Agronomic management practices that maximize monoculture switchgrass (Panicum virgatum L.) yield are generally well understood; however, little is known about corresponding effects of differing switchgrass management practices on near-surface soil properties and processes. The objective of the study was to evaluate the effects of cultivar (“Alamo” and “Cave-in-Rock”), harvest frequency (1- and 2-cuts per year), fertilizer source (poultry litter and commercial fertilizer), and irrigation management (irrigated and non-irrigated) on near-surface soil properties and surface infiltration in a Leadvale silt loam (fine-silty, siliceous, semiactive, thermic, Typic Fragiudult) after four years (2008 through 2011) of consistent management in west-central Arkansas. Irrigating switchgrass increased (P < 0.01) soil bulk density in treatment combinations where poultry litter was applied (1.40 g?cm?3) compared to non-irrigated treatment combinations (1.33 g?cm?3). Root density was greater (P = 0.031) in irrigated (2.62 kg?cm?3) than in non-irrigated (1.65 kg?cm?3) treatments when averaged over all other treatment factors. The total infiltration rate under unsaturated conditions was greater (P = 0.01) in the 1-cut (33 mm?min?1) than 2-cut (23 mm?min?1) harvest treatment combinations when averaged over all other treatment factors, while the total infiltration rate under saturated conditions did not differ among treatment combinations (P > 0.05) and averaged 0.79 mm?min?1. Results from this study indicate that management decisions to maximize switchgrass biomass production affect soil properties over relatively short periods of time, and further research is needed to develop local best management practices to maximize yield while maintaining or improving soil quality.
The United States (US) is the largest consumer of petroleum in the world at an estimated 18.6 million barrels per day [
Concerns related to the effects of global warming and possible shortages of finite fossil fuel sources have led to government-mandated regulations encouraging the development of alternative fuels. According to the Energy Policy Act of 2005 and the Renewable Fuel Standard Program of 2007, 36 billion gallons of renewable fuel must be blended into the nation’s transportation fuel supply by 2022 [
One potential solution is the conversion of plant biomass to fuel. Using plant biomass as a source of cellulosic biofuel as an alternative to fossil fuel is attractive because plants use sunlight for energy and capture existing CO2 from the atmosphere. Though bioenergy derived from lignocellulosic sources may not provide a complete alternative to the massive energy needs of the US, lignocellulosic bioenergy sources may alleviate a portion of the negative impacts of burning fossil fuels and contribute to an extended supply of fossil fuels for the future [
Plant biomass can be utilized as a fuel in two major ways: by burning or mixing biomass with coal (i.e., co- firing) to generate electricity [
Corn is also an annual crop that must be replanted every year and requires large amounts of fertilizer. The extensive preparation required for conventional corn production is regarded as causing more total soil erosion than any other crop grown in the US [
Switchgrass (Panicum virgatum L.) was recognized by the US Department of Energy in the 1990s as part of the Bioenergy Feedstock Development Program for its ability to produce large quantities of biomass on relatively poor sites. Annual yields of switchgrass in the US average 11.2 Mg∙ha−1, ranging from 4.5 Mg∙ha−1 in the northern plains to 23.0 Mg∙ha−1 in Alabama [
The western portion of Arkansas is largely unsuitable for traditional row-crop production due to steep topography and thin soil compared to the delta region of the Lower Mississippi River basin of eastern Arkansas. However, the average biomass yield for switchgrass grown in Booneville, Arkansas under a single annual harvest regime was 13.4 Mg∙ha−1 over a four-year period, which was above the national average yield of 11.2 Mg∙ha−1 estimated by McLaughlin and Kszos [
In addition to the projected increase in yield, western Arkansas is widely known for its forage and hay production and for its role in the beef cattle industry. Agricultural operators in western Arkansas are familiar with grass harvest, distribution, and storage and typically own conventional farm equipment that can be used to harvest switchgrass. The cost of producing one dry ton of switchgrass in Arkansas was estimated to be $26.73 in the third year of production, with an expected useful life of 12 years before having to replant [
Past research has identified agronomic procedures for maximizing switchgrass biomass production using varying systems, including cultivar choice, fertilizer application, irrigation, harvest frequency, and row spacing [
Since “Alamo” previously produced greater biomass yields in all treatment combinations [
