Spatial variability of soil properties and trace gas fluxes in reclaimed mine land of southeastern Ohio

Geoderma 136 (2006) 598 – 608 www.elsevier.com/locate/geoderma

Spatial variability of soil properties and trace gas fluxes in reclaimed mine land of southeastern Ohio P.-A. Jacinthe a,⁎, R. Lal b a

Department of Earth Sciences, Indiana University–Purdue University at Indianapolis (IUPUI), 723 W. Michigan Street, SL 122, Indianapolis, IN 46202, USA b Carbon Management and Sequestration Center, The Ohio State University, Columbus, OH 43210, USA Received 14 July 2005; received in revised form 27 March 2006; accepted 25 April 2006 Available online 22 June 2006

Abstract Uneven distribution of resources in reclaimed mine land could translate in heterogeneous gas fluxes in these ecosystems, but few studies have examined this linkage. In this study, the spatial variability of CO2, CH4 and N2O fluxes and soil properties was investigated at (i) a hay field (90 kg urea–N ha− 1 y− 1), and (ii) a meadow (without any grazing and fertilization since reclamation) established in 1978 and 1982, respectively on reclaimed mine land in southeastern Ohio. Using the static chamber method, gas fluxes were measured in July 2003 and May 2004 every 5 m along two 100-m long transects. Soil cores and disturbed soil samples were collected at each point to determine soil physical and chemical properties. Greater variability of soil properties at the meadow than at the hay site was found and, among the soil parameters, mineral N and N2O flux (CV up to 197%) exhibited the greatest variability. While no significant effect of landscape position on gas flux was found, several micro-depressions were identified as N2O hot spots. Relationships between soil properties and gas fluxes varied with site and gas species. The July 2003 data showed that both CO2 and N2O fluxes were significantly (P b 0.02) related to labile C at the hay site, and to mineral N at the meadow site. The PC1 of a principal component analysis of soil properties showed high loadings for total and labile C at the hay field, and for mineral N at the meadow. Inverse and significant relationships (r2: 0.17–0.24) were found between CH4 flux and macropore volume at the meadow site. These relationships were stronger (R2: 0.30–0.68) with inclusion of soil moisture in the regression. These results underscore the need to minimize soil compaction during reclamation in order to maintain macro-porosity and improve the CH4 uptake capacity of reclaimed mine soils. © 2006 Elsevier B.V. All rights reserved. Keywords: Mine land; Variability; Macro-pore; Nitrous oxide; Carbon dioxide; Methane

1. Introduction Surface mining of coal results in drastic landscape disturbances. Since adoption of the Surface Mining Control and Reclamation Act of 1977 (SMCRA, Public ⁎ Corresponding author. Tel.: +1 317 274 7969; fax: +1 317 274 7966. E-mail address: [email protected] (P.-A. Jacinthe). 0016-7061/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2006.04.020

Law 95–87), efforts have been made to restore mined areas to conditions similar to pre-mining through grading and topsoil application. Reclaimed mine land is generally heterogeneous, and this heterogeneity stems from partial mixing and irregular spreading of topsoil materials over chemically- and mineralogically-variable overburden. The spatial variability of reclaimed soil properties is well documented in the literature (Schroeder, 1995; Hossner et al., 1997; Mummey et al., 2002;

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Hangen et al., 2004; Shukla et al., 2004). Hangen et al. (2004) conducted a dye tracer study using soils from a reclaimed forest, and noted that only 15% of the topsoil volume participated in water transport due to preferential flow in the underlying overburden. Hossner et al. (1997) reported on the random distribution of zones of favorable pH and zones of high acidity in reclaimed mine land, and discussed the implication of that patchiness on exploration of the soil volume by grass roots. In shrub-dominated reclaimed sites in Wyoming, Mummey et al. (2002) showed that nutrient availability was spatial stratified with nutrient-depleted zones near the base of shrubs and nutrient-rich areas on the leeward side of shrubs. While much is known regarding the variability of soil physico-chemical properties associated with vegetation establishment and growth (Varela et al., 1993; Schroeder, 1995; Bendfeldt et al., 2001; Shukla et al., 2004), limited data exist with regard to the variability of soil properties related to the dynamics of greenhouse gases (GHG) such as carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) in reclaimed mine land. In undisturbed ecosystems, soil gas fluxes generally exhibit considerable variability (coefficient of variation NN 100% are not uncommon; Ambus and Christensen, 1994; Jacinthe and Dick, 1997; Röver et al., 1999; Yanai et al., 2003) because the dynamics of trace gases are controlled by several factors (e.g. temperature, moisture, nutrient availability and labile C) and the intensity of each of these factors varies in space and time. In reclaimed mine land, this variability would be further amplified due to the random distribution of soil properties introduced by the reclamation process. Heavy machinery is commonly used during the reclamation of mine land, and consequently these soils tend to be compacted. Both the volume and geometry of soil macropores are negatively affected by compaction (Schjonning and Rasmussen, 2000). As a result, soil– atmosphere exchange of gases could become impeded. Previous studies (Ball et al., 1997; Sitaula et al., 2000; Teepe et al., 2004) have shown that soil CH4 uptake diminishes with deterioration of soil structure and compaction. Further, compaction and related soil conditions (reduced macropore volume and continuity) could lead to the development, within the soil volume, of anaerobic micro-sites where production of CH4 (methanogenesis) and N2O (denitrification) is favored. Moisture accumulation in depressions and at downslope locations could also have a similar impact (Schroeder, 1995). Post-reclamation land-uses and management practices could also contribute to the variability of trace gas

