WETLANDS, Vol. 29, No. 4, December 2009, pp. 1153–1165 ’ 2009, The Society of Wetland Scientists
MICROBIAL AND GEOCHEMICAL RESPONSES TO ORGANIC MATTER AMENDMENTS IN A CREATED WETLAND Gregory L. Bruland1, Curtis J. Richardson2, and W. Lee Daniels3 1 Department of Natural Resources and Environmental Management College of Tropical Agriculture and Human Resources, University of Hawai‘i Ma¯noa 1910 East-West Rd., Honolulu, Hawai‘i, USA 96822 E-mail:
[email protected] 2
Duke University Wetland Center Nicholas School of the Environment, Duke University Box 90333, Durham, North Carolina, USA 27708-0328 3
Department of Crop and Soil Environmental Sciences 0404, Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA 24061 Abstract: Soil organic matter (OM) is an important feature of natural wetlands (NWs) often lacking in created wetlands (CWs). Some have suggested that OM amendments be used to accelerate development of edaphic conditions in CWs. Our objective was to investigate microbial and geochemical responses to compost amendments at a CW. Five levels of amendments were incorporated into drier and wetter zones of the CW to test two hypotheses: 1) microbial biomass carbon (MBC) and denitrification potential will increase with increasing levels of amendments; and 2) phosphorus (P) sorption will decrease with increasing levels of amendments. Regression indicated that pH, MBC, and P sorption had linear relationships, while bulk density (BD) had an exponential relationship with amendment level. Denitrification enzyme assay (DEA) had highest values at intermediate amendment levels. Analysis of variance indicated amendment effects for BD, MBC, DEA, and P sorption, and wetness effects for pH and MBC. Amendment levels between 60–180 Mg ha21 were ideal for microbial development and denitrification, while not sacrificing P sorption, and would be more logistically and economically feasible than levels of 200–300 Mg ha21. However, responses to amendments were complex and optimizing amendments for certain functions may detrimentally affect other functions. Key Words: compost, denitrification enzyme assay, microbial biomass carbon, mitigation, organic amendments, P sorption, soil properties
INTRODUCTION
This has been identified as a priority in wetland mitigation (Noon 1996). However, minimal data exist on the length of time needed to develop adequate soil conditions for natural biogeochemical cycling in CWs. The few studies on this topic suggest that key soil properties such as soil OM are not increasing with time as expected (Bishel–Machung et al. 1996, Shaffer and Ernst 1999); or if changes occur, they are slow and appear to level off at values well below those of adjacent NWs (Zedler and Calloway 1999). Thus, it has been suggested that OM amendments may be the best method for accelerating the development of many wetland functions at created wetland sites (Stauffer and Brooks 1997, Bailey et al. 2007). There are a variety of suitable organic wastes that could potentially serve as OM amendments including municipal leaf and lawn compost, certain sewage sludge/biosolids materials, food processing waste,
When compared to natural wetlands (NWs), soils of created wetlands (CWs) usually have higher bulk density (BD) and lower levels of organic matter (OM) (Bishel–Machung et al. 1996, Shaffer and Ernst 1999, Stolt et al. 2000, Bruland and Richardson 2006a). Such soil conditions can lead to poor growth and survival of planted and colonizing vegetation. The fact that litter layers in CWs are often poorly developed or absent further confounds this issue (Hunter and Faulkner 2001). As a result of low SOM and litter, it has been speculated that microbial communities in the soil of CWs are much less developed than those of NWs (Duncan and Groffman 1994). As microbes play a key role in nutrient cycling processes in wetlands, their development will be critical to development of nutrient transformation and retention functions in CWs. 1153
1154 and forest products (Stauffer and Brooks 1997). Instead of occupying valuable space in municipal landfills, these types of organic wastes could be used to improve soil conditions in CWs. For example, OM amendments applied to a CW in Pennsylvania (Stauffer and Brooks 1997) significantly increased soil moisture and nitrate-N availability compared to unamended control plots. The authors of this study concluded that amendments should be considered if soils of created wetland projects contain , 10% OM. Another study of a CW in Massachusetts reported that OM amendments produced high levels of microbial activity (Duncan and Groffman 1994). Despite the positive results of these preliminary studies, it remains to be seen whether OM amendments will produce similar results in different types of wetlands or in different regions of the USA. For example, another study of OM amendments at a created salt marsh in southern California revealed that soil carbon (C) and nitrogen (N) pools were not increased by amendments due to high decomposition rates in that site’s sandy soils (Gibson et al. 1994). Furthermore, the studies in Pennsylvania and Massachusetts only added a single level of OM. To our knowledge only one other study to date investigated the microbial and geochemical responses of a low OM soil to compost amendments (Walker and Shannon 2006), and this was done for a single soil type in laboratory mesocosms rather than a large-scale field experiment. As amendments are expensive, especially when applied to larger sites, it is imperative that additional studies be conducted to determine optimal amendment levels for different wetland types in different regions of the country. In response to these research needs, we investigated the effect of OM amendment levels on the nutrient transformation and retention functions of a CW. At this site, five different levels of OM were incorporated into the upper 15 cm of soil in both a wetter and a drier part of the site to determine how different hydrologic regimes would respond to the OM treatments (Bergschneider 2005, Bailey et al. 2007). Our study was part of a larger project conducted by researchers from Virginia Tech and the Virginia Institute of Marine Science that was designed to: 1) measure soil response to different amendment levels, and 2) correlate vegetative response (both herbaceous and woody) to amendment level (Bailey et al. 2007). Microbial biomass and denitrification are likely to increase with increasing amendment levels, as OM provides an energy source for the microbes, helps retain soil moisture, and contributes to decreases in redox potential. However, unlike microbial responses, geochemical responses such as P-sorption may show the opposite trend. For example, it is
