Gypsum and Coal-bed Methane Water Modify Growth Media Properties, Nutrient Uptake, and Essential Oil Profile of Lemongrass and Palmarosa

Zheljazkov, Valtcho D.; Astatkie, Tess; Norton, Urszula; Jeliazkova, Ekaterina A.

doi:10.3390/agronomy9060282

Open AccessArticle

Gypsum and Coal-bed Methane Water Modify Growth Media Properties, Nutrient Uptake, and Essential Oil Profile of Lemongrass and Palmarosa

by

Valtcho D. Zheljazkov

^1,*

,

Tess Astatkie

²

,

Urszula Norton

³ and

Ekaterina A. Jeliazkova

⁴

¹

Department of Crop and Soil Science, 431A Crop Science Building, 3050 SW Campus Way, Oregon State University, Corvallis, OR 97331, USA

²

Faculty of Agriculture, Dalhousie University, 50 Pictou Road, P.O. Box 550, Truro, NS B2N 5E3, Canada

³

Department of Plant Sciences, University of Wyoming, 4014 Agriculture Building, 1000 E. University Ave., Laramie, WY 82071, USA

⁴

Central Oregon Agriculture Research and Extension Center, 850 NW Dogwood Lane, Madras, OR 97741, USA

^*

Author to whom correspondence should be addressed.

Agronomy 2019, 9(6), 282; https://doi.org/10.3390/agronomy9060282

Submission received: 13 March 2019 / Revised: 28 May 2019 / Accepted: 30 May 2019 / Published: 31 May 2019

(This article belongs to the Section Soil and Plant Nutrition)

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Abstract

:

Coal-bed methane (CBM), an important energy source, coproduces a vast amount of saline-sodic wastewater, CBM water (CBMW), with environmental and economic disposal issues. This research evaluated under a greenhouse production system the influence of gypsum (CaSO₄·2H₂O) and CBMW on yields, essential oil (EO) content and composition in lemongrass (Cymbopogon flexuosus) and palmarosa (C. martinii), and on growth medium pH and available nutrients. CBMW treatments had higher pH than tap water treatment. Gypsum reduced pH in all CBMW treatments but did not affect pH in tap water treatment. While CBMW may increase the available Cu and Fe in growth medium, the application of gypsum may negate this effect. CBMW significantly increased growth medium Na. Gypsum increased growth medium S, and CBMW increased S in the high gypsum treatments. Palmarosa height, fresh weight, geranyl acetate, and isoneral in lemongrass EO were reduced, while geraniol in palmarosa EO increased with CBMW relative to tap water. In distillation waste plant tissue, CBMW increased Na in lemongrass and palmarosa by almost eight times; increased total P, S, and Mn in palmarosa; and reduced total N, S, Ca, and Mg in lemongrass and Ca in palmarosa, relative to tap water. This study demonstrated that CBMW may be used for greenhouse production of high-value crops, but it may affect the yields and oil content of some crops and growth medium characteristics.

Keywords:

Cymbopogon flexuosus; Cymbopogon martinii; essential oil composition; greenhouse; oil content; waste water

1. Introduction

In the last few decades, coal-bed methane (CBM) became an important energy source for many countries. In the United States alone, the production of CBM in 2014 was around 41.51 billion m³ of CBM gas compared to 2.5 billion m³ in 1990 [1], making it the largest producer (28.6%) of CBM in the world. About one-third of this amount was produced in Wyoming [1].

Coal-bed methane occurs naturally in coal seams and is compressed there by water above it. To release the pressure and pump out the CBM, the water in the seams is pumped out first [2]. This co-produced water is referred to as coal-bed methane water (CBMW) and is considered wastewater by the U.S. EPA. For example, the Powder River Basin in Wyoming and Montana contains the largest coal reserve in the United States [3,4] and there has been significant CBM production in this basin [4,5]. The U.S. Energy Information Administration reported that in 2010, Wyoming produced 16 billion m³ of CBM of the total U.S. output of 53.4 billon m³. However, due to price fluctuations, in 2015 Wyoming production of CBM was 5.86 billion m³, which is still significant.

Due to the relatively low quality of CBMW (contaminated with salts, minerals, methane, or heavy metals, e.g., Table 1) [6], the current practice is to re-inject the water back into the coal seams and pump it into creeks and rivers, or into specially designed evaporation ponds [2]. Each one of these practices has environmental impacts—they contaminate surface and ground water; soil, plant, and animal life; and are costly. Also, once the wells are functional, CBMW is pumped out every day, year-round (including winter), which presents additional challenges for its disposal. Unlike for other oil production, many wells per relatively small area may be required to take the CBMW out and make it economically feasible, due to the low pressure of CBMW in the coal seams. Hence, there is usually a high density of wells in CBM fields, further affecting the area biodiversity.

Wyoming and some other Western states are mostly arid, and water is an invaluable commodity; some of these regions receive less than 160 mm of precipitation per year. There is anecdotal evidence that some ranchers and farmers have been using CBMW for irrigation of forages and rangeland. However, the effects of CBMW on crops are largely unknown.

Previous research on CBMW in Wyoming found increased soil salinity and sodicity at places with CBMW application [7,8]. A field study using various amounts of CBMW and three soil treatments found that irrigation with CBMW alone elevated Na⁺ concentration in the soil profile, and electrical conductivity (EC) and sodium absorption ratio (SAR) increased in the A and Bt1 horizons [7]. The addition of gypsum and S resulted in a smaller increase of soil surface SAR [7]. In another field study, pre-treatment of CBMW in combination with elemental S or gypsum amendments resulted in lower soil pH; however, this treatment still resulted in higher concentrations of unspecified salts and Na in the soil surface layer [9].