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fluxes in reclaimed mine land. In the Appalachian coal region, reclaimed sites are most often seeded with grass with or without later incorporation of trees. Once established, grasslands are usually used either for grazing purposes or for hay production. In some cases, the grass is allowed to grow unharvested and un-grazed for many years (a grassland so managed is referred to as a meadow in this paper). In hay fields for example, the aboveground biomass is regularly exported out of the field. Biomass export could affect the amount of organic matter available for microbial decomposition, and thus indirectly impact the production of biogenic gases. In meadow systems (absence of grazing and haying) conversely, dead plant residue accumulates at the soil surface, and over time, leads to formation of a litter layer where conditions (nutrient and moisture) are likely conducive to sustained microbial activity and gas production. The objectives of the present study were to measure trace gas fluxes and related soil properties in reclaimed grasslands, and examine the effect of management and landscape features on the variability of gas fluxes. Soil physical (bulk density, moisture, macro-porosity), biochemical (basal soil respiration, pH, SOC, total and mineral N) properties and trace gas fluxes were measured at different landscape positions and at two reclaimed grassland sites — a hay field and a meadow maintained under contrasting management practices for ∼ 20 years. Management of the hay field involved annual N fertilization and aboveground biomass export, whereas none of these practices occurred at the meadow site. As a result of 25 years of fertilization, greater availability and evenness in the distribution of nutrients at the hay field than at the meadow are expected. Consistent with reports (Ambus and Christensen, 1994; Röver et al., 1999) of diminished variability with increased availability of nutrients in soils, it was hypothesized that emission of GHG is more intense but less spatially variable at the hay field compared to the meadow site. 2. Materials and methods 2.1. Study sites This field study was conducted at two reclaimed sites near McConnersville in southeastern Ohio. These sites included (i) a hay field (Long. 83° 01′ W and Lat. 40° 00′ N, Morgan county) established in 1978 on reclaimed coal-mine land, fertilized annually with urea (90 kg N ha− 1) and used exclusively for the production of hay, and (ii) a meadow (Long. 81°38′ W and Lat. 39°50′ N,

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Noble county) developed at a coal-mine site reclaimed in 1982. Grassland vegetation included orchard grass (Dactylis glomerata L.), blue grass (Poa annua L.), alfalfa (Medicago sativa L.) and various clover species (Trifolium sp.). At the hay site, the grass was harvested in the spring and in the fall with fertilization occurring in the summer (usually in June) between the two harvests. At the meadow site, grass was allowed to grow without any grazing, mowing or N fertilization since reclamation. Prior to mining, the dominant soil series were Lowell (fine, mixed, active, mesic Typic Hapludalfs), Gilpin (fine-loamy, mixed, active, mesic Typic Hapludults), Guernsey (fine, mixed, superactive, mesic Aquic Hapludalfs) and Morristown (loamy-skeletal, mixed, active, calcareous, mesic Typic Udorthents) developed from interbedded sandstone, shale, siltstone and limestone bedrocks.