WETLANDS, Volume 29, No. 4, 2009 commonly accepted that organic anions compete with phosphate anions for binding sites on the soil surface. Thus, for this study, we tested the following hypotheses: 1) microbial biomass and denitrification potential will increase with increasing levels of organic amendments; and 2) P-sorption will decrease with increasing levels of organic amendments. METHODS Study Site The study site was located in Charles City County, Virginia, USA, in the Coastal Plain physiographic region (Bailey et al. 2007) (Figure 1). This site is part of the Virginia Department of Transportation (VDOT) compensatory mitigation program and primarily offsets non-tidal forested wetland impacts. The pre-existing soil series at the site were a complex of Chickahominy (fine, mixed, semiactive, thermic Typic Endoaquults) and Newflat (fine, mixed, subactive, thermic Aeric Endoaquults) soils (Bergschneider 2005). The site was originally an upland mixed hardwood forest that had been partially converted to an agricultural field. Mitigation efforts during the winter of 1997–1998 attempted to convert the fields and remaining forest remnants to wetland status by removing O+A+E horizons and excavating into the subsoil (Btg horizon) to an elevation presumed to be indicative of the seasonal high water table. However, due to soil compaction, high silt and clay content, and lack of OM in the subsoil, vegetation that established over certain portions of the site was dominated by facultative upland or obligate upland species. This indicated that the hydrologic and edaphic conditions in certain parts of the site were not appropriate for supporting development of wetland vegetation. For example, while the wetter zone clearly supported wetland vegetation, there were areas of the drier zone that would not have met the vegetative success criteria. Much of the site away from our plots got ‘‘re-treated’’ in subsequent years with organic additions and tillage to improve vegetative growth and survival. During the summer of 2002, a group from Virginia Tech incorporated five different levels of compost in a wetter and a drier zone of the site (Bergschneider 2005). The two zones were differentiated by both elevation and vegetation, with the wetter zone being in slightly lower topographic position and supporting a plant community of more emergent marsh species such as Typha spp. and Scirpus spp. than the drier zone which contained a significant component of Lespedeza cuneata (Dum. Cours.) G. Don (Chinese lespedeza). Stable wood-
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Figure 1. Map of the study site showing the location of the wet and dry zones as well as the random locations of the amendment levels in each experiment.
fines compost was applied to replicate plots (4x) in each zone at the following levels: 0, 56, 112, 224, and 336 Mg ha21 after pre-existing vegetation was mowed and the surface was root-raked. The maximum amendment level used in this study is comparable to maximum application rates of 100 to 400 Mg ha21 as summarized in a review paper by Khaleel et al. (1981). The compost was incorporated
with an offset disk followed by a rototiller into the upper 10 cm of soil. Due to the absence of any obvious edaphic or wetness gradients, the amendments in the drier zone were established as a completely randomized design (CRD) (Bergschneider 2005) (Figure 1). There were indications that the wetter zone was located across a soil moisture gradient from northeast to southwest,
1156 so a randomized complete block design (RCBD) was established in this area (Bergschneider 2005, Bailey et al. 2007) (Figure 1). This resulted in two approximately 27 by 13 m rectangles adjacent to one another and blocked internally so that they spanned the potential moisture gradient (Figure 1). The amendments were applied on a volumetric basis (with volume:weight correlation) to square plots with an area of approximately 14 m2. There were four replicates of each treatment as well as four control plots for a total of 20 plots per zone and 40 plots in total. Plots were separated by 3-m wide alleys to allow for movement through the zones without compaction or disruption of the experimental units. Finally, a waist-high double electric fence was installed around each zone for further protection from herbivory by deer. Later, each zone was fenced with 40 cm tall chicken wire buried to a depth of 20 cm to prevent herbivory by rabbits. Soil Sampling and Laboratory Analysis Prior to Amendment Experiment In May 2002, prior to the initiation of the amendment experiment, five soil cores from the upper 0–20 cm of the soil profile were randomly collected from both the drier and wetter zones. Cores were collected in plastic sleeves with a piston corer. Sleeves were capped and stored on ice until being transported back to the Duke Wetland Center Laboratory. Upon arrival, the soil was extruded from the plastic sleeves, weighed, and placed into resealable plastic bags. In addition, a sample of the compost material to be used for the amendments was obtained. Subsamples from each soil core and the compost were oven dried at 105uC for 24 hours to determine the moisture content. A wet to dry conversion factor was then used to calculate the total dry mass of soil. The dry soil mass was divided by the sleeve volume to calculate bulk density (BD). The pipette method was used to determine the percent clay, silt, and sand of the soil cores (Sheldrick and Wang 1993). Finally the dry soils and compost were analyzed for total carbon (CT) and nitrogen (NT) by dry combustion with a Perkin Elmer 2400 CHNS autoanalyzer, and for total phosphorus (PT) by automated ascorbic acid reduction on a Lachat autoanalyzer following HNO3HClO4 digestion (O’Halloran 1993). Soil Sampling and Laboratory Analysis following Amendment Experiment In July 2003, approximately a year after the experiment had been initiated, 20 soil cores (one core