The US Department of Agriculture’s Natural Resource Conservation Service’s Plant Materials Center (PMC) was established in Booneville, AR in 1987 and serves the plant material needs of the Southern Ozarks, Arkansas River Valley, Boston, and Ouachita Mountains (Major Land Resource Areas [MLRAs] 116A, 118A, 117, and 119, respectively). The PMC is located along the Petit Jean River in Logan County, AR at an elevation of 146 m [
The study site at the PMC (
depth. The study site was a pasture dominated by tall fescue (Schedonorus arundinaceus [Schreb] Dumort., nom. Cons.) prior to being prepared for a yield study in 2006 [
“Alamo” and “Cave-in-Rock” switchgrass cultivars were planted in 12.2 by 12.2 m plots on 5 March 2007. Each plot contained one switchgrass cultivar and fertilizer source. Plots were seeded at a rate of 4.4 kg∙ha−1 pure live seed (457 seed∙m−2) and planted with a no-tillage native grass drill (Sukup 2050 series, Jonesboro, AR). After drilling, the seedbed was rolled with a water-filled roller to establish good seed-to-soil contact. Temporary sprinkler irrigation was applied to all plots for initial seed emergence and establishment. A permanent sprinkler irrigation system was installed in replicated irrigation treatments in summer 2007. Each 12.2 by 12.2 m plot was divided in half, with one subplot harvested once per year and the other subplot harvested twice per year. There were a total of 48 plots encompassing three replications of each cultivar-irrigation-fertilizer source-harvest frequency treatment combination.
Rain gauges were placed in the irrigated plots to calibrate the irrigation delivery system. Irrigated plots received 2.54 cm of irrigation water from a nearby surface pond per week during June through August from 2008 to 2011. The average annual precipitation at the study site was 131 cm [
The study site was burned each year in early March to remove residue stubble, stimulate switchgrass seed production for wildlife, remove surface residue for native pollinator nesting habitat, and to create corridors for other ground-nesting wildlife species [
Two harvest frequencies were imposed to test their effects on annual aboveground biomass production. A single harvest was made in November after the first killing frost for the 1-cut system. In the 2-cut system, harvests occurred twice per year. The first harvest occurred in June just prior to the boot stage when the seed head emerges and the second harvest occurred after a killing frost in November. Additional switchgrass harvesting details and results of the initial yield study were reported in Jacobs and King [
In July 2012, soil samples were collected in all plots from the 0- to 10- and 10- to 20-cm depth intervals for bulk density, extractable soil nutrients, and soil particle-size analyses. In the top 10 cm, bulk density samples were collected manually with a 5-cm outside diameter, stainless steel core chamber and a slide hammer. For the 10- to 20-cm depth, bulk density samples were collected with a 5-cm diameter, stainless steel mechanical hydraulic probe. Samples from both depths were dried in a forced-air dryer at 70˚C for 48 hours, and then weighed for bulk density determinations. Soil from the bulk density samples was sieved through a 2-mm mesh screen and used to measure particle-size distribution using a modified 12-hour hydrometer method [
In August 2013, soil samples were collected from the 0- to 5- and 5- to 10-cm depths using a 4.8-cm diameter core chamber and slide hammer to measure aggregate stability (AS) (i.e., water-stable aggregates > 0.25-mm diameter) using a wet-sieving procedure [
One soil sample for root density was collected per plot in September 2013 from the 0- to 15-cm depth interval using a 7.3-cm diameter core chamber and slide hammer, and prepared according to the procedures followed by Brye and Riley [
In November 2013, double-ring infiltrometers were used to measure surface infiltration rates two days after a soaking rainfall. Mature switchgrass was trimmed using hedge trimmers and residue carefully removed prior to placement of infiltrometers. One double-ring infiltrometer measurement was conducted in each cultivar-harvest frequency-fertilizer source treatment combination. In order to maximize potential infiltration differences among treatment combinations, infiltration measurements were only conducted in the non-irrigated treatment. Double- ring infiltrometers were placed between switchgrass crowns in the switchgrass rows after switchgrass was mowed to a height of 6 cm. The outer-ring diameter was 30 cm, the inner-ring diameter was 6 cm, and rings were 10 cm in height. The infiltrometer was inserted approximately 2 cm deep into the soil and the outer ring was filled with tap water to act as a buffer between dry soil outside the outer ring and saturated soil inside the inner ring. The inner ring was filled with tap water and the distance from the top of the soil to the water surface in the inner ring was measured at 0, 1, 2, 3, 4, 5, 8, 10, 12, 15, 18, and 20 minutes.