Information on how CBMW affects plant secondary metabolites is limited, despite some recent reports on CBMW’s effect on Artemisia annua [10], spearmint (Mentha spicata L.), Japanese cornmint (Mentha canadensis L.), lemongrass (Cymbopogon flexuosus [Nees ex Steud.] J.F. Watson]), common wormwood (Artemisia vulgaris L.) [11], dill, Anethum graveolens [12], peppermint (Mentha × piperita), and spearmint [13]. Overall, the effect of CBMW on plant secondary metabolites varied depending on the species, e.g., CBMW did not affect the major oil constituents in peppermint and spearmint; however, at a high application rate, CBMW increased total phenols and total flavonoids in spearmint but not in peppermint. Furthermore, CBMW also affected oil content in peppermint but not in spearmint [13]. Another unknown is whether CBMW could be used for irrigation of high-value greenhouse crops grown on typical greenhouse medium. Greenhouse medium and natural soils differ substantially in their chemical and physical properties; hence, the effects of CBMW on these two media could be significantly different. Given the limited water resources of Western states, the wide availability of CBMW may offer an inexpensive water supply to produce high-value specialty crops under greenhouse conditions if the sodic-saline nature of the water would not significantly alter the growth medium’s chemical and physical characteristics or plant produce quality. Furthermore, the agricultural use of CBMW in greenhouse production systems would prevent it from entering the surface and groundwater pools and may lessen its environmental impact.

The hypothesis of this study was that CBMW may affect greenhouse growth medium properties, plant growth, and essential oil (EO) accumulation and chemical profile. The specific objectives of the study were to assess how various amounts of CBMW and gypsum would affect plant and oil yields, plant secondary metabolites of lemongrass and palmarosa, and also growth medium chemical and biological properties. The two model plants are high-value crops with accelerated growth and large biomass production, and significant nutrient and water requirements. These crops are used in cooking and in traditional folk medicine preparations. The two plants are also grown commercially for EO production, and their oils have numerous applications in various industries and consumer products.

2. Materials and Methods

2.1. Plant Material and Growing Conditions

Two separate controlled-environment greenhouse experiments were carried out using the two high-value industrial crops: lemongrass (Cymbopogon flexuosus [Nees ex Steud.] Will. Watson) and palmarosa (C. martinii [Roxb.] Wats.), which are commercially grown as EO crops. Certified seeds of lemongrass and palmarosa were purchased from Richters Herbs (Goodwood, ON, Canada). Transplants were produced in a controlled-environment greenhouse with a day/night temperature regime of 22–25 °C and 18–20 °C, respectively, which lasted 50 days. Lemongrass and palmarosa seeds were planted in plastic cells (2–4 seeds/cell) filled with greenhouse growth medium (Sunshine Mix 1, Sun Gro Horticulture Canada Ltd, Seba Beach, AB, Canada). When the plants reached approximately 12 cm height, they were transplanted into 11.36 L plastic containers. Each container contained 3.3 kg of growth medium (Sunshine Mix 1). The experiments with lemongrass and palmarosa were conducted in the same greenhouse; the day/night temperature regime was 22–25 °C and 18–20 °C, respectively.

Nitrogen (N), phosphorus (P), and potassium (K) were provided prior to transplanting with a slow-release fertilizer (Osmocote Plus 15N-9P-12K; Scotts-Sierra Horticultural Products Co., Marysville, OH, USA) estimated to provide N comparable to 150 kg ha⁻¹ under field conditions. The fertilizer was applied to each container and mixed with the growth medium prior to transplanting. After transplanting, water-soluble fertilizer WSF (greenhouse grade NPK fertilizer containing 20N-8.8P-16.6K, Scotts-Sierra Horticultural Products Co., Marysville, OH, USA) providing 100 mg/kg of N was applied every 2 weeks to each container. We did not observe any pests, diseases or symptoms of nutrient deficiencies on lemongrass and palmarosa; therefore, no pesticides or additional nutrients were applied.

There were two factors in both studies: (1) two water sources: coal-bed methane water (CBMW) and tap (drinking) water, and (2) gypsum (CaSO₄·2H₂O) at four levels: calculated to provide 0, 500, 1500, and 4500 kg ha⁻¹ under field conditions. All treatment combinations were run for three replications. The rates of gypsum (CaSO₄·2H₂O—97%; Ca—22.5%; S—18%; Diamond K Gypsum, Inc. Richfield, Utah) were applied individually to each container prior to transplanting and thoroughly mixed with the growth medium. The water treatments were initiated immediately after transplanting; each container received 600 mL water/day. The water application rates were increased to 1200 mL/day once the plants reached approximately 40–50 cm height to meet the increased evapotranspiration needs.

2.2. Harvest and EO Extraction

The lemongrass and palmarosa plants were grown for 60 days in the large containers until harvesting. Lemongrass was in the vegetative stage; this species of lemongrass does not reach flowering under the environmental conditions of northern Wyoming, or even in Mississippi [14]. The palmarosa plants were harvested at flowering. At this stage, the EO content is high and the EO composition is desirable. Both lemongrass and palmarosa plants were cut at 3–4 cm above the growth medium level and processed immediately for distillation. Both lemongrass and palmarosa were distilled fresh. The sample size for distillation was 500 g and included all aboveground plant parts, chopped to 3–4 cm (stems and leaves for lemongrass, leaves, stems, and inflorescences for palmarosa). The EO was extracted for 60 min using steam distillation in 2-L steam distillation units [14,15]. The EO and accumulated water in the separator were transferred into glass vials and put in a freezer. The EOs were separated from the frozen water and the resulting oil samples were measured on an analytical scale, then stored in the freezer until analyzed using gas chromatography. The EO content (yield) was estimated as the gram of oil per gram of fresh biomass for both lemongrass and palmarosa.

