Methods

1. Correlates of Soil Respiration in Three Florida Ecosystems Paulo Brando (Department of Botany) and Helen C. Claudio (Department of Forestry) • Introduction • Human activities have severely altered the ecosystems of Florida. Of particular interest are the dynamics of the pine-savanna ecosystem and the drastic alteration of the fire regime. Lower fire frequency has been shown to facilitate the invasion of pine-savannas by woody vegetation dominated by oaks, which are usually less prone to fire than pines. Given that low intensity fires are essential to the natural regeneration of pine savannas, oak invasions will negatively impact the dynamics of pine savannas in the long run. Other alterations of the land cover in Florida may also have important effects to the dynamics of natural ecosystems. Deforestation and lower soil permeability, for example, are likely to increase runoff during the peak wet season, increasing the river stage and inundate otherwise dry areas. While changes in ecosystems of Florida have been occurring very rapidly, our understanding of its effects on the carbon cycle is still poor. Here we measured soil carbon in three different ecosystems of Florida: pine savanna, flooded hardwood forest, and a hardwood-invaded pine savanna. • Our hypotheses are: • the flooded forests have more soil carbon because of lower decomposition rates than in the pine savanna or the invaded area • soil respiration rates will be increased with a lower C:N ratio and soil moisture because these conditions favor microbial growth. Figure 2: Soil water content by depths. Note the similar patterns between both the 0-5cm and the 5-10cm depths. Methods Figure 2: Soil respiration rates, litter, and percent overhead cover. Note the similarities between longleaf and the flooded areas in overhead cover. See Table 1 for statistical test results. Field Sites: The study was conducted on a ranch bordered to the west by I-75 and to the north by the Santa Fe River. Three different ecosystems were explored: a floodplain dominated by hardwoods and cypress species, a sandy area further upland with both hardwoods and pines, and another sandy area dominated by a longleaf pine (Pinus palustris) restoration site. Other commonly found species in this region included saw palmettoes, various oak species, wiregrasses, pines, and understory shrubs and trees. Figure 6: Correlations between the soil respiration rates and % Overhead cover, litter depth, and CN ratio. The panels to the right are for the portable LICOR system and the panels to the left are for the incubation jars. Figure 3: Bulk soil densities. Note that the 0-5cm and the 5-10 cm depths are similar. The sandy soils of the invaded and the longleaf areas are similar in bulk density but the flooded region has a lower density. Figure 4: Soil C:N ratios by site and depth. Note that there is little difference between the three sites at the 5-10 cm depth and most of the differences are in the 0-5 cm depth. 30 m Table 2: r2 and p values for correlations between the respiration rates (chamber averages and incubation jars by date). The graph to the right shows the results. Bolded entries are significant. Discussion Figure 1: Experimental design at each of the three ecosystems with a 30 m transect and three places to take measurements. • Although %C was significantly higher in the floodplain (4%) than in the longleaf pine stand (0.3%) and in the invaded (1.3%), soil respiration measured in the field was lower in the floodplain than in the other two systems. This unexpected difference in soil respiration may be from higher autotrophic respiration in the invaded pine region. However, the incubation experiment showed that more carbon was being respired in the floodplain soils than in the sandy pine soils, which agreed with our hypothesis. Further investigation will be required. • The lower soil respiration rates in the field in the floodplain may also be from anaerobic conditions in the wet soil and constrain microbial activity and soil respiration. The floodplain soil may have developed higher respiration rates in the incubation jars because of improved availability of oxygen, facilitating microbial activity and increasing respiration rates. • These results suggest that if fires are excluded from longleaf pine savannas, an increase in soil carbon pools can be expected, which can then affect soil respiration. Based on this study, we can also expect that if more areas are flooded, anaerobic conditions could hinder decomposition, increasing carbon stocks in these forests and allow hardwood invasion to occur. • Field Methods • At each of the three sites, a 30 m transect was established and the following procedures were done every 10 m: • Soil respiration was measured with a portable LI-COR hand-held closed chamber • Litter depth was measured and collected in a ~380 cm2 area • Soil cores were taken at 0-5 cm and 5-10 cm depths • Percent overhead cover was measured Table 1: ANOVA results from comparing across the three sites along with p-values. Significant (< 0.05) values are in bold. Figure 5: Time-series of soil incubation jars by depth and area. Note the decreasing fluxes. • Lab Methods • The following were done in the lab after the collection of the materials: • Fresh soils and litter were weighed, dried in an oven at 60o C, then re-weighed to determine water content • Some of the soil samples were dried, ground into a fine powder, kept in a dessicator and then about 20mg placed into tins for C:N analysis in the mass spectrometer. • Approximately 20g of the soil samples were placed into sealed incubation jars, allowed to equilibrate, and then the CO2 fluxes were sampled using a LICOR three times over the study: 10/15, 10/18, and 10/23. Acknowledgments We would like to thank Jack and Kathy Ewel for allowing us to use their property as our field site, Silvia Alvarez and Michelle Mack for coordinating the methods of ecosystem ecology class, Grace Crummer for running the CN ratio, and the entire class for providing us with assistance in collecting field data and running lab experiments. References Myers, R.L., and J.J. Ewel, eds. 1990. Ecosystems of Florida. Univ. Central Florida Press, Orlando Davidson, E.A., E. Belk, and R.D. Boone. (1998). Soil water content and temperature as independent or confounded factors controlling soil respiration in a temperate mixed hardwood forest. Global Change Biology, 4, 217-227.