Biogeochemical cycling and retention of carbon and nutrients in a constructed wetland receiving agricultural runoff in the San Joaquin Valley, California

The biogeochemical cycles of carbon (C) and phosphorus (P) are intricately linked in aquatic systems, as P is often the limiting nutrient for primary productivity. The conversion of flood plain agroecosystems to flow-through wetlands is becoming a popular land-use practice for treating agricultural runoff nation wide, yet little information exists to document how these systems function in California where constructed wetlands dry out in late winter and spring. I examined the biogeochemical cycling of carbon and phosphorus in a small flow-through constructed wetland that receives agricultural runoff from approximately 2,300 ha of irrigated farmland. The first study (Chapter 2) examined the potential for wetland C sequestration via exogenous and endogenous C sources over a 13-year period. Results from this study showed that although surface sediments were enriched with endogenous sources of C (24 g kg -1), the long-term storage of C (∼14 g kg-1 ) was maintained at the same level as inflowing sediment (14 g kg -1) due to fluctuating cycles of flooding and drying. Thus additional C sequestration from endogenous sources may be limited in seasonally-saturated wetlands due to enhanced oxidation during drying cycles. The second study (Chapter 3) quantified wetland metabolism across a range of spatial scales (meters to 100s of meters) to better understand the linkages between ecosystem metabolism and biotic and abiotic factors. Results from this study showed that despite high rates of primary productivity, high respiration rates limited net C production in this wetland, resulting in heterotrophic conditions during most of the season. However, the addition of high C loads in inflowing water and moderate retention efficiencies resulted in a positive C mass balance during most sampling dates. The third study (Chapter 4) quantified the role of sulfides in inducing mobilization of P in constructed wetland soils. This study shows that sulfide induced P flux can play an important role in the retention and cycling of P in wetland soils. However, soils enriched in iron oxides can effectively buffer sulfide concentration in pore water. These studies illustrate the potential role small flow-though wetlands can play in sequestering carbon and improving water quality in agriculturally intensive watersheds.