(Jacinthe and Dick, 1997; Jacinthe and Lal, 2003). From variation in gas concentration inside the chamber, gas flux (F, mass of gas m− 2 d− 1) was computed using the equation:    DC V F¼ k ð1Þ Dt A

2.2. Measurement of gas fluxes

2.3. Soil sampling and assessment of properties

Measurement of gas fluxes was conducted on July 1, 2003 and May 5, 2004 at the hay site, and on July 29, 2003 and May 7, 2004 at the meadow site. During the measurement period, soil temperature (0–10 cm) ranged between 21.3 and 22.5 °C with only minor (ΔT b 0.8 °C) variation during the gas flux measurement period. Total rainfall recorded in Jackson, OH (∼ 100 km SW of study sites) was 134, 189 and 106 mm during the months of June 2003, July 2003 and April 2004, respectively. Long-term normal precipitation during these months are 95, 110 and 97 mm, respectively. At each study site, two 100-m long transects (east– west and north–south) were delineated with the north– south transect intersecting the east–west transect at its mid-point. Along each transect, there were 20 sampling points (every 5-m). Gas fluxes were measured using the static chamber method. Measurements took place between 11 AM and 2 PM when fluxes were expected to be maximum (Benasher et al., 1994). At each sampling point, ground elevation was recorded and, to facilitate installation of gas chambers, the above-ground vegetation was cut (but litter layer remained) in a 40 × 40 cm area. At least 2 days prior to sampling, a chamber (diameter: 15 cm; height above ground: 30 cm; depth of insertion into the ground: 5 cm) made of polyvinyl chloride (PVC) was installed at each sampling point. A lid, also made of PVC material, and fitted with a sampling port, was used to close the chamber. Air samples (∼10 ml) were taken from each chamber headspace at 0, 20 and 40 min and stored in pre-evacuated 7-ml vials fitted with butyl rubber septa. Details regarding the construction, ground insertion and operation of the gas chambers are available elsewhere

At each measurement point, duplicate soil cores (5 cm diam. by 5 cm deep) and a composite disturbed soil sample (0–10 cm) were taken (from vegetation-free area) and carried on ice to the laboratory for determination of soil physical and biochemical properties. Fresh soil (∼20 g moist) was extracted with 2 M KCl and the extract analyzed for NO3 and NH4 using a Quickchem autoanalyzer (Lachat Instruments, Milwaukee, WI, USA). Soil cores were first water-saturated and then equilibrated successively in a pressure plate extractor (Soil Moisture Equipment) at a matric potential of −33 kPa (field capacity). Upon cessation of water dripping out of the extractor, cores were removed and their weight recorded. The loss of water between saturation and −33 kPa was taken as a measure of macropore volume. In other words, macro-pore volume corresponds to the volume of soil pores that is water-free at field capacity. Then each core (at field capacity) was transferred into a Mason jar with the lid fitted with a sampling port. Jar was incubated for 48 h at 25 °C in an incubator and its headspace analyzed for CO2. The amount of CO2 produced during the incubation period was used as an index of labile organic substrate available for soil respiration. At the end of the incubation, soil cores were placed in an oven (105 °C, 72 h) and dry weight determined. Bulk density (ρb) was computed as the ratio of soil dry weight to core volume and discounting for gravel (coarse materials N 2 mm diam). Soil pH was determined with a pH electrode using a 2:1 water to soil suspension. Total carbon and total nitrogen (TN) was determined by dry combustion techniques using a VarioMax C-N analyzer (Elementar

where ΔC / Δt: rate of change in GHG concentration inside the chamber (mass GHG m− 3 air min− 1) obtained by linear regression, V: chamber volume (3.5 × 10− 3 m3), A: area circumscribed by the chamber (1.77 × 10− 2 m2), and k: time conversion factor (1440 min d− 1). A positive value of F corresponds to a net emission of gas from soil into the atmosphere. Conversely, a negative F value corresponds to a net transfer (uptake) of gas from the atmosphere into the soil.

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Americas, NJ). Inorganic C was obtained using the procedure of Bundy and Bremner (1972) with modifications involving gas chromatographic measurement of CO2 evolved from acid-decomposition of carbonates. Soil organic C (SOC) was computed as the difference between total and inorganic C. Soil texture was determined by the hydrometer method. All results are reported on the basis of dry soil. Gravimetric moisture content at time of sampling was determined by oven drying of moist soil samples at 105 °C for 48 h in an oven. During the May 2004 gas flux measurements, only soil moisture and inorganic N contents were determined. Minor variations in the other soil properties were expected in the 8-month period between the two sampling occasions. 2.4. Air samples analysis Gas samples were analyzed for CO2 and CH4 by gas chromatography (Shimadzu, GC 14A) using He (20 ml

