limitation near ridge-tops (where soils have longer residence times and N is abundant) towards greater N constraints on steeper, eroding slopes. To test this hypothesis, we measured rates of free-living N fixation, the potential activities of extracellular enzymes involved in N and P mineralization, and the substrate-induced respiration response to factorial N and P amendments in soil incubations. 

Results/Conclusions

Free-living N fixation rates conformed to our expectations and were two to three times higher in slope soils (1.2 ± 0.35 nmols/g/day) compared to ridgetops (0.4 ± 0.2 nmols/g/day). However, the activity of NAG, an enzyme that degrades N-bearing chitin oligomers, was not significantly different in slope soils compared to ridges. Contrary to expectations, slopes had ~ 1.3 x as much phosphatase activity and a lower NAG-to-phosphatase ratio than ridges. CO₂ emissions from incubated soils were suppressed by N addition at all landscape positions, yet respiration was suppressed less from slope soils than ridges. Ridges and slopes both responded to the addition of phosphorus with two-fold increases in CO₂ production. Interestingly, the addition of N and P together did not elicit a strong, immediate respiration response from any soils tested, yet N + P treatments finished the experiment with a larger microbial biomass, suggesting higher growth efficiency when both nutrients are added. Taken as a whole, our study indicates that topography caused some expected variation in indices of nutrient limitation, but also underscores the complexity of determining such limitation in heterogeneous systems. Different indices may lead to different conclusions, highlighting the need to employ multiple methods in any given site.