Double-ring infiltration measurements were paired with mini-disk tension infiltrometer (Decagon, Pullman, Washington) measurements, where infiltrometer tension was set at −2 cm for each measurement. Two mini-disk infiltrometer measurements were conducted in each plot, with one measurement collected from the center-ring area immediately following double-ring infiltrometer measurements and one measurement collected in a nearby location. The infiltrative surface of the mini-disk infiltrometer was 4.5 cm. The two mini-disk measurements were performed between switchgrass crowns in the switchgrass rows in each plot for a total of 48 measurements. The height of the water inside the 2.1-cm diameter mini-disk infiltrometer water reservoir was recorded at 0, 1, 2, 3, 4, 5, 8, 10, 12, 15, 18, and 20 minutes. The total surface infiltration rate was calculated for both sets of infiltration measurements by dividing the total amount of infiltrated water by 20 minutes.
The effects of cultivar, harvest frequency, irrigation, fertilizer source, soil depth, and their interactions on soil bulk density, soil chemical properties, particle-size distributions, and AS were evaluated by analysis of variance (ANOVA) using PROC GLM in SAS (version 9.3, SAS Institute, Inc., Cary, NC). A separate ANOVA was conducted to evaluate the effects of cultivar, harvest frequency, irrigation, fertilizer source, and their interactions on root density using SAS. A separate ANOVA was also conducted to evaluate the effects of cultivar, harvest frequency, fertilizer source, and their interactions on total surface infiltration rate for both sets of infiltration data. All treatments were treated as fixed effects for all ANOVAs conducted. An analysis of covariance (ANCOVA) was conducted to evaluate the effects of cultivar, harvest frequency, and fertilizer source on the relationship between the natural logarithm of the infiltration rate and the natural logarithm of the mid-point of the measurement time interval. When appropriate, means were separated by least significant difference (LSD) at α = 0.05.
Throughout the study site, sand, silt, and clay in the top 20 cm varied somewhat. Though sand, silt, and clay distributions remained within the range of a silt-loam texture in all treatment combinations, sand, silt, and clay contents in the top 20 cm differed slightly among various treatment combinations (P < 0.05), with the largest differences occurring between soil depths, which was expected. In both depth intervals, sand content was greatest in the irrigated-“Alamo” treatment combinations (
Averaged over harvest frequency, fertilizer source, and irrigation, clay content also differed between cultivars within soil depths (P < 0.01). As expected, clay content increased with increasing depth from the top 10 cm (19.5%) to the 10- to 20-cm depth interval (22.9%) for both cultivars (
The observed differences in sand, silt, and clay contents between soil depths were expected based on the reported textural classes of the top two horizons of the Leadvale silt-loam official series description [
Soil property | Soil depth (cm) | Alamo | Cave-in-Rock | ||
---|---|---|---|---|---|
Irrigated | Non-irrigated | Irrigated | Non-irrigated | ||
Sand (%) | 0 - 10 | 32.6 a* | 31.4 ab | 30.3 abc | 26.1 d |
10 - 20 | 30.3 ab | 27.5 cd | 29.0 bc | 28.9 bc | |
Mn (kg∙ha−1) | 0 - 10 | 183.2 b* | 170.9 b | 179.3 b | 169.4 b |
10 - 20 | 230.7 a | 168.9 b | 178.7 b | 185.5 b | |
EC (dS∙m−1) | 0 - 10 | 0.070 a* | 0.072 a | 0.071 a | 0.070 a |
10 - 20 | 0.060 bc | 0.053 c | 0.053 c | 0.067 ab |
*For each soil property, means followed by different letters differ significantly at P < 0.05.
accumulation with increasing depth was expected, as the surface Ap horizon grades to an argillic Bt horizon at approximately 20 cm [
Bulk density was affected by several experimental treatment factors. As expected, averaged over all other treatment factors, soil bulk density was greater (P < 0.001) in the 10- to 20-cm depth (1.44 g∙cm−3) than in the top 10 cm (1.30 g∙cm−3). Averaged over soil depths and cultivars, soil bulk density was also greater (P = 0.004) in the irrigated-PL treatment combination with either harvest frequency (1.40 g∙cm−3) than the non-irrigated-1-cut treatment combination with either fertilizer source (1.33 g∙cm−3). Soil bulk density was unaffected (P > 0.05) by switchgrass cultivar.