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min− 1) as the carrier gas and an Hayesep DB column (300 cm long, 0.3 cm id) connected in series to a thermal conductivity detector (100 °C, for CO2 detection) and a flame ionization detector (FID at 150 °C, for CH4 detection). Hydrogen and hydrocarbon-free air were used as flame gases for the FID. For analysis of air samples for N2O the GC configuration was modified and the gas chromatograph was fitted with a pre-column (100 cm × 0.3 cm id) and an analytical column (300 cm × 0.3 cm). Both columns were packed with 80/ 100 mesh Porapak Q and linked through a timeprogrammed 8-port valve (Valco, Texas). With this configuration, most O2 was vented out as it elutes the precolumn while N2O was directed to the electron capture detector (ECD) via the analytical column. Argon– methane (95:5) mixture (30 ml min− 1) was used as the carrier gas. Oven and detector temperatures were 70 and 350 °C, respectively. Standard gases obtained from Alltech (Deerfield, IL) were used for instrument calibration.

Fig. 1. Mean elevation (above sea level) at the study sites.

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Table 1 Summary statistics for the biochemical soil properties (0–10 cm) measured at the study sites (40 sampling points per site) Variable

pH NH+4 (mg N kg− 1 soil)b NH+4 (mg N kg− 1 soil)c NO+3 (mg N kg− 1 soil)b NO+3 (mg N kg− 1 soil)c Organic C (g C kg− 1 soil) Total N (g N kg− 1 soil) Respiration at field capacity (mg CO2–C− 1 kg− 1 d) CO2 flux (g C m− 2 d− 1)b CO2 flux (g C m− 2 d− 1)c CH4 flux (g C m− 2 d− 1)b CH4 flux (g C m− 2 d− 1)c N2O flux (g C m− 2 d− 1)b N2O flux (g C m− 2 d− 1)c a b c d

Hay field

Meadow a

Mean

Median

CV %

Skewness

Outliers

Mean

Median

CV %

Skewness

Outliers

7.74 0.14 0.22 4.20 2.84 3.26Ad 0.30A 18.8

7.80 0.12 0.15 3.69 2.77 3.10 0.29 17.1

4 78 94 55 61 19 21 36

− 0.92 1.25 2.64 1.97 1.41 0.5 0.83 0.64

4 3 3 3 4 4 4 1

(8.38) (0.32) (0.45) (7.21) (4.87) (4.21) (0.39) (32.9)

7.69 0.17 0.28 3.71 2.73 1.86B 0.16B 19.5

7.77 0.09 0.18 3.32 2.63 1.71 0.16 17.7

7 155 107 81 59 33 27 46

−0.52 4.71 4.05 1.76 0.79 1.37 0.63 1.13

0 (8.9) 6 (0.27) 5 (0.49) 2 (9.41) 1 (6.48) 5 (2.68) 3 (0.25) 4 (31.9)

3.26A 5.10A 0.47A 0.39A 0.48 0.36

3.01 5.01 0.44 0.29 0.28 0.24

32 31 51 36 98 20

0.93 0.12 0.50 0.14 2.21 2.84

4 3 4 1 5 5

(4.56) (7.75) (5.20) (2.98) (0.98) (0.82)

2.73B 3.31B −1.02B −0.48B 0.75 0.42

2.45 3.16 − 0.87 − 0.58 0.50 0.26

45 29 96 76 98 197

1.69 0.30 −0.75 −0.14 1.95 3.30

3 (4.70) 1 (5.24) 3 (2.60) 1 (1.21) 3 (1.75) 6 (0.92)

Number of outliers in the data set. Outliers are greater than the values (median + 1.5 inter-quartile range) listed in parentheses (Ott, 1984). Samples collected in July 2003. samples collected in May 2004. Within a row, means followed by different letters are statistically different at P b 0.05.

2.5. Data analysis The data were analyzed using descriptive statistics, principal component analysis, and regression models to assess the variability of soil properties and detect relationships between soil properties and gas fluxes. Principal component analysis of the soil properties was conducted using the statistical software Systat11 (SPSS, 2004). The Variomax rotation factor was selected for the analysis. The gas flux data were grouped by landscape positions (summit, backslope and footslope), and analysis of variance (ANOVA) was performed to assess the effect of landscape position. ANOVA and regression analysis were conducted with the SAS (SAS Institute, 2001)

software using Proc GLM (general linear modeling) and Proc REG, respectively. Statistical significance was determined at the 95% confidence level. 3. Results 3.1. Landscape patterns The two sites exhibited differences in sloping patterns (Fig. 1) and these terrain attributes could impact soil moisture, nutrient availability and trace gas production. The north–south transect at the hay field, and the east–west transect at the meadow site were located on a generally uniform area with slope ranging

