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                                                                                                                                                                    Results and Discussion                                                             Results and Discussion
                                                                                                                                        Carbon storage                                                                                                                 Water holding capacity
      Public concern about extensive losses of wetlands during the                                                                                                          There was a progressive increase in total organic carbon                                         As a consequence of increased soil organic C (from 0.7 to 5% C)
last 200 years has prompted institution of a “No Net Loss” goal in the                                                                  storage in soil and litter after conversion of croplands to wetlands (Fig.                                                     and decreased soil bulk density (from 1.23 to 0.73 g cm-3), there was a
US. The main strategy to achieve this goal is compensatory mitigation,                                                                  2a), which was attributed to increased deposition of recalcitrant                                                              progressive increase in soil water holding capacity during ecosystem
which refers to the restoration or creation of new wetlands to                                                                          organic matter that accompanied the shift from primarily herbaceous                                                            development (from 27 to 36%) (Fig. 2b). Increased soil water holding
                                                                                                Western      Kentucky
compensate for unavoidable losses of natural wetlands. Even though                                                                      to woody vegetation (data not shown). From Fig. 2a, approximately 4                                                            capacity is critical in these seasonal wetlands because it helps to extend
thousands of mitigated sites have been built nationwide over the last                                                                   times more cropland was required to store the same amount of C as                                                              the period of soil anaerobiosis beyond the typical two week inundation
two decades, their capacity to perform normal wetland functions such                                                                    climax wetlands. Also, it will take about 42 years for mitigated sites to                                                      period. The implication is that reduced water holding capacity at early
as water quality improvement, flood control, and aquatic habitat is still                     Webster                                   accumulate 95% of the C stored at climax wetlands.                                                                             successional wetlands reduces their capacity to perform most wetland
                                                                                             R    Hopkins           24 km   N
in question (National Research Council, 2001).                                                                                                                                                                                                                         functions that are carried out by climax wetlands, including flood control,
                                                                                              RS      ST                                                                                                                                                    (a)
                                                                                                                                                                                               5000             Li tter C                                              aquatic     life   support,   and    water   quality   improvement   through

                                                                                                                                         TOC in litter and soil,
      In western KY, thousands of mitigated wetland acres were built                                                                                                                                                                                                   denitrification and other biochemical reactions. Based on Fig. 2b, it will
                                                                                                                                                                                                                Soi l C
over the last 20 years to compensate for losses of bottomland
                                                                                                                                                                                                                Total C                                                take 25 years for mitigated sites to recover 95% of the water holding
hardwood forest wetlands caused by surface coal mining activities (Fig.                                                                                                                        3000

                                                                                                                                                 g m-2
                                                                                                                                                                                                                                                                       capacity of climax wetlands.
                                                                               Cropland (0 y)               Early successional (4 y)                                                                                                               0. 31
1). Typically, wetland mitigation consists of removing tracts of poorly                                                                                                                                                        y =1168 x
drained croplands from agricultural production and planting with                                                                                                                                                                  r   2    =0.96                       Microbial community composition
appropriate tree species. As a result of mitigation activities carried out                                                                                                                     1000
                                                                                                                                                                                                                                                                             The relative distribution of phospholipid fatty acids (PLFAs) was
over the last two decades, much of the region is a patchwork of                                                                                                                                     0                                                                  used to assess soil microbial community composition at mitigated and
mitigated wetlands at different successional stages.                                                                                                                                                CF ES        LS                            RS
                                                                                                                                                                                                        0             20         40                   60               climax wetland sites (Fig. 3). Mature wetlands consistently had greater
                                                                                                                                                                                               40                                                          (b)         fractions of PLFA markers for anaerobic bacterial groups (i17, i17:1n7,