Similar to bulk density, aggregate stability was affected by several experimental treatment factors. Averaged over soil depth, cultivar, and harvest frequency, AS was greater (P = 0.03) under irrigated (0.93 g∙g−1) than non-irrigated (0.86 g∙g−1) treatments with PL, but did not differ between irrigation treatments with CF (0.86 g∙g−1;
Soil pH was unaffected by switchgrass cultivar, irrigation, harvest frequency, or soil depth (P > 0.05), but differed between fertilizer sources (P = 0.001). As expected, averaged over all other treatment factors, soil pH was greater in the PL (pH = 6.1) than in the CF (pH = 5.9) treatment. Though soil pH differed between fertilizer sources by only 0.02 units, nutrient availability and buffering capacity were likely different as well between fertilizer sources.
Similar to bulk density and aggregate stability, soil EC was affected by several experimental treatment factors. Averaged over fertilizer source and soil depth, soil EC was greatest (P = 0.016) in “Cave-in-Rock” (0.070 dS∙m−1) and least in “Alamo” (0.056 dS∙m−1) treatments without irrigation in the 2-cut harvest frequency. Averaged over harvest frequency and fertilizer source, soil EC was generally greater (P = 0.006) in the top 10 cm than in the 10- to 20-cm depth and was the lowest in the non-irrigated-“Alamo” and irrigated-“Cave-in-Rock” treatment combinations in the 10- to 20-cm depth interval, which did not differ (
Similar to bulk density, aggregate stability, and soil EC, numerous extractable soil nutrients were affected by multiple experimental treatment factors. Averaged over harvest frequency and fertilizer source, extractable soil Mn was greater (P = 0.026) in the irrigated-“Alamo” treatment combination in the 10- to 20-cm depth interval (230.2 kg∙ha−1) than in all other treatment combinations, which did not differ and averaged 176.6 kg∙ha−1
Soil property | Soil depth (cm) | Harvest frequency | Alamo | Cave-in-Rock | ||
---|---|---|---|---|---|---|
PL† | CF† | PL | CF | |||
AS (g∙g−1) | 0 - 10 | 1 | 0.90 bcde* | 0.91 abc | 0.87 f | 0.89 cdef |
2 | 0.89 cdef | 0.92 ab | 0.91 abc | 0.88 def | ||
10 - 20 | 1 | 0.90 bcde | 0.92 ab | 0.88 ef | 0.89 cdef | |
2 | 0.92 ab | 0.93 a | 0.90 bcde | 0.90 bcd |
†PL, poultry litter; CF, commercial fertilizer. *Means followed by different letters differ significantly at P < 0.05.
Soil property | Harvest frequency | 0 - 10 cm depth | 10 - 20 cm depth | ||
---|---|---|---|---|---|
PL† | CF† | PL | CF | ||
Cu (kg∙ha−1) | 1 | 2.9 a* | 2.0 c | 1.0 d | 0.9 de |
2 | 2.5 b | 2.01 c | 1.0 de | 0.8 e | |
EC (dS∙m−1) | 1 | 0.081 a* | 0.067 b | 0.059 c | 0.059 c |
2 | 0.068 b | 0.067 b | 0.060 bc | 0.055 c |
†PL, poultry litter; CF, commercial fertilizer. *For each soil property, means followed by different letters differ significantly at P < 0.05.