Table 2 Summary statistics for the physical soil properties (0–10 cm) measured at the study sites (40 sampling points per site) Variable Bulk density (g soil cm− 3) Macropore (cm− 3 void cm− 3 soil) Clay (g kg− 1 soil) Silt (g kg− 1 soil) Soil moisture (g H2O g− 1 soil)b Soil moisture (g H2O g− 1 soil)d a b c d

Hay field

Meadow

Mean

Median

CV %

Skewness

Outliers

1.33 0.29 388 529 0.23Bc 0.24

1.33 0.29 392 530 0.24 0.24

13 15 3 4 10 25

−0.01 −0.99 −1.4 −0.8 −0.17 0.62

3 0 0 1 0 1

a

(1.58) (0.37) (411) (549) (0.28) (0.38)

Mean

Median

CV %

Skewness

Outlier

1.26 0.19 352 571 0.28A 0.28

1.27 0.19 351 572 0.29 0.28

13 20 10 8 15 23

− 0.13 0.71 − 0.01 − 0.4 0.28 0.01

1 (1.60) 6 (0.24) 0 (451) 0 (689) 2 (0.37) 0 (0.45)

Number of outliers in the data set. Outliers are greater than the values (median + 1.5 inter-quartile range) listed in parentheses (Ott, 1984). Samples collected in July 2003. Within a row, means followed by different letters are statistically different at P b 0.05. Samples collected in May 2004.

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Table 3 Results of the principal component analysis for soil properties at the study sites Hay field

Meadow

PC1

PC2

PC3

Physical properties Soil moisture Bulk density Macropore volume

0.67 − 0.50 0.00

− 0.22 − 0.10 0.21

0.27 − 0.62 0.77

Bio-chemical properties pH Electrical conductivity Organic carbon Total nitrogen Inorganic carbon Extractable NH+4 Extractable NO−3 Basal soil respiration Eigenvalue % of total variance

− 0.12 − 0.10 0.90 0.90 0.05 0.10 − 0.02 0.49 2.59 23.5

0.84 0.26 0.32 − 0.30 0.90 − 0.19 0.02 0.17 1.94 17.7

0.26 0.33 0.04 − 0.05 0.13 0.24 − 0.34 0.77 2.01 18.3

PC4

PC1

PC2

PC3

PC4

0.37 0.12 0.22

0.40 − 0.10 − 0.02

0.08 − 0.17 0.48

0.58 −0.77 0.43

0.11 − 0.18 0.08

0.03 0.66 0.02 0.01 0.02 0.70 0.61 0.23 1.55 14.1

− 0.20 0.57 0.43 0.64 − 0.15 0.77 0.83 − 0.26 2.50 22.8

0.87 0.57 − 0.21 − 0.50 0.35 0.03 − 0.21 − 0.38 1.94 17.7

−0.07 0.25 0.21 0.17 0.05 −0.06 0.07 0.76 1.85 16.8

0.04 0.20 0.83 0.41 0.87 − 0.13 0.19 − 0.08 1.76 15.9

Variomax rotated component loadings, eigenvalues and percentage of variance explained by each principal component are listed.

between 2% and 4% at the hay field, and 4–6% at the meadow site. The east–west transect at the hay field encompassed a prominent backslope (slope 6%) and a

broad summit in the middle. At the meadow site, the north–south transect becomes strongly concave in the lower landscape positions (Fig. 1).

Fig. 2. Carbon dioxide fluxes along the east–west (open circles) and north–south (closed circles) transects at the study sites.

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3.2. Variability of soil physical and chemical properties Soil organic C (SOC) was significantly (P b 0.003) higher (1.5 times) at the hay field than at the meadow site (Table 1) suggesting no negative impact of biomass removal as hay on SOC storage. The amount of biomass exported as hay may have been compensated by increased biomass production at this site due to N fertilization. However, N fertilization appears to have no effect on labile C as results of the short-term (48 h) soil respiration assay (Table 1) showed no significant difference (P b 0.73) between the fertilized hay and non-fertilized meadow sites. At both study sites, surface soil (0–10 cm) texture was silty clay loam with clay content between 265 and 433 mg kg− 1 soil (Table 2). The meadow soil was wetter (0.28 vs. 0.24 g H2O g− 1 soil) than the hay field soil, but difference was significant (P b 0.01) only at the second sampling event in May 2004 (Table 2). Macropore volume at the hay site (range: 0.17–0.35, mean: 0.29 cm3 air cm− 3 soil) was higher than at the meadow (range: 0.13–0.29, mean: 0.20 cm3 air cm− 3 soil). Macropore