WETLANDS, Volume 29, No. 4, 2009 per plot) from the upper 0–10 cm of the soil profile were collected from each zone for a total of 40 cores. For this sampling, cores were collected, transported, and extruded just as they had been in the preexperiment sampling. The same procedures described in the pre-experiment sampling were also used to determine soil moisture and BD. In addition, representative subsamples were taken from the field moist soils and analyzed for soluble organic carbon (SOC) based on a deionized water extraction method (Kaiser and Zech 1996, Hunter and Faulkner 2001, Bruland et al. 2006), pH (Hendershot et al. 1993), and for microbial biomass carbon (MBC) by the chloroform fumigation extraction method (Brookes et al. 1985, Allen 1999, Bruland and Richardson 2004a). We also measured the denitrification enzyme assay (DEA) (Tiejde 1982) as an index of denitrification potential. Briefly, 20 grams of homogenized field moist soil were measured into mason jars fitted with septa to allow for collection of gas samples with syringes. Soil samples were amended with solutions of 1 mM glucose, 1mM potassium nitrate, and 1 g L21 chloramphenicol. The slurries were made anaerobic by repeated flushing with N2 gas, followed by the injection of H2SO4-scrubbed acetylene to inhibit N2 production (Hyman and Arp 1987). Jars were vigorously shaken by hand and then placed on an orbital shaker and shaken at 100 rpm for 90 minutes. At 20, 40, and 60 minutes, gas samples were collected from each jar with a syringe. Nitrous oxide concentrations were determined with a Shimadzu (Columbia, MD) GC-14A 63Ni electron capture detector gas chromatograph (GC). Operating conditions and calibration of the GC were according to Whalen (2000). Nitrous oxide fluxes were calculated as the time-linear rate of concentration increase in the headspace of the mason jars. The DEA was calculated as the short-term (2 h) rate of N2O production in the jars and is indicative of the size of the denitrifying enzyme pool present in the soil. Corrections for N2O dissolved in pore water and the decreases in the volume of gas in the jars with repeated sampling were calculated according to Whalen (2000). Finally, due to the significant differences in BD across the treatments, the DEA data were expressed on a volumetric rather than a mass basis as in Bruland et al. (2006). The phosphorus (P) sorption capacity of the soils was measured with the P sorption index (PSI) (Richardson 1985). Previous studies have established that the PSI is a reliable gauge of a wetland soil’s P sorption potential, is less time-consuming to measure than multiple-point P sorption isotherms, and facilitates comparison with related soil properties
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Table 1. A comparison of the mean values (6 1 standard deviation, n 5 5) of the soil properties of the wetland prior to the establishment of the amendment experiment as well as of the compost used as the source of organic matter. Means followed by different letters were significantly different according to a post-hoc least-squared differences test. NA 5 not applicable. Soil type Soil Property 23
Bulk Density (g?cm ) Moisture (%) Sand (%) Silt (%) Clay (%) Total Carbon (%) Total Nitrogen (%) Total Phosphorus (%) C/N C/P N/P
Drier Zone
Wetter Zone
1.29 15.0 27.6 37.7 34.7 1.36 0.08 0.02 17.3 61.9 3.62
1.25 18.3 25.7 42.0 32.3 1.14 0.08 0.02 14.8 50.8 3.41
(0.07) a (1.6) a (4.03) a (6.12) a (9.49) a (0.56) a (0.03) a (0.00) a (3.71) a (20.7) a (1.00) a
(Richardson 1985, Axt and Walbridge 1999, Bridgham et al. 2001, Bruland and Richardson 2004b, Bruland and Richardson 2006b). The PSI was determined by shaking 2.0 grams (dry weight equivalent) of field moist soil with a 25 mL solution of 130 mg PO4-P L21 for 24 hours. Soils were sterilized with 2 drops of toluene to prevent microbial uptake of P (Richardson and Vaithiyanathan 1995). After shaking, samples were centrifuged for 20 minutes and filtered with Whatman 42 filter paper. Samples were analyzed for PO4-P by Ion Chromatography (Dionex, Sunnyvale, CA). The difference in concentration of inorganic P between the initial (130 mg L21) and final concentration represents the amount of P sorbed. The index was then calculated as the amount of P sorbed (mM P) per a given soil volume (100 cm23) (Darke and Walbridge 2000). It has been recommended that when comparing P sorption across wetland soils of varying BD or organic matter content, it is more appropriate to express the data on a volumetric than a mass basis (Walbridge and Struthers 1993), and since our treatments varied considerably in both BD and organic matter, we followed this recommendation. Statistical Analyses Means and standard deviations for basic soil properties of the wetter zone drier zone, and compost sample were calculated for the pre-experiment soils data. Differences in the mean values of the pre-experiment data were determined with a least-squared difference (LSD) test following a oneway analysis of variance (ANOVA).