2.3. Growth Medium and Distillation Waste Plant Tissue Analyses of Lemongrass and Palmarosa

The growth medium samples were taken at harvest in each of the experiments and analyzed as described in Zheljazkov et al. [13]. Briefly, six core samples were taken from each container immediately after harvest using a soil probe, composited, and sent immediately to the Soil Testing Laboratory of the American Agriculture Laboratory Inc. (https://www.amaglab.com/) (McCook, NE, USA) for the analyses of plant available nutrients, EC, and pH in growth medium. The concentration of plant-available nutrients in the growth medium was determined by extracting the samples with Mehlich 3 [16].

The growth medium samples were analyzed following the procedures and methods in Brown [17]. The growth medium pH was measured in soil slurry potentiometrically using an electron pH meter [18]. The growth medium EC was measured in a 1:1 soil/water dilution slurry [19]. Growth medium organic matter was determined using the loss of weight on ignition method [20]. Potassium was determined by the NCR-13–exchangeable K procedure [21]; Ca and Mg were determined by atomic absorption; Na was determined by emission [21]; and cation exchange capacity was calculated using those values. The distillation waste plant tissue was collected at the end of each distillation, dried at 65 °C for 48 to 72 hours until dry, then sent to the American Agricultural Laboratory, Inc. (McCook, NE, USA) for nutrient analyses using common procedures for total elemental concentration in plant tissue.

2.4. Microbial Biomass and N Determination

Microbial biomass C (MBC) and N (MBN) were determined using 48-h fumigation followed by extraction with a 0.5 molar potassium sulfate (K₂SO₄). Microbial BC and MBN were calculated as a the difference in dissolved organic C and N concentrations between fumigated and non-fumigated soils and multiplied by a fumigation coefficient of 0.35.

2.5. Gas Chromatography-Mass Spectrophotometer (GC-MS) Analysis of the EO

The lemongrass and palmarosa EOs from all treatments were sent to the commercial company Citrus and Allied Essences Ltd. which specialize in essential oil analyses. The samples were analyzed by using GC-MS methods and the conditions for analysis were identical to those previously described for lemongrass [14].

2.6. Quantitative Analysis

Commercial standards (R)-(+)-limonene and (+)-δ-cadinene were purchased from Fluka (Switzerland); citral, (−)-trans-caryophyllene, and caryophyllene oxide were purchased from Sigma-Aldrich (St. Louis, MO). With five concentration points, an external standard least squares regression for quantification was used. Each specific analyte was used to formulate a separate calibration curve using MS total ion chromatogram (TIC) data. Linearity was imposed by using response factors and regression coefficients independently. Response factors were calculated using the equation RF = DR/C, where DR was the detector response in peak area (PA) and C was the analyte concentration. Since citral was available only as a mixture of E and Z isomers, the TIC area from both isomers was added together to generate the response factor used for the two individual isomers that were quantified separately.

The chromatograms of each of the EO samples from the field experiments were compared to the chromatograms from standards. Target analytes were confirmed by retention time and mass spectra. Confirmed integrated peaks were used to determine the percentage of each chemical constituent in the EO itself. The RF of the target chemical constituent was used to determine the percentage of oil for each sample using the equation PA/RF/C × 100 = % analyte in the oil on a wt/wt basis.

2.7. Statistical Analyses

For lemongrass, the effect of water (CBM, tap) and gypsum (0, 500, 1500, 4500 kg ha⁻¹) on height, fresh weight, oil yield, and oil content; oil composition (6-Methyl-5-Hepten-2-one, Beta-Caryophyllene, Geranial, Geraniol, Geranyl Acetate, Isogeranial, Isoneral, and Neral); soil microbial indices (dissolved organic carbon [DOC] and dissolved organic nitrogen [DON]); available nutrients in the growth medium (pH, Soluble salts, P, Nitrate, B, Ca, Cu, Fe, K, Mg, Mn, Na, S, and Zn); and nutrient content in the plant tissue from distillation waste [P, N, B, Ca, Cu, Fe, K, Mg, Mn, Na, S, and Zn) was determined by completing Analysis of Variance (ANOVA) of a 2 × 4 factorial design with three replications.

For palmarosa, the effect of Water (CBM, Tap) and Gypsum (0, 500, 1500, 4500 kg ha⁻¹) on Height, Fresh weight, Oil yield, Oil content; oil composition (Beta-Caryophyllene, Geraniol, Geranyl Acetate, Linalool, and Trans-Ocimene); soil DOC, and DON; available nutrients in the growth medium (pH, Soluble salts, P, Nitrate, B, Ca, Cu, Fe, K, Mg, Mn, Na, S, and Zn); and nutrient content in the plant tissue from distillation waste (P, N, B, Ca, Cu, Fe, K, Mg, Mn, Na, S, and Zn) was determined by completing Analysis of Variance (ANOVA) of a 2 × 4 factorial design with three replications.