volume was inversely related to bulk density, but the relationship was only significant (r2 = 0.19, P b 0.005) at the hay site. Based of the coefficient of skewness and the coefficient of variation (CV), variability in soil properties was generally greater at the meadow than at the hay field. The soil physical properties showed a lesser degree of variability (CV b 25%) compared to mineral N and N2O fluxes which exhibited the greatest variability (CV up to 197%). Principal component analysis (PCA) of the soil properties identified 4 principal components with eigenvalue N1, and these components explained 72–74% of the total variance (Table 3). The first component (PC1) showed high loadings for SOC, TN and respiration at the hay field, and for mineral N at the meadow site. The reverse was observed for the fourth component (PC 4). At both study sites, the second component (PC2) loaded heavily on soil chemical properties (pH, EC and inorganic C), while the third component (PC3) showed high loadings for soil physical properties (macropore volume and bulk density).

Fig. 3. Methane fluxes along the east–west (open circles) and north–south (closed circles) transects at the study sites.

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3.3. Trace gases fluxes Measured gas fluxes exhibited considerable variability (Figs. 2–4) but no significant effect of landscape position on gas fluxes was found. The mean daily flux of CO2 was 1.2–1.5 times greater (3.3–5.5 vs. 2.7–3.3 g CO2–C m− 2 d− 1) at the hay field than at the meadow site (Table 1 and Fig. 2). Nitrous oxide fluxes (Table 1 and Fig. 4) exhibited the most variability. Coefficients of variation of the mean N2O emission at the hay (0.42 mg N2O–N m− 2 d− 1) and meadow (0.58 mg N2O–N m− 2 d− 1) were 135% and 145%, respectively. Sampling points that could be considered as “hot-spots” were located next to sampling points exhibiting near zero emission (Fig. 4). The median (0.26) and inter-quartile range (IQR: 0.44) of N2O fluxes at the meadow site for the two sampling occasions indicate that N2O fluxes N1 mg N2O–N m− 2 d− 1 (median + 1.5 IQR) should be considered as outliers (Ott, 1984). More than half (70%) of the sampling points with N2O emission N 1 mg N2O–N m− 2 d− 1 were located in the first 40 m of the north–south transect — a generally flat area (slope b 2%; Fig. 1) with several micro-depressions.

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Sampling points exhibiting positive as well as negative CH4 fluxes were found at both study sites. However, both the frequency of occurrence and the emission strength of the CH4-emitting points were much greater at the hay site than at the meadow (Fig. 3). CH4 fluxes averaged 0.46 and 0.39 mg CH4–C m− 2 d− 1 at the hay site and − 1.02 and − 0.47 mg CH4–C m− 2 d− 1 at the meadow site during the July 2003 (P b 0.02) and May 2004 sampling (P b 0.01), respectively. Although no significant effect of landscape position on CH4 fluxes was found, there were a few noteworthy patterns in the July 2003 CH4 flux data. At the hay field, the highest CH4 uptake rate was recorded at the uppermost landscape position (side of convex slope, 45 m along the north–south transect). Conversely, at the meadow site (north–south transect), net CH4 emissions (albeit small positive fluxes) were recorded in the downslope position (where slope becomes concave). Few statistically significant relationships between gas fluxes and soil properties were found with the May 2004 data. However, analysis of the July 2003 data showed statistically (hay: r2 = 0.27, P b 0.001; meadow:

Fig. 4. Nitrous oxide fluxes along the east–west (open circles) and north–south (closed circles) transects at the study sites.

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Table 4 Regression models for methane flux at the meadow site Predicted Variable variable CH4 flux (July 2003) CH4 flux (May 2004)

Coefficient Standard error of coefficient

Macropore − 29.04 θ 30.94 Constant − 4.14 Macropore − 8.49 θ 2.76 Constant 0.41

4.82 4.31 1.45 2.75 1.63 0.65

Probability Model R2 b0.0001 b0.0001 0.0073 0.004 0.091 0.539

0.68

0.23

Units: CH4 flux (F): mg CH4–C m− 2 d− 1; macropore: cm3 void space cm-3 soil; soil moisture (θ): g water g− 1 soil.