(0.10) a (1.7) b (2.92) a (4.85) a (7.37) a (0.53) a (0.02) a (0.00) a (2.03) a (12.9) a (0.44) a
Compost Sample NA 14.1 (2.4) a NA NA NA 35.3 (0.78) b 0.93 (0.02) b 0.09 (0.01) b 37.8 (1.16) b 378 (28.0) b 9.98 (0.47) b
As no significant block by treatment interactions were noted for the wetter zone for any of the soil properties quantified following the amendment experiment, the data from the two zones could be analyzed jointly. This was accomplished using a two-way ANOVA with amendment level (0, 56, 112, 224, 336 Mg?ha21), wetness regime (wetter, drier), and the amendment 3 wetness interaction as factors. This model was used to account for the variability in the BD, pH, SOC, MBC, DEA, and PSI data. The data were not transformed as all variables except for PSI were normally distributed according to the Shapiro-Wilk test (Shapiro and Wilk 1965). In terms of PSI, ANOVA is robust in dealing with non-normal data particularly when there are multiple treatments and the samples of each treatment are balanced (Underwood 1997), as was the case in this study. Furthermore, the two-way ANOVA of the log-transformed PSI data was very similar to the ANOVA of the raw PSI data. Differences across the amendment levels and wetness regimes were analyzed with a LSD procedure. We also used linear, exponential, and polynomial regression to model the response of BD, pH, MBC, DEA, and P sorption to the amendment levels in the drier and wetter zones. The statistical significance for the ANOVAs and regressions was set at a 5 0.05. RESULTS Characterization of the Soils and Compost Prior to Amendment Experiment Prior to the initiation of the amendment experiment, there were no significant differences in BD,
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Figure 2. Regressions of A) bulk density (BD), B) pH, C) soluble organic carbon (SOC), D) microbial biomass carbon (MBC), E) denitrification enzyme assay (DEA), and F) the phosphorus sorption index (PSI) as a function of organic amendment loading rate. Circles represent the mean value at each loading rate and error bars represent 6 1 standard error. Open circles represent data from the drier zone while shaded circles represent data from the wetter zone. For each panel,
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Table 2. Results of the two-way ANOVAs testing for significance of the amendment, wetness, and amendment 3 wetness effects for bulk density (BD), pH, soluble organic carbon (SOC), microbial biomass carbon (MBC), the denitrification enzyme assay (DEA), and the phosphorus sorption index (PSI). All significant effects (a 5 0.05) are indicated by bold type. Degrees of freedom for amendment 5 4, wetness 5 1, amendment 3 wetness 5 4, and error 5 30. Soil Property 23
BD (g?cm
)
pH
SOC (mg?cm23)
MBC (mg?cm23)
DEA{
PSI{
Source of Variation
F
p
Amendment Wetness Amend 3 Wet Amendment Wetness Amend 3 Wet Amendment Wetness Amend 3 Wet Amendment Wetness Amend 3 Wet Amendment Wetness Amend 3 Wet Amendment Wetness Amend 3 Wet
49.95 5.22 1.27 2.65 6.27 0.58 15.26 0.65 0.65 4.21 13.56 0.39 3.12 0.00 1.42 9.33 2.92 1.45
, 0.001 0.03 0.30 0.05 0.02 0.68 , 0.001 0.63 0.43 , 0.001 , 0.001 0.81 0.03 0.99 0.25 , 0.001 0.10 0.24
{ The denitrification enzyme assay expressed as ng N2O produced per cm3 of soil per hour. { The phosphorus sorption index expressed as mM P sorbed per 100 cm3 soil.
sand, silt, clay, CT, NT, PT, C/N, and N/P between the drier and wetter zones (Table 1). As expected, the wetter zone had significantly higher moisture than the drier zone. There were significant differences between the unamended soils of the wetter and drier zones and the compost sample. For example, mean CT, NT, PT, C/N, C/P, and N/P were all significantly higher in the compost than in the mineral soils from either experimental zone (Table 1). Edaphic Response to the Amendment Experiment The soil properties measured in this study displayed a variety of responses to amendment levels. As there were significant differences in BD across the amendment levels, we chose to express the post-amendment data on a volumetric rather than a mass basis. Specifically, BD decreased exponentially with increasing amendment level (Figure 2A). The model for the drier zone (BDd 5 1.07e20.003Loading Rate (LR)) had an r2 of 0.92 (p 5
0.02). The model for the wetter zone (BDw 5 1.03e20.004LR) had an r2 of 0.97 (p 5 0.0001). The two-way ANOVA indicated that both amendment and wetness accounted for a significant proportion of the variance in the BD data while their interaction did not (Table 2, Figure 2A). The amendment effect F-statistic for BD (49.95) was the highest reported for any of the soil properties measured in this study (Table 2). Across both zones, the control plots had significantly higher mean BD values than the other four levels (Figure 2A) and the 56 Mg ha21 plots had higher mean BD values than the 224 or 336 Mg ha21 levels. Mean BD in the 336 Mg ha21 level of the wetter zone was lower than all other treatments. Soil pH displayed a linear decrease with amendment level in both wetter and drier zones (Figure 2B); the slope and y-intercept were both slightly greater in the drier zone. The ANOVA indicated that the relative strength of the wetness effect (p 5 0.02, Table 2) was stronger than that of the amendment effect (p 5 0.05, Table 2). Specifically, mean pH values were higher in the drier control
r the upper equation and r2 value corresponds to plots from the drier zone and the lower equation and r2 corresponds to plots from the wetter zone. Circles with different letters are significantly different according to the ANOVA post-hoc LSD test.