For each significant (p-value < 0.05) or marginally significant (0.05 < p-value < 0.1) effect, further multiple means comparison was completed by comparing the least squares means of the corresponding treatment combinations using the lsmeans statement of Proc GLM with pdiff option to produce p-values for all pairwise differences. Letter groupings were generated using a 5% level of significance. For each response, the validity of model assumptions on the error terms was verified by examining the residuals as described in Montgomery [22]. The statistical analysis was completed using the GLM Procedure of SAS [23].

3. Results

As expected, the CBMW and tap water had very dissimilar properties such as pH, EC, and in some cases, very different concentrations of various elements (Table 1).

Iron (Fe), Cu, and Mn were not quantified in the CBMW as their concentrations were below the detection limit, whereas these elements were quantified in the growth medium.

The effect of water (tap or CBMW) treatment was highly significant on lemongrass fresh weight (p = 0.001) and marginally significant on oil yield and isoneral concentration in lemongrass EO; whereas the interaction effect of water and gypsum was significant on geranyl acetate and marginally significant on the concentration of beta-caryophyllene in lemongrass EO (Table 2). Similarly, the water effect was significant on palmarosa height (p = 0.042) and fresh weight (p = 0.045) and on concentrations of geraniol (p = 0.015) and geranyl acetate (p = 0.007) in palmarosa EO (Table 2). Also, for both crops, water had a significant effect on soil microbial C flush only. Gypsum had a significant effect on beta-caryophyllene concentration in palmarosa EO (p = 0.043) (Table 2).

3.1. Water Treatment: Lemongrass Experiment

For the lemongrass experiment, neither the main effects of water and gypsum nor their interaction were significant on the concentrations of soluble salts and growth medium-available Nitrate-N (Table 3). Except for the growth medium available K in the lemongrass experiment, which was marginally affected by gypsum, the interaction effect of water and gypsum was marginally significant for B and Zn and significant for all other growth medium-available nutrients (Table 3).

Water treatment effect was significant on N, Ca, Mg, Na, and S content of lemongrass distillation waste tissue, while gypsum had a marginally significant and significant effect on K and Mn content of lemongrass distillation waste tissue, respectively (Table 3). The interaction effect of water and gypsum was significant on the P, Fe, and Zn content of lemongrass distillation waste tissue (Table 3).

3.2. Water Treatment: Palmarosa Experiment

For the palmarosa experiment, growth medium-available P and Na concentrations were significantly affected by both water and gypsum, and the effect of the gypsum treatment was also significant on growth medium-soluble salts (Table 3). Furthermore, the main effects were significant on growth medium pH, B, Ca, Fe, Mg, Mn, and S (Table 3). In addition, the effect of water treatment was significant on growth medium-available Cu (Table 3). The interaction effect of water and gypsum was significant on growth medium pH and available B, Ca, Cu, Fe, Mg, Mn, S, and Zn (Table 3).

Water significantly affected the P, Ca, Na, and S content of palmarosa distillation waste tissue, while the gypsum treatment significantly affected Ca, K, Mg, Na, and S content of palmarosa distillation waste tissue (Table 3). No significant interaction of water and gypsum effects was observed for the nutrient content of palamrosa distillation waste tissue (Table 3).

The effect of water was significant on microbial C and N flush in palmarosa, but only on microbial C flush in lemongrass (Table 2). The average DOC was higher with CBMW in both plants (Table 4 and Table 5), but the average DON was higher with CBMW only in palmarosa (Table 5).

The application of CBMW reduced palmarosa plant height, fresh weight, and concentration of geranyl acetate in palmarosa EO (Table 5). However, CBMW application resulted in increased geraniol concentration in palmarosa EO and increased growth medium-available P and Na (Table 5). Also, the application of CBMW caused increased P, S, Mn, and Na and reduced Ca content in palmarosa distillation waste plant tissue (Table 5).

3.3. Gypsum Amendment: Lemongrass Experiment

Overall, the application of CBMW significantly reduced lemongrass oil yield, fresh weight, and the concentration of isoneral in lemongrass EO relative to the tap water treatment (Table 4). Coal-bed methane water application also significantly decreased lemongrass distillation waste tissue concentrations of N, S, Ca, and Mg, while it significantly increased the Na concentration of lemongrass distillation waste tissue relative to the corresponding tap water treatments (Table 4). The Na concentration in the lemongrass distillation waste tissue of the CBMW treatment was approximately eight times greater than that in the tap water treatment.

Lemongrass fresh weight was greater in the 0 and 500 kg ha⁻¹ gypsum treatments than in the 4500 kg ha⁻¹ gypsum treatment; however, lemongrass fresh weight in the 1500 kg ha⁻¹ gypsum treatment was similar to the fresh weights in the 0, 500, and 4500 kg ha⁻¹ gypsum treatments (Table 6). Growth medium-available K concentration and lemongrass plant tissue K and Mn contents were higher in the 4500 kg ha⁻¹ gypsum treatment than in the 0 kg ha⁻¹ gypsum treatment of the lemongrass experiment (Table 6).

Overall, the growth medium pH of the lemongrass experiment was significantly higher in the CBMW treatment than in the tap water treatment (Table 7). Gypsum application reduced growth medium pH in the CBMW treatment, but did not have an effect on growth medium pH in the tap water treatment. Gypsum application could not fully neutralize the alkaline growth medium pH caused by the application of CBMW.