(
Averaged over irrigation, cultivar, and harvest frequency, extractable soil P, K, Mg, S, Na, and Zn contents followed similar trends related to treatment combinations and were generally greater (P < 0.04) in the top 10 cm compared to the 10- to 20-cm depth. Extractable soil P (
Averaged over irrigation, cultivar, and fertilizer source, extractable soil P, K, Mg, and Zn contents also differed between soil depths within harvest frequency. Extractable soil P (
hypothesis that harvesting once after switchgrass senescence rather than twice per year allows greater retention of extractable soil nutrients. Extractable soil P (
In addition, averaged over cultivar, harvest frequency, and soil depth, extractable soil Ca (
Averaged over cultivar, harvest frequency, and fertilizer source, extractable soil Zn content differed between irrigation treatments within soil depths. While the greatest (P = 0.025) extractable Zn content was in the irrigated-0- to 10-cm depth compared to all other treatment combinations, extractable Zn content was generally greater in the top 10 cm than in the 10- to 20-cm depth (
Averaged over irrigation and soil depth, extractable soil Cu (P = 0.009) was greatest in the “Alamo”-PL-1-cut and in the “Cave-in-Rock”-PL treatment combinations under both harvest frequencies (2.0 kg Cu ha−1) compared to all other treatment combinations. Averaged over cultivar and irrigation, extractable soil Cu content was greater (P = 0.038) in the 1-cut-PL treatment combination in the top 10 cm (2.9 kg Cu ha−1) than in all other treatment combinations, which were <2.0 kg Cu ha−1 (
Soil property | Harvest frequency | Alamo | Cave-in-Rock | ||
---|---|---|---|---|---|
PL† | CF† | PL | CF | ||
Cu (kg∙ha−1) | 1 | 2.0 a* | 1.5 b | 1.9 a | 1.5 b |
2 | 1.6 b | 1.4 b | 2.0 a | 1.4 b | |
Fe (kg∙ha−1) | 1 | 212.5 a* | 184.2 b | 193.3 ab | 189.5 b |
2 | 195.5 ab | 188.8 b | 192.6 ab | 160.9 c |
†PL, poultry litter; CF, commercial fertilizer. *For each soil property, means followed by different letters differ significantly at P < 0.05.
In contrast to the initial hypothesis that irrigation would have minimal effect on belowground switchgrass growth in the climatic region of west-central Arkansas, averaged over all other treatment factors, switchgrass root density was greater (P = 0.031) in the irrigated (2.62 kg∙m−3) than in the non-irrigated (1.65 kg∙m−3) treatment.
It was also hypothesized that increasing the harvest frequency from 1 to 2 cuts per year would significantly decrease switchgrass root density. However, results did not support this initial hypothesis as root density was also unaffected (P > 0.05) by switchgrass cultivar and fertilizer source.
Total water infiltration rates measured with the double-ring infiltrometer did not differ (P > 0.05) among any treatment combination. Total infiltration rates ranged from a low of 0.2 mm∙min−1 in the non-irrigated-“Alamo” treatment combination with either fertilizer source to a high of 2.4 mm∙min−1 in the non-irrigated-“Cave-in- Rock”-PL treatment combination and averaged 0.79 mm∙min−1 across all treatment combinations.
Unlike total infiltration rates measured with the double-ring infiltrometer, total infiltration rates measured with the mini-disk infiltrometer at a tension of −2 cm were greater (P = 0.034) in the 1-cut (33 mm∙min−1) than in the 2-cut (24 mm∙min−1) treatment. The total tension infiltration rate was unaffected (P > 0.05) by cultivar, irrigation, or fertilizer source.
1) Double-ring infiltrometer
Though total infiltration measured with the double-ring infiltrometer did not differ among experimental treatments, averaged over fertilizer sources, the slope of the linear regression characterizing the natural logarithm of the infiltration rate against the natural logarithm of the mid-point time was greatest (P = 0.004) for the “Cave-in-Rock”-2-cut and “Alamo”-1-cut treatment combinations, which did not differ (
2) Mini-disk infiltrometer
Unlike the double-ring infiltrometer, the slope of the linear regression equations characterizing the natural logarithm of the infiltration rate against the natural logarithm of the mid-point time was positive and numerically greatest (P = 0.026) for the “Alamo”-CF-2-cut and numerically smallest and negative for the “Alamo”-PL-2-cut and “Cave-in-Rock”-CF-2-cut treatment combinations, which did not differ (
Cultivar | Fertilizer source | Harvest frequency | Slope (m) | Y-intercept |
---|---|---|---|---|
Alamo | Commercial fertilizer | 1 | −0.088 ab* | 1.109 b |
2 | 0.047 a | 0.253 c | ||
Poultry litter | 1 | −0.126 abcd | 1.270 ab | |
2 | −0.212 cd | 1.266 ab | ||
Cave-in-Rock | Commercial fertilizer | 1 | 0.131 abcd | 1.190 ab |
2 | −0.196 bcd | 0.873 bc | ||
Poultry litter | 1 | −0.126 abcd | 1.852 a | |
2 | −0.106 abc | 0.909 bc |
*Means followed by different letters differ significantly at P < 0.05.