1160 treatment than in the drier 336 Mg ha21 treatment and the wetter zone 56, 112, 224, and 336 Mg ha21 treatments (Figure 2B). Soluble organic carbon exhibited linear increases with amendment level across both wetter and drier zones (SOCd 5 0.35*LR + 57, r2 5 0.93, p 5 0.008; SOCw 5 0.39*LR + 62, r2 5 0.93, p 5 0.03) (Figure 2C). The slopes and y intercepts for the models for SOC were similar in each experiment. ANOVA indicated that only amendment effects were significant for SOC (Table 2). Specifically, the control treatments had lower SOC than the 336 Mg?ha21 treatments in both zones and the 224 Mg?ha21 treatment in the wetter zone (Figure 2C). While not statistically significant, the model for MBC in the drier zone (MBCd 5 0.25*LR + 213, r2 5 0.54, p 5 0.16) showed a trend of increasing MBC with amendment level (Figure 2D). In the wetter zone, MBC showed a significant linear relationship with amendment level (MBCw 5 0.27*LR +145, r2 5 0.86, p 5 0.02). The slopes of the models for MBC were nearly identical, whereas the y intercept for MBC in the drier zone was considerably higher than the intercept of the wetter zone. Microbial biomass carbon was the only soil parameter for which the F-statistic for amendment effects was lower than the F-statistic for wetness effects. Specifically, the mean MBC in the drier zone 224 Mg ha21 treatment was significantly higher than the mean values from 0, 56, 112, and 224 Mg ha21 treatments in the wetter zone (Figure 2D). The DEA exhibited a non-linear relationship to amendment level (Figure 2E), best modeled by a second-order polynomial function. While the model for the drier zone was not statistically significant (DEAd 5 20.01*LR2 + 2.88*LR + 477, r2 5 0.49, p 5 0.10), there was a second-order trend in the data. The model for the wetter experiment (DEAw 5 20.01*LR2 + 3.39*LR + 369, r2 5 0.91, p 5 0.02) was significant and had a strong fit with the experimental data (Figure 2e). According to the ANOVA, there was a significant amendment, but not a wetness effect for DEA (Table 2). For example, the drier zone 112 Mg?ha21 plots and the wetter zone 224 Mg?ha21 plots had higher DEA rates than the control plots (Figure 2E). The 112 Mg?ha21 treatment from the drier zone was also higher than both the 336 Mg?ha21 treatments. The two treatments with the highest mean DEA rates were the 112 Mg?ha21 drier plots and the 224 Mg?ha21 wetter plots. At lower amendment levels, DEA rates tended to be higher in the drier plots, while at higher levels, DEA rates tended to be higher in the wetter plots.
WETLANDS, Volume 29, No. 4, 2009 In both zones, the PSI displayed a linear decrease with amendment level (PSId 5 20.002*LR +1.6; r2 5 0.87; p 5 0.02; PSIw 5 20.004*LR + 2.1; r2 5 0.92; p 5 0.01) (Figure 2F). The slope and y intercept of the model for PSI in the wetter zone were slightly greater than the slope and intercept for the drier zone (Figure 2F). The ANOVA indicated that there were significant amendment effects for the PSI data (Table 2). For example, in the control plots the mean PSI was higher in the wetter than in the drier zone (Figure 2F).