Growth medium-available P was highest in the tap water with 4500 kg ha⁻¹ gypsum treatment and lowest in the CBMW with 1500 kg ha⁻¹ gypsum treatment of the lemongrass experiment (Table 7). However, growth medium-available P in tap water with no gypsum was similar to growth medium-available P in all gypsum treatments with CBMW. The highest soluble B in the growth medium of the lemongrass experiment was in the CBMW with no gypsum application treatment; gypsum at 1500 or 4500 kg ha⁻¹ reduced the available B in the CBMW treatments. However, gypsum application did not affect the available B in the tap water treatments (Table 7). As expected, growth medium Ca in the lemongrass experiment increased with increasing gypsum application. The increase in growth medium Ca was more pronounced in the gypsum applications with tap water treatment; growth medium Ca was always higher in the respective tap water with gypsum application treatments than in the CBMW treatments, suggesting that CBMW reduced the growth medium Ca (Table 7).

Growth medium-available Cu in the lemongrass experiment was significantly higher in the CBMW with no gypsum treatment than in all other treatments (Table 7). Similarly, growth medium Fe was significantly higher in the CBMW with no gypsum treatment compared to the other treatments. These results suggest that while CBMW application may increase the available Cu and Fe in the growth medium, adding gypsum may negate this effect.

Growth medium Mg was highest in tap water with 500 and 1500 kg ha⁻¹ gypsum and lowest in the CBMW with no gypsum treatment in the lemongrass experiment (Table 7). Overall, the CBMW application reduced growth medium-available Mg in most gypsum treatments relative to the tap water application, except in the 4500 kg ha⁻¹ gypsum treatment (Table 7).

Growth medium Mn in the lemongrass experiment was highest in the tap water with 500 kg ha⁻¹ gypsum treatment and lowest in the CBMW with 1500 and 4500 kg ha⁻¹ gypsum treatments (Table 7). As expected, growth medium-available Na in the lemongrass experiment was significantly higher in CBMW with gypsum application relative to the tap water with gypsum application treatments (Table 7). Gypsum application increased the growth medium-available Na in the CBMW with 500 and 1500 kg ha⁻¹ gypsum relative to the CBMW with zero and 4500 kg ha⁻¹ gypsum treatments (Table 7).

Also, as expected, gypsum increased growth medium-available S in the lemongrass experiment; the highest available S was found in the CBMW with 1500 and 4500 kg ha⁻¹ gypsum treatments (Table 7). Growth medium-available S was greater in the CBMW with 1500 and 4500 kg ha⁻¹ gypsum treatments than in the corresponding tap water treatments, suggesting that CBMW application may increase S in the higher gypsum treatments. Growth medium-available S in the tap water with 500, 1500, and 4500 kg ha⁻¹ gypsum treatments was similar to the CBMW with 500 kg ha⁻¹ gypsum treatment (Table 7).

Growth medium-available Zn in the lemongrass experiment was highest in the tap water with 500 kg ha⁻¹ gypsum treatment and lowest in the corresponding CBMW treatment (Table 7). Growth medium-available Zn was similar in all other treatments.

The concentration of geranyl acetate in the lemongrass oil was highest in the CBMW with 4500 kg ha⁻¹ treatment and lowest in CBMW with no gypsum (Table 8). The concentration of beta-caryophyllene in lemongrass EO was highest in the CBMW with 4500 kg ha⁻¹ gypsum treatment and lowest in the corresponding tap water treatment (Table 8). Lemongrass distillation waste plant tissue P content was lowest in the tap water with 4500 kg ha⁻¹ gypsum treatment and similar in all other treatments (Table 8). Lemongrass distillation waste plant tissue Zn content was highest in the CBMW with 4500 kg ha⁻¹ gypsum treatment and lowest in the CBMW with 0 and 500 kg ha⁻¹ gypsum treatments (Table 8). In all other treatments, lemongrass plant tissue Zn content was similar. Lemongrass distillation waste plant tissue Fe content was highest in the CBMW with 4500 kg ha⁻¹ gypsum treatment and lowest in the CBMW with 500 kg ha⁻¹ gypsum treatment (Table 8).

3.4. Gypsum Amendment: Palmarosa Experiment

Gypsum application of 500 and 1500 kg ha⁻¹ reduced the concentration of beta-caryophyllene in palmarosa EO (Table 9). The amount of soluble salts in the growth medium was significantly higher at the 4500 kg ha⁻¹ gypsum treatment than at the 0 or 500 kg ha⁻¹ gypsum treatments (Table 9). Also, gypsum at 1500 and 4500 kg ha⁻¹ increased growth medium-available Na relative to the no gypsum treatment (Table 9).

Gypsum at 4500 kg ha⁻¹ increased palmarosa distillation waste tissue concentration of K relative to the 500 or 1500 kg ha⁻¹ gypsum applications, increased distillation waste plant tissue concentration of S relative to the 0 or 500 kg ha⁻¹ gypsum treatments, and also resulted in the highest Ca content in palmarosa distillation waste tissue relative to the 0, 500, or 1500 kg ha⁻¹ treatments. The content of Mg in palmarosa distillation waste plant tissue was significantly higher in the no gypsum treatment than in the 1500 or 4500 kg ha⁻¹ gypsum treatments; Fe content was highest in the 500 kg ha⁻¹ gypsum and lowest in the 1500 kg ha⁻¹ gypsum treatments; and Na content in palmarosa distillation waste plant tissue was highest in the 1500 kg ha⁻¹ gypsum and lowest in the 4500 kg ha⁻¹ gypsum treatments (Table 9).