Overall, results of this study indicated treatments imposed on the switchgrass production system affected near- surface properties in the top 20 cm and surficial processes. However, treatment combinations that produced significant differences in the amount of aboveground biomass production noted previously by Jacobs and King [
Switchgrass cultivar was not expected to affect most soil properties; however, AS and silt concentration, and Na, Fe, P, and K contents differed between cultivars when averaged over all other treatment factors. In addition, clay concentration differed by depth within cultivars when averaged over irrigation, harvest frequency, and fertilizer source. While the silt and clay differences were likely residual inherent differences from before the study was initiated, and assuming similar nutrient concentrations between cultivars, greater soil extractable P and K contents in “Cave-in-Rock” treatments may have resulted from lower nutrient removal rates according to lower biomass production compared to “Alamo” treatments [
While extractable soil nutrient differences were relatively straightforward, AS depends on many factors related to soil, water, and plant relationships. Previous research in Arkansas has shown that aggregate stability in the top 10 cm is greater in undisturbed native prairie or prairie restorations than in cultivated, row-crop agroecosystems. Aggregate stability in row-crop production varied from 0.05 g∙g−1 in a non-irrigated, wheat (Triticum aestivum)-soybean (Glycine max), double-cropped system managed consistently for 10 years [
Differences in root density could influence AS, but results from this study showed that root density did not differ between cultivars. Soil organic matter (SOM) may also influence AS, but SOM was not measured in this study. The 0- to 15-cm depth interval where root density was measured in this study may not have captured enough of each cultivar’s unique rooting characteristics, which may explain why AS and not root density differed between cultivars when averaged over all other factors. Frank et al. [
Similar to cultivar effects, it was hypothesized that irrigation treatments would have little influence on soil properties. However, irrigation treatments produced significant differences in root density. Though application of irrigation to the switchgrass crop did not significantly increase biomass yields [
Averaged over all other treatment factors, AS did not differ between irrigation treatments. However, increased AS been linked to greater root density, as roots encourage the formation and stabilization of soil aggregates [
Whalen and Chung [
The dispersive effects of monovalent cations typically present in PL, such as Na, may have affected soil properties evaluated in this study. For example, irrigation of plots fertilized with PL may have caused greater dispersion by further mobilizing monovalent cations. In this study, extractable soil Na was greater in the 10- to 20-cm depth than in the top 10 cm. Results from this study also further support the conclusions of Whalen and Chung [
The application of two different fertilizer sources over a four-year period [
Poultry litter was applied at a rate of 4.5 Mg∙ha−1 and corresponding amounts of N, P, and K were applied as CF. Poultry litter contains more than just N, P, and K, and significant differences between the levels of extractable soil nutrients were apparent in this study. Only one soil nutrient tested (i.e., Mn) did not differ between fertilizer sources when averaged across all other treatment factors. Soil collected from treatment combinations fertilized with PL contained significantly greater contents of extractable soil P, K, Ca, Mg, S, Na, Fe, Zn, and Cu than CF treatment combinations. Edaño [
In addition, both soil pH and EC were significantly greater in PL than in CF treatment combinations, which were expected. Results from this study regarding soil pH changes in “Alamo” switchgrass from fertilizer sources were similar to those reported by Edaño [
The previous yield study by Jacobs and King [
Despite the increased yield for switchgrass harvested in the 2-cut system, removing greater amounts of biomass also decreased near-surface extractable soil K and Mg levels. In addition, AS and mini-disk total infiltration rates were significantly lower in the 2-cut than in the 1-cut system. In a similar study, Edaño [
The lack of significant effects due to increased harvest frequency on root density are in contrast to harvest frequency effects on aboveground biomass yield. Jacobs and King [
In general, results indicated total infiltration rates were greater under tension when measured with the mini-disk infiltrometer compared to the double-ring infiltrometer because each method measured different mechanisms of infiltration. Infiltration was only measured in the inner ring for the double-ring infiltrometer while adjacent soil was nearly saturated throughout the 20-minute measurement period. Infiltration measured with the double-ring infiltrometer measured downward or vertical water infiltration (i.e., one-dimensional water infiltration). In contrast, soil around the mini-disk infiltrometer was unsaturated throughout the 20-minute measurement period; thus infiltration was likely both vertically downward and lateral (i.e., more three-dimensional water infiltration).