DISCUSSION Characterization of the Soils and Compost Prior to Amendment Experiment The pre-experiment BD values at the study site were comparable with BD values reported in other studies of mitigation wetlands in the Southeastern Coastal Plain (Cummings 1999, Bruland and Richardson 2004b, Bruland and Richardson 2005). While these values are less than what would be considered ‘‘root-limiting’’ for this texture class (i.e., 1.4 g cm23, W. L. Daniels, Virginia Polytechnic Institute and State University, pers. comm.), these values are higher than those of natural forested wetlands in this region (i.e., 0.24–1.0 g cm23; Bruland and Richardson 2005). Soil sampling in the winter of 2003 and spring of 2004 revealed especially high BD values of the subsoil at this site (Bergschneider 2005), which may also help to explain the poor vegetative growth and survival rates that were observed. In addition to soil compaction, pre-experiment soil moisture values were fairly low in the summer of 2002, indicative of the drought conditions that occurred across this region at the time of the study. Created wetlands with compacted soils may also result in episaturated, perching conditions in which surface soils are not hydrologically connected to ground water and are prone to excessively dry summer conditions (Whittecar and Daniels 1999, Bergschneider 2005). The CT and NT values reported for this site were also comparable to CT and NT values reported in mitigation wetlands in this region (Cummings 1999, Stolt et al. 2000). The C/N ratios of the drier and wetter soils (17 and 15, respectively) prior to the amendment experiment were considerably higher than the C/N of the average B horizon (9), slightly higher than the median C/N ratio for cultivated surface horizons (12), and lower than the C/N of average forest A horizons (20) as summarized in Brady and Weil (2008). The CT of the compost used for the amendment experiment (35%) was compa-
Bruland et al., MICROBIAL AND GEOCHEMICAL RESPONSE TO AMENDMENTS rable to typical CTs of other types of organic material such as bluegrass from fertilized lawns (42%), wheat straw (38%), digested municipal sewage sludge (31%), and finished household compost (30%) (Brady and Weil 2008). C/N (38) for the compost used in this study were lower than C/N ratios of sawdust (792–211) (Fang et al. 1999, McGuckin et al. 1999), wood chips (653) (Martinez and Otten 1999), and wheat straw (92–51) (Liang et al. 1999, Barrington et al. 2002); comparable to C/N ratios for rye cover crops (37) (Brady and Weil 2008), and compost from municipal solid and tobacco waste (29) (Egelkraut et al. 2000); and lower than C/N ratios for cotton gin compost (16) (Tejada and Gonzalez 2008), finished household compost (15) (Brady and Weil 2008), poultry manure (9) (Tejada and Gonzalez 2008), or digested municipal sewage sludge (7) (Brady and Weil 2008). Edaphic Response to the Amendment Experiment While a few previous studies have examined the effects of amendments on plant growth in created wetlands, this study is one of the first to investigate the effects of organic amendments on microbial and geochemical processes such as denitrification and P sorption. The exponential models used to relate BD to amendment level suggested that initial reductions in BD may be achieved at low amendment levels, but that continued reductions would require increasingly greater amounts of OM or larger and better equipment for incorporation. Such levels would not be logistically nor economically favorable. Our results were similar to previous studies of organic amendments in various soil types and landuses that showed a reduction in compaction with an increase in amendment level (Khaleel et al. 1981, Bendfeldt et al. 2001, Cogger 2005). We expect that the decreases in compaction reported with organic matter amendments at this site will stimulate root penetration and bioturbation by soil fauna. Even the 56 Mg?ha21 amendment level showed significantly lower BD than the control plots, suggesting that it may not take large amounts of OM amendments to get ecologically meaningful reductions in BD. Over time, we expect that the wetter zone may experience greater rates of SOM accumulation as waterlogged conditions prevent aerobic decomposition of litter and surface SOM. However, these pedogenic processes are slow, and such differences would be difficult, if not impossible, to detect in a short time period. The linear models used to relate pH, SOC, and MBC to the amendment level suggested that increases or decreases in these parameters were
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directly proportional to the amendment level. Our results of decreasing pH with increasing amendment level were opposite those of a previous study of OM amendments in a mitigation wetland (Stauffer and Brooks 1997), but consistent with expected effects of decomposition of woody residues. In this previous study, amended plots showed an increase in pH that the authors attributed to an increase in NH4-N during wet periods from the amended plots. The linear increase in SOC with amendment level was expected as the organic amendments were the predominate source of soil C in these low OM mineral soils. The slope of the increase in SOC was higher than that of the slope of the increase in MBC, perhaps suggesting that the microbes had yet to utilize all of the SOC provided by the amendment or that some of the C in the amendment was not readily available to the microbes due to the lack of mixing of subsoil and compost at some of the higher loading rates. Further research is needed to determine the amount of time required for microbial populations to reach equilibrium with the available C. As hypothesized, MBC had a positive response to amendment level in both zones. With five amendment levels spread over a wide range of values, we are confident in the linearity of these relationships. At higher loading rates than those used in this study, these relationships may cease to be linear, or reach an asymptote, but such rates would probably not be used at an actual wetland mitigation site or at least be at the upper end of OM loading rates. However, we did not expect to see higher MBC levels in the plots from the drier experiment than in the plots from the wetter experiment. We expected the microbes to be moisture limited, however this did not appear to be the case. This may be due to the energetics of aerobic versus anaerobic respiration. Drier plots would have more aerobic soil conditions where microbes would utilize oxygen as a terminal electron acceptor, which yields much more energy than other terminal electron acceptors that must be used when oxygen is consumed. Given the MBC results, the second-order relationship between DEA and amendment level was somewhat surprising. However, additional research of the soil surface elevation, redox potential (Eh), and hydrology at this site would help to provide an explanation for this result. For example, Bergschneider (2005) tracked changes in redox potential (Eh) in the wetter and drier zones at this site in 2003 and 2004 and found that Eh values were generally higher in the two highest OM treatments across both zones. This was probably a result of the positive relationships between loading rate and soil surface
