Patterns of Soil Properties and Respiration along an Elevation Gradient in the Luquillo Mountains, Notheastern Puerto Rico
Yu, Mei (Advisor)
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Soil respiration is one of the most important components in the global carbon cycle. It is also one of the primary paths to release carbon fixed by vegetation to the atmosphere. Climate change has direct impacts on soil respiration, and the responses of soil respiration also have important feedback to future climate change. Tropical forests store about 11% of the global soil carbon pool. The amount of carbon stored in soils is twice of that in either the atmosphere or terrestrial vegetation. Thus, understanding how soil respiration responds to climate change in tropical forest ecosystems has profound ecological significance. Soil temperature, moisture, and carbon substrate are three main factors influencing soil respiration. Soil carbon substrate is significantly correlated to the distribution of vegetation and soil fauna. Abiotic factors such as solar radiation, topography, parent material, soil temperature and moisture also partly determine the spatial variation in soil carbon. This thesis was developed to study the variation in soil organic matter (SOM) and respiration rate as responses to environmental changes in a tropical wet montane forest. In general, microclimate conditions, vegetation, and SOM vary with elevation. We hypothesized that SOM increase with elevation from low to upper, whereas soil respiration rate decreases due to the limitation of low temperature. To test these two hypotheses, we analyzed soil properties including texture, SOM, and other physicochemical properties along an elevation gradient in the Luquillo Experimental Forest (LEF), northeastern Puerto Rico. We then conducted a soil translocation experiment and measured monthly soil respiration rate to investigate the impacts of environmental changes on soil respiration, as well as the responses in soil carbon. XIV As the results showed, soil properties were strongly related to the elevation gradient in the LEF. Especially, SOM content increased significantly along the elevation gradient with altered temperature and precipitation. The soil translocation experiment indicated that soil carbon processes were affected by climate change, especially the variation in temperature and moisture. Soil cores translocated from the plots with high elevation to the plots with lower elevation showed increased soil respiration rates and decreased soil C content at the end of the experiment, mainly because of the increased temperature. Soil cores translocated from the low elevation to the top also showed increased soil respiration rates and decreased soil C content, which might have been related to the increased soil moisture and altered soil microbes at the top. The experiment indicated that both soil respiration rate and soil C content changed with altered climate. Therefore, the predicted climate change in the Caribbean region, warmer and drier, might bring significant impacts on C sequestration in tropical forests.