Due to the differences associated with measuring infiltration in unsaturated and nearly saturated conditions, only mini-disk infiltration rates were directly affected by harvest frequency. The mini-disk infiltration rate was greater in the 1-cut than in the 2-cut treatment, which supported the hypothesis that more intensive harvest frequencies decrease water infiltration through soil micropores. Increases in wheel traffic from tractors and other harvesting equipment in 2-cut systems may not consistently increase bulk density, as previous data showed, but may reduce soil micropore abundance and volume and consequently hydraulic conductivity. Removal of the switchgrass canopy cover during the growing season (i.e., at the June harvest in the 2-cut system) likely exposed more soil than in the 1-cut treatment and may have allowed soil micropores to become sealed or clogged by temporary crusting from precipitation or irrigation.
The slope of the linear regression relating the natural logarithm of the infiltration rate and the natural logarithm of time provided additional information about what could happen to surface water (i.e., precipitation or irrigation) in each treatment combination. Large (i.e., steep) slopes represented greater surface infiltration per unit time, while smaller (i.e., flatter) slopes represented slower surface infiltration per unit time. This relationship was readily apparent when comparing the slopes averaged over fertilizer source for double-ring infiltration, which differed by cultivar within harvest frequency (
The mean double-ring infiltration rate into a silt-loam soil, which was measured in this switchgrass production study after four years of continuous and consistent management, was less than that reported by Bonin et al. [
Studies that evaluate soil aggregate stability may give ancillary information toward interpreting water infiltration rates because soil aggregate size influences soil pore space. In this study, though AS did not systematically differ between harvest frequencies, the total tension infiltration rate was greater in the 1-cut than in the 2-cut system, which supports the supposition of Edaño [
Producers and other private landowners engaged in feedstock production must make informed management decisions to maximize production while protecting soil and water resources. A first step in providing information to producers and landowners lies in pairing aboveground management strategies to maximize biomass yield, such as cultivar choice, irrigation management, harvest frequency, and choice of fertilizer, with belowground consequences.
Similar to the results of this study, Jacobs and King [
Biomass yield information coupled with soil hydraulic properties provide valuable information to landowners for making management decisions. For example, landowners may choose to harvest twice per year only if “Cave-in-Rock” is used, knowing that increased biomass yield could be balanced by the trade-off of decreased water infiltration rates. Similarly, landowners may only harvest “Alamo” once per year, as harvesting twice per year may only increase yields by 35%, while also decreasing infiltration rates. Landowners who are interested in converting highly erodible cropland to switchgrass production may choose to plant “Alamo” instead of “Cave- in-Rock” to decrease the potential for soil erosion due to more water-stable soil aggregation. Subtleties associated with these management strategies may become important if switchgrass is grown on the vast areas needed to support industrial facilities for renewable fuel production.
Results showed that four years of consistent agronomic management with various strategies to maximize switchgrass production in west-central Arkansas produced significant soil property differences, some of which were small and of limited practical use, while other differences were larger and had practical management implications. Results from this study also illuminated new opportunities for producers and landowners to customize switchgrass management systems to address specific natural resource goals. In general, “Alamo” produced greater biomass yields and greater AS, which resulted in greater depletion of extractable soil P and K contents over time compared with “Cave-in-Rock”. Irrigating switchgrass, though not currently cost-effective or recommended for marginal sites, significantly increased AS and switchgrass root densities when averaged over all other treatment factors. Harvesting switchgrass in a 2-cut system significantly decreased AS, extractable soil K and Mg contents, and total tension infiltration rates. Fertilizing switchgrass with PL rather than CF generally increased soil pH, EC, and extractable soil P, K, Ca and Mg contents, though both PL and CF appear to be viable options for landowners interested in switchgrass production.
Further investigations are needed to evaluate the effect of management strategies when no fertilizer is applied to assess soil property change if or when the bulk of fertilizers are diverted for growing food crops to feed a growing world population. More information is also needed about the fate of surface water and infiltration characteristics associated with switchgrass production strategies, as the importance of surface water and groundwater recharge only increase with time. Finally, this study demonstrated the need to enlarge the scope of discussion when assessing the feasibility and natural resource consequences of producing energy crops in the US.
The authors are grateful for the field and laboratory assistance provided by Bryan Jacobs, Eddie Pratt, Debbie Orick, Dale Goff, Michele Helton, Julie Osborne, and Taylor Adams.