Also, in the palmarosa experiment, growth medium pH significantly increased with the application of CBMW, reaching 8.98 in the CBMW no gypsum treatment (Table 10). The application of gypsum at 1500 and 4500 kg ha⁻¹ with the CBMW treatments reduced pH relative to no gypsum, indicating gypsum can alleviate to some extent the pH increase caused by CBMW application. Gypsum application had no effect on growth medium pH in the tap water treatments (Table 10).

Growth medium B was highest in the CBMW with no gypsum treatment and lower in the other treatments (Table 10). Generally, the application of high amounts of gypsum (1500 or 4500 kg ha⁻¹) increased growth medium-available Ca and S relative to the no gypsum treatment in both CBMW and tap water treatments (Table 10). However, CBMW treatments had lower growth medium-available Ca relative to the corresponding tap water treatments. The reverse trend was observed in growth medium-available S, which was higher in the CBMW treatments relative to the available S in the corresponding tap-water treatments. Hence, the application of CBMW may increase S but reduce Ca in the growth medium. Overall, the growth medium-available Cu, Fe, and Mn were high in the CBMW with no gypsum treatment and low in most other treatments. Gypsum at 1500 and 4500 kg ha⁻¹ increased growth medium-available Mg, while CBMW application reduced growth medium Mg relative to the respective tap water treatments (Table 10). The growth medium-available Zn was highest in the CBMW with no gypsum treatment and lowest in the tap water with no gypsum treatment (Table 10).

4. Discussion

In this study, fresh biomass yields from lemongrass and palmarosa were reduced by 20% and 12%, respectively, by the application of CBMW compared to tap water. The application of CBMW reduced the concentration of isoneral in lemongrass and the concentration of geranyl acetate in the palmarosa EO. These results are logical because the plants in the CBMW treatments were exposed to high pH, Na, and salts accumulation in the growth medium. Agricultural crops can be sensitive to pH but also to salt stress. Generally, environmental stresses are thought to affect secondary metabolites synthesis and accumulation in higher plants.

Coal-bed methane water quality may vary depending on the region and the particular well [24]. Some wells may have elevated concentrations of trace elements, which may depress normal plant growth and development. The CBMW used in this study represented the middle range for soil quality and was representative if not typical for the CBMW in the Powder River Basin.

There are some reports on the use of CBMW for irrigation of rangeland and forage plants [7,25]. In a previous study with peppermint and spearmint, Zheljazkov et al. [13] demonstrated that horticultural crops grown on commercial medium could be watered with CBMW as long as it was mixed with half-good-quality (tap) water. Irrigating peppermint and spearmint with only CBMW reduced herbage yields and mint oil concentration compared to plants watered with tap water [13]. This study is the next logical step, using gypsum to mitigate the negative effect of CBMW on plant productivity and accumulation of plant secondary metabolites (EO). As indicated in Zheljazkov et al. [13], care must be taken for the spent growth medium that was subject to CBMW irrigation; it accumulates salts and some nutrients above optimal levels for plant growth and development.

Recent studies with peppermint (Mentha x piperita L.) and spearmint (M. spicata L.) grown in growth medium [13] and with dill (Anethum graveolens L.) grown in field soil [12], found that if plants were watered with CBMW, the chemical profile of plant chemicals may be altered slightly. Burkhadt et al. [10] in a field study reported that lemongrass and spearmint EO concentrations were significantly affected by CBMW. Sintim et al. [26] in a study with three cultivars of the oilseed camelina (Camelina sativa L.) grown in field soil reported that CBMW may have reduced total saturated fatty acids, but it had no effect on mono- and poly-unsaturated fatty acids. Since CBMW contains salts and several elements in elevated concentrations, it is difficult to separate which of these would be responsible for the observed effects on plant secondary (EO) and primary (FA) synthesis and accumulation. The summative effect of any of these factors may be different compared to a single effect of one factor (element or another characteristic of CBWM).

4.1. Lemongrass

The concentration of EO varied between 0.54 and 0.96%, and concentrations of the major oil constituents in the lemongrass study were similar to those reported previously. Idrees et al. [27] reported that the EO content of lemongrass irradiated with gamma-rays varied between 0.53 and 0.72% in fresh leaves, while the citral content in the oil varied from 40 to 56%. However, in their study, the distillation time was 3 h, which might have affected EO composition. Moncada et al. [28] used 2-h water distillation of chopped and ground lemongrass material. The EO content was 0.75% in dried (to 10% moisture) biomass, which is low and may have been due to EO losses during grinding prior to distillation. The main EO constituents in the lemongrass EO included citral (72.32%), myrcene (14.28%), farsenol (10.37%), and nerol (3.03%). Other authors reported oil content in lemongrass to vary between 0.55 and 1.03%, with 78–95% citral [29]; however, some clones may have up to 1.5% oil concentration [30]. In a study in Mississippi with the same genotype of lemongrass, Zheljazkov et al. [14] reported EO concentration of 0.35 to 0.6% of the dried biomass, with major oil constituents neral (20–45%), geranial (25–53%), and caryophyllene oxide (1.3–7.2%). The latter authors extracted the oil via a 60-min steam distillation of 300 g of dried biomass using the same distillation setup as in this study. In another study in Mississippi with the same genotypes of lemongrass and palmarosa, Joyce et al. [31] reported a 0.61–0.67% oil concentration in lemongrass dried biomass.

4.2. Palmarosa

Rajeswara Rao and Rajput [32] distilled palmarosa for 3 h in a Clevenger-type glass apparatus, whereas in this study, the palmarosa oil was extracted for 60 min based on our preliminary studies. The palmarosa oil composition included 79.7–85.8% geraniol, 4.5–10.3% geranyl acetate, and 1.5–3.2% linalool [32].