1162 elevation reported for this site (Bailey et al. 2007). These results suggested that the lower three OM treatments experienced inundated/ponded conditions for at least part of the growing season, whereas the two higher OM treatments were not ponded during the growing season and likely were only saturated following rain events (Bailey et al. 2007). Thus the decrease in DEA at the higher OM loading rates may have been due to the slightly higher soil surface elevations, which lead to increased soil: atmosphere gas exchange, higher Eh values, and conditions that were more favorable to aerobic versus anaerobic processes in these plots. It should also be noted that these results are from a single mitigation wetland; other sites with different BD, moisture, and OM contents may produce different results. If high OM loading rates tend to cause increases in soil surface elevations at CW sites, this suggests that increasing the loading rate would not necessarily result in increased denitrification. Furthermore, microbial processes such as denitrification, Fe reduction, or methanogenesis may exhibit more complex relationships to amendment loading rates, as these processes often show high rates above a threshold or within a certain window of soil conditions (Wu and Patrick 2003). Thus, our first hypothesis suggesting that denitrification potential would increase with increasing levels of organic amendments was correct for DEA at low to intermediate OM levels, but not at higher OM levels. We do note, however, that when considered on a mass basis, DEA rates continued to increase linearly with amendment level. The choice to display data on a mass or volumetric basis can thus have a considerable impact on the interpretation of the relationship of amendment levels to soil parameters. While in certain cases it is more appropriate to express data on a mass/concentration basis (Bruland and Richardson 2004b), in other cases it may be more appropriate to express data on a volumetric basis (Walbridge and Struthers 1993, Smith et al. 2001, Bruland et al. 2006). Volumetric data perhaps provide interpretation from a more ecological perspective as microbes and plant roots occupy soil in three-dimensional space rather than by a dimensionless weight (Smith et al. 2001). It is also important to point out that microbial communities may respond differently to different types of OM amendments such as compost, salvaged marsh surface, and municipal sewage sludge, further complicating the types of relationships reported here. For example, another study of the denitrification potential of restored agricultural wetlands in
WETLANDS, Volume 29, No. 4, 2009 Louisiana reported increasing DEA rates with increasing moisture and soluble organic carbon (Hunter and Faulkner 2001). A mesocosm study of the effects of organic amendments on an upland mineral soil also reported significant increases in nitrate removal with OM loading rates (Walker and Shannon 2006). The mean values for DEA (300– 800 ng N2O cm23 hr21) reported in this study were higher than mean values (10–120 ng N2O cm23 hr21) reported for a series of paired restored/created and natural wetlands in the North Carolina Coastal Plain (Bruland et al. 2006), which may be related to the high levels of labile carbon from the amendments compared to other wetlands in the region. The negative relationship of amendment level to PSI supported our second hypothesis that organic amendments would decrease P sorption. The results for PSI in the wetter zone more fully matched our hypothesis in that PSI values displayed a monotonic decrease with amendment level. In the drier zone, on the other hand, PSI did not exhibit significant decreases from the control plots until the 224 Mg ha21 amendment level. Soluble organic anions/ complexes in the pore water of the wetter zone may have outcompeted phosphate for binding sites, whereas in the drier zone such anions may have been less prevalent in the pore water. Our PSI values in the amendment plots were comparable to those reported for a forested floodplain wetland in Georgia and higher than those reported for an upland forest adjacent to that floodplain (Darke and Walbridge 2000). Our results were similar to several previous studies that examined the effects of organic matter on P sorption. For example, Walker and Shanon (2006) reported decreasing P removal in compost amendment mesocosms compared to unamended control mesocosms. Ohno and Erich (1997) found that dissolved organic matter from crop residues inhibited P sorption in an Aquic Haplorthod in Maine. Ohno and Crannel (1996) determined that dissolved OM from manure inhibited P precipitation in acid soils. Fox et al. (1990) illustrated that organic acids with high Al solubility constants tended to increase P solubility and decrease P sorption. Thus, the use of organic amendments in wetland creation or restoration may force site designers and managers to make difficult choices such as compromising P sorption in order to promote tree survival or stimulate microbial communities. However, in this study, as decreases in P sorption in the amended plots were only significantly different from the control plots at the highest levels, amendment levels between 50–200 Mg?ha21 should not have major impacts on P sorption. We expect this to hold true for sites with
