Carbon and Microbial Community Composition at Mitigated Bottomland Forest Wetlands * Elisa M. D’Angelo, A.D. Karathanasis, S.A. Ritchey, and S.W. Wehr UK Department of Agronomy Introduction Public concern about extensive losses of wetlands during the last 200 years has prompted institution of a “No Net Loss” goal in the US. The main strategy to achieve this goal is compensatory mitigation, which refers to the restoration or creation of new wetlands to compensate for unavoidable losses of natural wetlands. Even though thousands of mitigated sites have been built nationwide over the last two decades, their capacity to perform normal wetland functions such as water quality improvement, flood control, and aquatic habitat is still in question (National Research Council, 2001). In western KY, thousands of mitigated wetland acres were built over the last 20 years to compensate for losses of bottomland hardwood forest wetlands caused by surface coal mining activities (Fig. 1). Typically, wetland mitigation consists of removing tracts of poorly drained croplands from agricultural production and planting with appropriate tree species. As a result of mitigation activities carried out over the last two decades, much of the region is a patchwork of mitigated wetlands at different successional stages. In this investigation, we wanted to determine the relative capacity of these mitigated wetlands to perform water quality improvement processes (C, N, P storage and transformations) compared to natural wetlands. It was expected that wetlands at different successional stages would have different microbial community composition and functional performance due to shifts in vegetation types (herbaceous vs woody), organic matter content and quality, and hydrology. This research will provide a scientific basis for setting mitigation ratios and judging the adequacy of mitigation projects, which is essential for achieving no net loss of bottomland hardwood forest wetlands in western KY. Fig. 1. Study site locations and photographs of mitigated bottomland forest wetlands of western KY *This work was sponsored by a grant from the USDA National Research Initiative Competitive Grants program Kentucky Webster Hopkins ST RS R Pond River Tradewater River 24 km Western Coalfield Cropland (0 y) Late successional(15 y) Early successional (4 y) Materials and Methods Litter and soil were collected from nine locations at four successional stages, including a cultivated field (CF), early successional (ES), late successional (LS) and climax wetland (RS) (Fig. 1). Samples were collected in the wet and dry seasons, and evaluated for C, N, P storage and transformations and microbial community composition. Nutrient storage Carbon:Total C, forest product analysis Nitrogen: Total N, NH 4, NO 3 +NO 2 Phosphorus: Total P, oxalate (P, Fe, Al), Mehlich III (P, Fe, Al), pH Microbial community composition by phospholipid fatty acid analysis (PLFA) Nutrient cycling C, N, P mineralization: laboratory batch experiments Denitrification, phosphatase activity P sorption capacity Nutrient flux from flooded soils Results presented here will focus mainly on organic C storage and microbial community composition changes as a function of wetland mitigation age. Enriched at climax wetlands Enriched at early successional wetlands Fig. 3. Typical phospholipid fatty acid (PLFA) profile of soils collected from bottomland hardwood forest wetlands. Soils from climax wetlands were enriched in PLFA biomarkers for anaerobic bacteria (i17:1n7,10me16, i17, b18, cy19, b20) and soils from early successional wetlands were enriched in PLFA biomarkers for aerobic fungi (18:2n6), protozoa (20:4n6), and bacteria (16:1n5). Fig. 2. Changes in organic C storage in the litter and soil to a 12 cm depth (a) and soil water holding capacity (b) as a function of wetland mitigation age. Water holding capacity was determined from the difference in saturated and wilting point volumetric water contents, as described by Vereecken et al. (1989). CFESLSRS CFESLSRS (a) (b) Mitigation age, years Water holding capacity, % TOC in litter and soil, g m -2 N Results and Discussion Water holding capacity As a consequence of increased soil organic C (from 0.7 to 5% C) and decreased soil bulk density (from 1.23 to 0.73 g cm -3 ), there was a progressive increase in soil water holding capacity during ecosystem development (from 27 to 36%) (Fig. 2b). Increased soil water holding capacity is critical in these seasonal wetlands because it helps to extend the period of soil anaerobiosis beyond the typical two week inundation period. The implication is that reduced water holding capacity at early successional wetlands reduces their capacity to perform most wetland functions that are carried out by climax wetlands, including flood control, aquatic life support, and water quality improvement through denitrification and other biochemical reactions. Based on Fig. 2b, it will take 25 years for mitigated sites to recover 95% of the water holding capacity of climax wetlands. Microbial community composition The relative distribution of phospholipid fatty acids (PLFAs) was used to assess soil microbial community composition at mitigated and climax wetland sites (Fig. 3). Mature wetlands consistently had greater fractions of PLFA markers for anaerobic bacterial groups (i17, i17:1n7, b18:1, b20:1, 10me16, cy19) and decreased markers for fungi (18:2n6) and aerobic Gram-negative bacteria (16:1n5). These results indicated that mature sites were wetter for longer periods than immature sites. It is suggested that the shift from aerobic to anaerobic microbial communities was linked to changes in soil properties that occurred during ecosystem development: the transition from predominantly herbaceous to woody vegetation resulted in increased surface litter, greater labile and non-labile soil organic accumulation, decreased soil bulk density, and increased soil water holding capacity. After flooding episodes, climax wetland soils remained wetter for longer duration, which lead to anaerobic bacteria enrichment at these sites. Conclusions The climax wetlands stored about 4 times more organic carbon than early successional sites. Thus, the current mitigation ratio of 2 should probably be doubled in order to achieve no net loss of this wetland function. It can expected that it will take about 42 years for mitigated sites to accumulate 95% of the C stored at climax wetlands. Organic C accumulation influenced several other soil properties, including bulk density, porosity, and water holding capacity. These factors govern the capacity of wetlands to perform most water quality improvement and other functions. Climax wetlands were enriched in anaerobic microbial populations compared to immature sites, which likely reflected differences in water holding capacity and hydrologic conditions that occurred during ecosystem development. As an environmental condition integrator, microbial community composition may be an important new tool for monitoring the success of compensatory mitigation wetland projects. Results and Discussion Carbon storage There was a progressive increase in total organic carbon storage in soil and litter after conversion of croplands to wetlands (Fig. 2a), which was attributed to increased deposition of recalcitrant organic matter that accompanied the shift from primarily herbaceous to woody vegetation (data not shown). From Fig. 2a, approximately 4 times more cropland was required to store the same amount of C as climax wetlands. Also, it will take about 42 years for mitigated sites to accumulate 95% of the C stored at climax wetlands.