In another study with the same genotype of palmarosa as in this study, the EO concentration in dried biomass was reported to be 0.48–0.55%, from the first and the second cut, respectively, with geraniol as the main oil constituent [31]. They extracted the EO from dried palmarosa biomass for 60 min in a setup similar to this study. Previous reports found the concentration of geraniol in palmarosa oil to be 70–78% [33], 82% [34], and even up to 93% [35]. Geraniol concentration in palmarosa oil varied from 70% to 85% and geranyl acetate was 4–15% [33]. Essential oil with 82% geraniol and 10% geranyl acetate was reported previously [34], and Rajeswara [35] reported EO with up to 93% geraniol, 3–4% linalool, and 2% geranyl acetate.

The above studies were conducted in various and different environmental conditions; oil concentration and composition of lemongrass and palmarosa may vary as a response to the environment and are a function of postharvest management such as oil extraction [36].

Previous research demonstrated that lemongrass and palmarosa produce high yields of biomass under the environmental conditions found in the southeastern United States and could be used as lignocellulosic feedstock for ethanol production [14,31]. Indeed, in a 2-year study in Mississippi, Joyce et al. [31] found that the biomass production of lemongrass was around 12.83 Mg ha⁻¹ whereas the biomass yield of palmarosa was 15.11 Mg ha⁻¹ during the second harvest year, which translated into theoretical biofuel yields of 2541 and 2569 L ethanol ha⁻¹ respectively compared to a reported 1749–3691 L ethanol ha⁻¹ for switchgrass.

5. Concluding Remarks

This study showed the feasibility of using low-quality CBMW and gypsum for irrigation of lemongrass and palmarosa. The production of a high-value natural product, such as EO from these two species, would offset some or most of the production costs and would provide double the benefit for growers and society, if these two crops could be developed as feedstock for biofuel production in the United States. This in turn would improve the agronomic, environmental, and economic sustainability of biofuel production in the United States while utilizing the low-quality CBMW. The use of CBMW along with gypsum for non-food biofuel crops would have further benefits to the environment and society. However, although the long-term effects of CBMW on soil and crops need to be further investigated, this study provides important, relevant practical information that could be used by field or greenhouse producers during their decision-making processes.

Author Contributions

Conceptualization, V.D.Z. and E.A.J.; methodology, V.D.Z., U.N., T.A. E.A.J.; software, T.A.; validation: V.D.Z. and E.A.J.; formal analysis: V.D.Z., T.A., U.N., and E.A.J.; investigation: V.D.Z., T.A., U.N., and E.A.J.; resources: V.D.Z., U.N. and T.A.; data curation: V.D.Z.; writing—original draft preparation, V.D.Z.; writing—review and editing: T.A., U.N., and E.A.J.; visualization: T.A.; supervision: V.D.Z.; project administration: V.D.Z.; and funding acquisition: V.D.Z.

Funding

This research was supported by the University of Wyoming School of Energy funds awarded to Valtcho D. Zheljazkov (Jeliazkov).

Acknowledgments

The authors thank Derek Lowe of BeneTerra LLC, in Sheridan, Wyoming, for providing access to coal-bed methane water, and Dan Smith, farm manager at the University of Wyoming’s Sheridan Research and Extension Center, Sheridan.

Conflicts of Interest

The authors declare no conflict of interest.

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Table 1. Selected properties of coal-bed methane water and tap water used in this study.

Selected Properties	Units	Coal-Bed Methane Water	Tap Water	Analytical Method
General parameters
pH		8.3	7.6	SM 4500HB
Electrical conductivity	dS/m	2.0	0.12	SM 2510B
Total dissolved solids	mg/L	1393.3	70	SM 2540
Alkalinity, total (as CaCO₃)	mg/L	1157	ND	SM 2320B
Hardness (as CaCO₃)	mg/L	53.0	33	SM 2340B
Nitrogen, ammonia (as N)	mg/L	1.6	-
E. coli	MPN/100 mL	ND ¹	ND	SM 9223B
Total coliform	MPN/100 mL	- ²	ND
Sodium adsorption ratio		32.6	-	Calculation
Anions
Alkalinity, bicarbonate as HCO₃⁻	mg/L	1393	ND	SM 2320B
Alkalinity, carbonate as CO₃⁻²	mg/L	19	ND	SM 2320B
Chloride	mg/L	6	-	EPA 300.0
Nitrate + Nitrite as N	mg/L	ND	ND	EPA 300.0
Sulfate	mg/L	85	7	EPA 300.0
Cations
Barium	mg/L	0.4	-	EPA 200.8
Calcium	mg/L	9	9	EPA 200.7
Magnesium	mg/L	8	2	EPA 200.7
Sodium	mg/L	545	9	EPA 200.7
Zinc	mg/L	0.01	-	EPA 200.7
Cation/Anion—Milliequivalents
Hydroxide as OH	meq/L	ND	-	SM 1030E
Chloride	meq/L	0.18	-	SM 1030E
Fluoride	meq/L	ND	-	SM 1030E
Nitrate + Nitrite as N	meq/L	ND	-	SM 1030E
Sulfate	meq/L	1.76	-	SM 1030E
Calcium	meq/L	0.44	-	SM 1030E
Magnesium	meq/L	0.61	-	SM 1030E
Sodium	meq/L	23.70	-	SM 1030E
Radiochemistry
Gross Beta (dissolved)	pCi/L	2		SM 7110B
Radium 226 (dissolved)	pCi/L	0.2 ± 0.1	-	Ra-05
Dissolved Metals/Metalloids
Boron	mg/L	0.2	-	EPA 200.7

¹ ND = Not Detected; ² = Not Analyzed.