Bruland et al., MICROBIAL AND GEOCHEMICAL RESPONSE TO AMENDMENTS similar soil properties to those of this study. While the subsoils below OM incorporation zones at our site in Virginia will continue to have high sorption capacities, this may not be the case at CWs with sandier surface or subsurface horizons. Regardless of texture, it is also important to point out that the microbial biomass in soils of CWs would also presumably sequester considerable P over time which would contribute to overall P retention. Thus we suggest that the optimal organic amendment level for mitigation wetlands in the Coastal Plain of Virginia is between 60–180 Mg?ha21 as this range of rates appeared to give the optimal reductions in BD, and increases MBC and denitrification potential, without resulting in detrimental decreases in pH or P sorption. Thus, the 60–180 Mg?ha21 range appeared to balance economic constraints with nutrient transformation and retention processes to provide the created wetland with maximum functional benefits. In studying the response of the vegetation to the organic amendment loading rates at this same site, Bailey et al. (2007) concluded that the amendment loading rate of 112 Mg?ha21 was optimal as it provided soil nutrient levels similar to natural wetlands in this region as well as minimized changes in the soil surface elevation due to the added amendment material. Management Implications While it may be too costly to amend large mitigation sites in their entirety (as current values for compost are in the $10–15 m23 range), we believe there is value in amending certain sections or subplots of these sites. Interestingly, soil amendments are now a standard recommendation for created wetlands in the state of Virginia (Daniels et al. 2005). Ultimately, just as there are hydrologic and vegetative success criteria for created wetland, we believe that there should be edaphic success criteria. When poor soil conditions can lead to inadequate hydrology and low plant survival, establishing proper substrate conditions may be as important as reestablishing wetland hydrology and hydrophytic vegetation. With the development of edaphic success criteria, sites that did not meet an OM threshold would need to be amended during mitigation to improve functionality. Such OM thresholds are also now a standard recommendation in Virginia (Daniels et al. 2005). Without further quantitative evaluation of mitigation options such as OM amendments, wetlands will continue to be created and restored without the knowledge that management actions can be taken,
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albeit with some possible tradeoffs, to ‘‘jump-start’’ wetland function. This research is even more relevant because over the 2001–2008 period, the North Carolina Department of Transportation (NCDOT) alone was required to mitigate for approximately 320 km of stream impacts and 2,400 ha of wetland impacts (D. Schiller, North Carolina Department of Transportation, pers. comm.), for which no OM additions were required. Since our study shows that OM amendments result in more functional created wetlands, VDOT, NCDOT, and other groups involved in wetland mitigation should be encouraged to utilize such practices in future wetland creation to stimulate vegetative diversity and biogeochemical processes than might otherwise not develop. ACKNOWLEDGMENTS Funding was provided by the Duke Wetland Center Case Studies Program and by a Graduate Research Fellowship from the Center for Transportation and the Environment (Raleigh, NC). The field experiment sampled here was funded by the VDOT in collaboration with Dr. J. Perry at the Virginia Institute of Marine Sciences and Dr. G. R. Whittecar at Old Dominion University. C. Bergschneider was the primary monitor of field experiments and aided in the collection of the soil cores. W. Willis, J. Rice and P. Heine assisted with the chemical analyses at the Duke Wetland Center Laboratory. Dr. S. Whalen and E. Fischer provided critical assistance with the DEA measurements at University of North Carolina-Chapel Hill. Comments on earlier versions of this manuscript by H. Bruland, J. Pahl, and A. Sutton-Grier are also gratefully acknowledged. LITERATURE CITED Allen, A. S. 1999. Effects of elevated atmospheric CO2 on soil nitrogen availability. Ph.D. Dissertation. Duke University, Durham, NC, USA. Axt, J. R. and M. R. Walbridge. 1999. Phosphate removal capacity of palustrine forested wetlands and adjacent uplands in Virginia. Soil Science Society of America Journal 63:1019–31. Bailey, D. A., J. E. Perry, and W. L. Daniels. 2007. Vegetation dynamics in response to organic matter loading rates in a created freshwater wetland in southeastern Virginia. Wetlands 27:936–50. Barrington, S., D. Chiniere, M. Trigui, and W. Knight. 2002. Compost airflow resistance. Biosystems Engineering 81:433–41. Bishel-Machung, L., R. P. Brooks, S. S. Yates, and K. L. Hoover. 1996. Soil properties of reference wetlands and wetland creation projects in Pennsylvania. Wetlands 16:532–41. Bendfeldt, E. S., J. A. Burger, and W. L. Daniels. 2001. Quality of amended mine soils after 16 years. Soil Science Society of America Journal 65:1736–44.
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Bruland et al., MICROBIAL AND GEOCHEMICAL RESPONSE TO AMENDMENTS Underwood, A. J. 1997. Experiments in Ecology: Their Logical Design and Interpretation using Analysis of Variance. Cambridge University Press, Cambridge, United Kingdom. Walbridge, M. R. and J. P. Struthers. 1993. Phosphorus retention in non-tidal palustrine forested wetlands of the Mid-Atlantic region. Wetlands 13:84–94. Walker, C. W. and R. D. Shannon. 2006. Nitrate and phosphate removal effects of compost amendments in wetland mesocosms. Transactions of the American Society of Agricultural and Biological Engineers 49:1773–78. Whalen, S. C. 2000. Nitrous oxide emission from an agricultural soil fertilized with liquid swine water or constituents. Soil Science Society of America Journal 64:781–89.
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Whittecar, G. R. and W. L. Daniels. 1999. Use of hydrogeomorphic concepts to design created wetlands in southeastern Virginia. Geomorphology 31:355–71. Wu, K. and W. H. Patrick. 2003. Redox range with minimum nitrous oxide and methane production in a rice soil under different pH. Soil Science Society of America Journal 67:1952–58. Zedler, J. B. and J. C. Calloway. 1999. Tracking wetland restoration: do mitigation sites follow desired trajectories? Restoration Ecology 7:69–73. Manuscript received 3 October 2008; accepted 27 May 2009.
