Carbon and Microbial Community Composition at Mitigated Bottomland Forest Wetlands*

Elisa M. D’Angelo, A.D. Karathanasis, S.A. Ritchey, and S.W. WehrUK 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

STRS

R

Pon

d R

iver

Trad

ewat

er R

iver 24 km

Western Coalfield

Cropland (0 y)Cropland (0 y)

Late successional(15 y)Late successional(15 y)

Early successional (4 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, NH4, NO3+NO2

•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.

y = 26x 0.077

r 2 = 0.8325

30

35

40

0 20 40 60

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).

CFES LS RS

CFES LS RS

(a)

(b)

Mitigation age, years

Wa

ter

ho

ldin

g c

ap

acit

y, %

TO

C in

litt

er

and

so

il,g

m-2

N

y = 1168 x 0.31

r 2 = 0.96

0

1000

2000

3000

4000

5000

0 20 40 60

Litter C

Soil C

Total C

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.