r2 = 0.18, P b 0.01) significant relationships between CO2 and CH4 fluxes. The stronger relationship at the hay field is consistent with the observation that, at this site (Figs. 2 and 3, north–south transect), the sampling points with the highest CO2 fluxes corresponded to points with strong rates of CH4 emission. Significant relationships were also found between CO2 flux and labile C (r2 = 0.26, P b 0.001) at the hay site, and between CO2 flux and mineral N (r2 = 0.14, P b 0.02) at the meadow site. Emission of N2O was also related to labile C at the hay field (r2: 0.24, P b 0.001) and to mineral N at the meadow site (r2: 0.14, P b 0.01). With the exception of macropore volume and soil moisture, most of the soil properties considered in this study were weakly correlated with CH4 flux. At the meadow site, macropore volume (x), either alone or in combination with soil moisture, was the soil property most strongly correlated with CH4 fluxes (y). Inverse and significant relationships between these two variables were

found (July 2003: y = −24.8x + 3.8, r2 = 0.24, P b 0.002; May 2004: y = −7.7x + 1.0, r2 = 0.17, P b 0.008, n = 40). The strength of these relationships (R2: 0.68 and 0.23, respectively) improved when the soil moisture data were added to the model (Table 4). Regardless of landscape position, no significant relationship between CH4 flux (y) and macropore volume was found at the hay site. Relationships between CH4 fluxes and labile C were detected when sampling points were grouped by macropore volume. Relationships between the two variables were positive for macropore volume b 0.2 and negative for macropore volume N 0.3 cm3 void cm− 3 soil (Fig. 5). 4. Discussion 4.1. Nutrient availability, landscape features and variability of gas fluxes In this study, measurements of gas fluxes occurred 2 and 11 months after urea application to the hay field. The pool of mineral N was very low with no difference between the fertilized hay and the meadow (Table 1). Previous studies (Jacinthe and Dick, 1997; Röver et al., 1999; Jacinthe and Lal, 2003) have shown that N fertilization generally results in short-lived bursts of N2O emission during the first 4 weeks following application of NH4-based fertilizer to soils. Thus, the N2O fluxes recorded at the hay site were probably not affected by fertilization but rather reflect rainfall variation and soil attributes generally associated with N2O production. The July 2003 measurement was made during a relatively wet period with above normal

Fig. 5. Relationships between methane fluxes and soil respiration measured with intact soil cores maintained at field capacity.

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precipitation (June 2003 rainfall: 134 mm vs. normal: 95 mm). As a result, the average daily flux of N2O (0.48–0.75 mg N2O–N mg d− 1, Table 1) was 1.3–1.7 times higher compared to the average N2O flux recorded in May 2004 when precipitation was near normal (actual rainfall: 106 mm; normal: 97 mm). Although relationships were not very strong (r2: 0.24, P b 0.01), regression analysis consistently showed that both N2O and CO2 fluxes were significantly related to labile C at the hay field and to mineral N at the meadow. The results of the PCA (Table 3) also indicated distinct groupings of the soil properties at the 2 sites: PC1 was associated with moisture, total and labile C at the hay field, and with mineral N factor at the meadow. Association of soil properties with PC4 was just the opposite. These soil properties are known drivers of trace gas dynamics in soils and the data of Yanai et al. (2003) further demonstrated a strong dependency of N2O flux on the organic matter component of a PCA (component showing high loadings for total C, N, moisture and temperature). In light of these results, it is proposed that the biological processes leading to CO2 and N2O production are linked probably through decomposition of labile C, but that the intensity of these processes are controlled by different soil attributes at each site. 4.2. Methane fluxes and soil macro-porosity Relationships between labile C and CH4 fluxes were also found to vary with macropore volume. Relationship (Fig. 5) was (i) positive at low macropore volume (b0.2 cm3 void cm− 3 soil) and negative at the high (N0.3 cm3 void cm− 3 soil) macropore volume. This variation in the relationship between these 2 variables suggests that when mineralization of labile C occurs under conditions of restricted air exchange hypoxic soil condition is induced (resulting in more positive CH4 fluxes), whereas when air exchange is not restricted, the labile C pool could sustain the activity of methanotrophs (hence negative fluxes or CH4 uptake). This interpretation is consistent with previous studies (Goldman et al., 1995; Jacinthe and Lal, 2005) documenting positive relationships between mineralizable C and CH4 oxidation capacity of soils. The finding that CH4 fluxes were inversely related to macropore volume is significant. That is, CH4 uptake (more negative fluxes) was greater at sampling points where gas transfer was not impeded by soil physical condition. Consistent with reports from other ecosystems (Ball et al., 1997; Sitaula et al., 2000; Ding et al., 2004; Teepe et al., 2004), this finding highlights the