Table 2. ANOVA p-values for the main and interaction effects of Water and Gypsum on lemongrass and palmarosa Height (in), Fresh weight (FW, g), Oil yield (g), and Oil content (%); oil composition [6-Methyl-5-Hepten-2-one (%), Beta-Caryophyllene (%), Geranial (%), Geraniol (%), Geranyl Acetate (%), Isogeranial (%), Isoneral (%), Linalool (%), Neral (%), and Trans-Ocimene (%)]; and soil DOC (µg/g OD soil) and DON (µg/g OD soil).

Response Variable	Lemongrass			Palmarosa
	Source of Variation			Source of Variation
	Water	Gypsum	Water*Gypsum	Water	Gypsum	Water*Gypsum
Height	0.101	0.184	0.760	0.042	0.441	0.139
FW	0.001 ¹	0.060	0.698	0.045	0.696	0.461
Oil yield	0.075	0.721	0.964	0.263	0.700	0.422
Oil content	0.642	0.543	0.959	0.608	0.515	0.355
6-Methyl-5-Hypten-2-one	0.607	0.945	0.815	-	-	-
Beta-Caryophyllene	0.197	0.534	0.051	0.163	0.043	0.396
Geranial	0.628	0.936	0.116	-	-	-
Geraniol	0.647	0.525	0.174	0.015	0.952	0.453
Geranyl Acetate	0.681	0.002	0.031	0.007	0.770	0.673
Isogeranial	0.597	0.761	0.463	-	-	-
Isoneral	0.097	0.562	0.653	-	-	-
Linalool	-	-	-	0.240	0.566	0.485
Neral	0.499	0.396	0.160	-	-	-
Trans-Ocimene	-	-	-	0.588	0.671	0.633
Soil DOC	0.001	0.487	0.292	0.001	0.993	0.214
Soil DON	0.223	0.545	0.100	0.055	0.238	0.626

¹p-values shown in bold face indicate significance of the effects that need multiple means comparison.

Table 3. ANOVA p-values for the main and interaction effects of Water and Gypsum on growth medium pH, Soluble salts, growth medium available nutrients, and nutrient content of plant tissue from distillation waste of the lemongrass and palmarosa experiments.

Response Variable	Lemongrass			Palmarosa
	Source of Variation			Source of Variation
	Water	Gypsum	Water*Gypsum	Water	Gypsum	Water*Gypsum
Growth medium
pH	0.001	0.001	0.005 ¹	0.000	0.044	0.002
Soluble salts	0.209	0.460	0.257	0.309	0.011	0.493
P	0.001	0.043	0.004	0.048	0.030	0.194
Nitrate	0.129	0.679	0.825	0.742	0.563	0.913
B	0.587	0.004	0.068	0.000	0.001	0.016
Ca	0.001	0.001	0.001	0.001	0.001	0.047
Cu	0.001	0.001	0.002	0.003	0.063	0.039
Fe	0.001	0.001	0.001	0.001	0.001	0.001
K	0.911	0.060	0.576	0.649	0.112	0.626
Mg	0.001	0.001	0.001	0.001	0.001	0.001
Mn	0.001	0.235	0.004	0.003	0.002	0.021
Na	0.001	0.001	0.001	0.001	0.035	0.220
S	0.001	0.001	0.002	0.001	0.001	0.001
Zn	0.033	0.734	0.083	0.768	0.611	0.028
Plant tissue from distillation waste
P	0.002	0.222	0.017	0.006	0.812	0.212
N	0.049	0.118	0.413	0.729	0.940	0.197
B	0.264	0.756	0.414	0.637	0.200	0.851
Ca	0.001	0.523	0.399	0.046	0.003	0.321
Cu	0.958	0.265	0.195	0.905	0.540	0.642
Fe	0.829	0.256	0.023	0.381	0.074	0.933
K	0.116	0.075	0.494	0.842	0.022	0.574
Mg	0.012	0.324	0.212	0.261	0.005	0.114
Mn	0.113	0.001	0.273	0.058	0.101	0.524
Na	0.001	0.695	0.466	0.001	0.021	0.109
S	0.023	0.745	0.191	0.013	0.002	0.611
Zn	0.074	0.004	0.026	0.469	0.638	0.671

¹p-values shown in bold face indicate significance of the effects that need multiple means comparison.

Table 4. Mean Oil yield, Fresh weight (FW), Isoneral content in lemongrass essential oil, and soil microbial C flush [DOC (µg/g OD soil)]; and nutrient content in plant tissue from distillation waste [Nitrogen (N), Sulfur (S), Calcium (Ca), Magnesium (Mg), and Sodium (Na)] obtained from lemongrass irrigated with Coal-Bed Methane (CBM) water and Tap water (Tap).

Water	Oil Yield g	FW g	Isoneral %	DOC	Plant Tissue
Water	Oil Yield g	FW g	Isoneral %	µg/g OD Soil	N %	S %	Ca %	Mg %	Na %
CBM	0.98 b¹	379 b	1.34 b	1938 a	2.10 b	0.203 b	0.404 b	0.388 b	0.639 a
Tap	1.19 a	470 a	1.39 a	841 b	2.21 a	0.257 a	0.646 a	0.452 a	0.081 b