Effects of Nutrients and Salinity on Soil Organic Matter

Project Goals Coastal wetlands are a globally significant sink for carbon due to their high rates of plant growth and low rates of microbial organic matter breakdown. These systems are increasingly threatened by salinization, nutrient enrichment and other human disturbances. Salinization and nutrient enrichment are known to alter plant growth and microbial breakdown, however there is uncertainty regarding their effect on the carbon balance of coastal wetlands. Because plants produce the soil organic matter consumed by microbes, changes in plant and microbial communities brought on by human disturbances can act either antagonistically to increase carbon release or synergistically to preserve carbon as soil organic matter. The soil organic matter pool, therefore, reflects feedbacks between plants and microbes and integrates them over long time-scales, providing a useful tool for assessing the cumulative impacts of salinization and nutrient enrichment on the wetland carbon cycle. In order to assess the impacts of salinization and nutrient enrichment on wetland carbon cycling, soil will be collected from GCE 7, where Dr. Craft has maintained a fertilization experiment for 9 years and a formerly freshwater marsh that has been salinized over the last 15 years on the Darien River. In addition, the interactive effects of salinization and nutrient enrichment will be explored by transplanting vegetated soil cores between nutrient-enriched and salinized field sites and allowing transplanted cores to incubate for one year. Soils will be analyzed for 1) the quantity and composition of soil organic matter 2) the microbial demand for different types of soil organic matter and 3) the rate of microbial organic matter breakdown. Results from this study will be integrated with previously collected data on plant and microbial processes at each site. SOM Survey Soil Collection and Storage 16 replicate cores will be collected from each study site. Cores will be collected at three seasonally important time points May, July, and October in 2014 (168 total cores). Briefly, a piston corer will be used to extract intact cores 20 cm in depth. Each core will extruded and divided into the top 0-10 and bottom 10-20 increments under water (to preserve anaerobic conditions) and stored under site water in anaerobic jars on ice. Immediately after collection, I will remove the coarse roots from and homogenize each core sections roots in a glove bag under N2 in the GCE LTER laboratory facility at the University of Georgia Marine Institute field station. A subsample of each core will be immediately frozen at -80°C for EEA analysis and the remainder of the soil will be returned to anaerobic jars on ice. All samples will be transported back to Indiana University for analysis. To examine the interaction between salinity and nutrient enrichment, in July of 2014, I will transplant 8 cores (2 from each replicate plot) from the long-term fertilization site (at Hammersmith Creek, GCE 7) to the long-term salinization site (on the Darien River) and vice versa from the salinization site to the fertilized site. 8 additional cores from each site will be removed and replaced in their home soil as a procedural control (32 cores total). Each 8.5 cm in diameter by 20 cm deep core will be placed in a mesh bag to allow materials and microbes to pass in and out of the bag, while maintaining the integrity of the core. Cores will be marked with PVC pipe. Cores will be removed in May of 2015 and transported back to Indiana University for analysis.