Topography, elemental ratios, and nutrient limitation in tropical montane ecosystems

Our understanding of biogeochemical processes in tropical ecosystems is incomplete. While the factors regulating biogeochemical processes in tropical ecosystems are not well understood, the likelihood that ecosystem responses to environmental changes will be severely constrained by nitrogen (N) and phosphorus (P) availability is almost certain. In particular, our understanding of the forests and grassland ecosystems found in the foothills of the Andes mountains is notably poor, though these represent land areas with exceedingly high rates of anthropogenic land conversion. The research presented here addressed the interacting effects of topography, land cover change, and nutrient limitation in the Amboro National Park region in Santa Cruz, Bolivia.;I established transects in five land use types (primary forest, secondary forest, grasslands, pasture, and slash and burn agriculture) to examine patterns of nutrient loss as a consequence of land cover change, and I measured how these patterns were influenced by topographic position. Additionally, I evaluated the importance of nutrient limitation in regulating primary production across topographic gradients in grasslands using a fertilization experiment. Finally, I conducted a laboratory incubation experiment to measure the sensitivity of eastern Bolivian soils to changes in temperature and moisture gradients, such as those predicted to result from climate change.;Primary forests in the Amboro region contained significantly more carbon (C), N, and P than the secondary forest site, and grasslands. Unexpectedly, neither primary, secondary forest, nor grasslands exhibited patterns of nutrient accumulation down slope suggesting rapid erosional losses in bottom slope positions that maintain lowered soil nutrient status. However, the pasture and slash plot were able to retain soil C, N, and P concentrations through a redistribution of nutrients from the ridge tops to lower slope positions. I hypothesized that grassland aboveground production would be limited by N and after two years of fertilization treatments, aboveground production in grasslands responded to N amendments. However, by the third year of the fertilization, aboveground productivity was limited by P availability. I tested the applicability of foliar elemental ratios (N: P) in identifying nutrient limitation, and foliar N: P ratios were in agreement with the finding of an initial N limitation followed by P limitation in these grasslands. These findings suggest that these grasslands exhibit transient nutrient limitations following grazing and fire suppression.;Finally, I used a laboratory incubation study to assess the effect of changes in soil moisture and temperature on tropical soil C cycling. The effects of temperature and soil moisture on soil CO2 were dependent upon one another, such that the effect of one was dependent upon the status of the other. Additionally, interactions between C quality and soil moisture were a strong regulator of the temperature dependence of soil decomposition. Alterations to global patterns in temperature, soil moisture, and soil C cycling are unlikely to occur in isolation and my results suggest that the interactions between these regulating factors may account for the lack of consensus found in climate studies examining the temperature sensitivity of soil organic matter decomposition. Taken together, these results suggest that patterns of ecosystem function described by temperate ecosystem models may not adequately depict biogeochemical processes in tropical ecosystems.