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impact of soil compaction (resulting in high soil bulk density, diminished macropore volume, restricted water movement and gas exchange) on CH4 oxidation in reclaimed mined soils. The results indicate that the meadow management is favorable to CH4 uptake but that the process may have been limited by soil–atmosphere gas transfer. Although the presence of a litter layer could be a contributing factor, restriction in gas exchange at this site probably results from soil compaction at the time of topsoil application more than 20 years ago. Therefore, techniques must be developed to minimize compaction during soil reclamation. To limit the impact of compaction and improve land reclamation success, several techniques have been proposed including subsoiling and deep ripping (Chong and Cowsert, 1997; Rokich et al., 2001) and loose tipping (Moffat and Bending, 2000). While the first two techniques seek to alleviate compaction, loose tipping is a topsoil application technique developed with a goal of avoiding compaction altogether (Moffat and Bending, 2000). With loose tipping, topsoil is spread in consecutive strips using an excavator (or other appropriate machinery) that works exclusively from the overburden surface, but never travels over the applied topsoil. These techniques have, for the most part, been evaluated for their impact on soil water movement, root development, tree survival and growth. The information presented in this paper suggests that compaction alleviation and avoidance techniques may also improve the capacity of reclaimed lands to act as CH4 sinks. 5. Conclusions The spatial variability of soil properties and trace gas (CO2, CH4 and N2O) fluxes was studied along transects at two grassland sites (a N-fertilized hay field and nonfertilized meadow) established more than 20 years ago on reclaimed coal-mine land. Of the soil properties evaluated in the study, mineral N and gas fluxes were the most variable, and as hypothesized, the meadow site exhibited greater variability than the hay field for most properties. While no significant effect of landscape position (summit, backslope and toeslope) on gas fluxes was detected, marked variations in N2O fluxes were observed at sampling points located in micro-depressions at the meadow site. Significant relationships were found between CO2 flux and labile C at the hay site and between N2O fluxes and mineral N pool at the meadow. At the meadow site, CH4 flux was positively related to soil moisture and inversely related to macropore volume. In other words, an increase in macro-porosity results in a decrease in CH4 emission. This finding

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suggests that reclamation techniques leading to soil compaction and loss of macropore volume could have long-lasting effect on the ability of reclaimed mine lands to mitigate CH4 emission. Acknowledgments This research was funded through a grant of the Ohio Coal Development Office to the Ohio State University Research Foundation. The authors acknowledge the logistical support provided by Gary Kasper and Brian Cox of American Electric Power in McConnelsville, OH. Field and laboratory assistance of Joseph Philips and Yogendra Raut is gratefully acknowledged. References Ambus, P., Christensen, S., 1994. Measurement of N2O emission from a fertilized grassland — an analysis of spatial variability. J. Geophys. Res., Atmos. 99, 16549–16555. Ball, B.C., Dobbie, K.E., Parker, J.P., Smith, K.A., 1997. The influence of gas transport and porosity on methane oxidation in soils. J. Geophys. Res., Atmos. 102, 23301–23308. Benasher, J., Cardon, G.E., Peters, D., Rolston, D.E., Biggar, J.W., Phene, C.J., Ephrath, J.E., 1994. Determining root activity distribution by measuring surface carbon dioxide fluxes. Soil Sci. Soc. Am. J. 58, 926–930. Bendfeldt, E.S., Burger, J.A., Daniels, W.L., 2001. Quality of amended mine soils after sixteen years. Soil Sci. Soc. Am. J. 65, 1736–1744. Bundy, L.G., Bremner, J.M., 1972. A simple titrimetric method for determination of inorganic carbon in soils. Soil Sci. Soc. Am. Proc. 36, 273–275. Chong, S.K., Cowsert, P.T., 1997. Infiltration in reclaimed mined land ameliorated with deep tillage treatments. Soil Tillage Res. 44, 255–264. Ding, W.X., Cai, Z.C., Tsuruta, H., 2004. Cultivation, nitrogen fertilization, and set-aside effects on methane uptake in a drained marsh soil in Northeast China. Glob. Chang. Biol. 10, 1801–1809. Goldman, M.B., Groffman, P.M., Pouyat, R.V., McDonnel, M.J., Pickett, S.T.A., 1995. CH4 uptake and N availability in forest soils along an urban to rural gradient. Soil Biol. Biochem. 27, 281–286. Hangen, E., Gerke, H.H., Schaaf, W., Huttl, R.F., 2004. Flow path visualization in a lignitic mine soil using iodine–starch staining. Geoderma 120, 121–135. Hossner, L.R., Shahandeh, H., Birkhead, J.A., 1997. The impact of acid forming materials on plant growth on reclaimed minesoil. J. Soil Water Conserv. 52, 118–125.

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