                                                                                                                                                                   Water holding capacity, %
      In this investigation, we wanted to determine the relative capacity                                                                                                                                                                                              b18:1, b20:1, 10me16, cy19) and decreased markers for fungi (18:2n6)
of these mitigated wetlands to perform water quality improvement
                                                                                        Late successional(15 y)                                                                                35                                                                      and aerobic Gram-negative bacteria (16:1n5). These results indicated that
                                                                                                                                                                                                                                          y =26x       0.077
processes (C, N, P storage and transformations) compared to natural                                                                                                                                                                                                    mature sites were wetter for longer periods than immature sites.
wetlands.   It was expected that wetlands at different successional                                                                                                                                                                            2
                                                                                                                                                                                                                                           r        =0.83
stages would have different microbial community composition and                                                                                                                                25                                                                            It is suggested that the shift from aerobic to anaerobic microbial
functional performance due to shifts in vegetation types (herbaceous vs                                                                                                                             CF ES       LS                             RS                      communities was linked to changes in soil properties that occurred
woody), organic matter content and quality, and hydrology.
                                                                                                                                                                                                    0                20           40                       60
                                                                                                                                                                                                                                                                       during ecosystem development: the transition from predominantly
                                                                                                                                                                                                                 Mitigation age, years                                 herbaceous to woody vegetation resulted in increased surface litter,
      This research will provide a scientific basis for setting mitigation                                                                     Fig. 2. Changes in organic C storage in the litter and                                                                  greater labile and non-labile soil organic accumulation, decreased soil
ratios and judging the adequacy of mitigation projects, which is                                                                               soil to a 12 cm depth (a) and soil water holding                                                                        bulk density, and increased soil water holding capacity. After flooding
essential for achieving no net loss of bottomland hardwood forest                                                                              capacity (b) as a function of wetland mitigation age.                                                                   episodes, climax wetland soils remained wetter for longer duration,
                                                                             Fig. 1. Study site locations and photographs of                   Water holding capacity was determined from the
wetlands in western KY.                                                                                                                                                                                                                                                which lead to anaerobic bacteria enrichment at these sites.
                                                                             mitigated bottomland forest wetlands of western KY                difference in saturated and wilting point volumetric
                                                                                                                                               water contents, as described by Vereecken et al.
                      Materials and Methods                                                                                                    (1989).                                                                                                                                                     Conclusions
                                                                                                                                                                                                                                                                       • The climax wetlands stored about 4 times more organic carbon than
       Litter and soil were collected from nine locations at four                                                                                                                                                                                                      early successional sites. Thus, the current mitigation ratio of 2 should
                                                                                                                                                                                                            Enriched at climax
successional stages, including a cultivated field (CF), early                                                                                                                                                                                                          probably be doubled in order to achieve no net loss of this wetland
successional (ES), late successional (LS) and climax wetland (RS)                                                                                                                                                                                                      function.
(Fig. 1). Samples were collected in the wet and dry seasons, and                                                                                                                                            Enriched at early                                          • It can expected that it will take about 42 years for mitigated sites to
evaluated for C, N, P storage and transformations and microbial                                                                                                                                             successional wetlands
                                                                                                                                                                                                                                                                       accumulate 95% of the C stored at climax wetlands.
community composition.                                                                                                                                                                                                                                                 • Organic C accumulation influenced several other soil properties,
                                                                                                                                                                                                                                                                       including bulk density, porosity, and water holding capacity. These
Nutrient storage                                                                                                                                                                                                                                                       factors govern the capacity of wetlands to perform most water quality
   •Carbon:Total C, forest product analysis
                                                                                                                                                                                                                                                                       improvement and other functions.
   •Nitrogen: Total N, NH4, NO3+NO2
   •Phosphorus: Total P, oxalate (P, Fe, Al), Mehlich III (P, Fe, Al), pH                                                                                                                                                                                              • Climax wetlands were enriched in anaerobic microbial populations
                                                                                                                                                                                                                                                                       compared to immature sites, which likely reflected differences in water
Microbial community composition by phospholipid fatty acid
analysis (PLFA)                                                                                                                                                                                                                                                        holding capacity and hydrologic conditions that occurred during
                                                                                                                                                                                                                                                                       ecosystem development.
Nutrient cycling
   •C, N, P mineralization: laboratory batch experiments                                                                                                                                                                                                               • As an environmental condition integrator, microbial community
   •Denitrification, phosphatase activity
                                                                                                                                                                                                                                                                       composition may be an important new tool for monitoring the success of
   •P sorption capacity
   •Nutrient flux from flooded soils                                                                                                                                                                                                                                   compensatory mitigation wetland projects.

                                                                               Fig. 3. Typical phospholipid fatty acid (PLFA) profile of soils collected from bottomland hardwood
       Results presented here will focus mainly on organic C storage
                                                                               forest wetlands. Soils from climax wetlands were enriched in PLFA biomarkers for anaerobic bacteria
and microbial community composition changes as a function of                                                                                                                                                                                                      *This work was sponsored by a grant from the USDA National Research
                                                                               (i17:1n7,10me16, i17, b18, cy19, b20) and soils from early successional wetlands were enriched in PLFA                                                                             Initiative Competitive Grants program
wetland mitigation age.                                                        biomarkers for aerobic fungi (18:2n6), protozoa (20:4n6), and bacteria (16:1n5).

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