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Biofuels

Switchgrass and Soil Carbon Sequestration Response to Ammonium Nitrate, Manure, and Harvest Frequency on Conservation Reserve Program Land

Plant Science Dep., South Dakota State Univ., 1110 Rotunda Ln. N., Brookings, SD 57007, USA

^* Corresponding author (vance.owens{at}sdstate.edu)

Received for publication May 16, 2006.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Conservation Reserve Program (CRP) land on which perennial warm-season grasses are grown could be a resource for bioenergy feedstock production and C sequestration. A 4-yr field experiment was conducted to determine the response of switchgrass (Panicum virgatum L.) and soil C sequestration to N fertility and harvest frequency on switchgrass-dominated CRP land in eastern South Dakota. Soil at the site is an Egan silty clay loam. Three N rates (0, 112, and 224 kg ha^–1) were applied as NH₄NO₃ (NH₄NO₃–N) and cattle (Bos taurus L.) manure (manure-N). Switchgrass was harvested at anthesis every year (EY) or alternate years (AY) from 2001 to 2004. Soil samples were collected before starting the experiment (fall 2000) and after 4 yr (fall 2004) to determine C sequestration. Averaged across N rate, the proportion of switchgrass was higher with manure-N (64.7%) than NH₄NO₃-N (46.8%). Total (switchgrass plus other herbaceous material) biomass production tended to be higher when harvested EY (average 5.0 Mg ha^–1 yr^–1) compared with AY (average 4.0 Mg ha^–1 yr^–1). However, by 2004, the proportion of switchgrass was 75% higher in plots harvested AY compared with those harvested EY. The concentration of structural components was greater in biomass harvested AY, whereas total N and ash tended to be lower. Total-N and ash concentrations in biomass were higher with NH₄NO₃-N than manure-N. Soil C was sequestered at a rate of 2.4 ± 0.9 and 4.0 ± 1.0 Mg C ha^–1 yr^–1 at the 0- to 90-cm depth with NH₄NO₃–N and manure-N, respectively. There were no changes in soil organic C without N fertilization. Manure could be used as an alternate N source for switchgrass biomass production on CRP land with an added benefit of increased C sequestration.

Abbreviations: ADF, acid detergent fiber • ADL, acid detergent lignin • AY, alternate years • CRP, Conservation Reserve Program • DM, dry matter • EY, every year • NDF, neutral detergent fiber • SOC, soil organic carbon • TN, total N • SOC, change in SOC • 112M, 112 kg available-N ha^–1 with manure • 112N, 112 kg NH₄NO₃–N ha^–1 • 224M, 224 kg available-N ha^–1 with manure • 224N, 224 kg NH₄NO₃–N ha^–1 • 0N, 0 kg N ha^–1 (control)

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
SWITCHGRASS grown on Conservation Reserve Program (CRP) land could be used as a biomass feedstock rather than converting to traditional row crops after the contracts expire. Many of the environmental benefits associated with CRP may be lost if land is converted to traditional row crops (Gewin et al., 1999).

For sustainable switchgrass production to occur, nitrogen (N) fertility and harvest management must be understood in local and regional environments. Switchgrass biomass responded positively to N rates up to 168 kg ha^–1 in low organic matter and in low fertility soils in Texas (Muir et al., 2001), and Vogel et al. (2002) reported that each Mg of switchgrass biomass required 10 to 12 kg N ha^–1 in the midwestern USA. However, Mulkey et al. (2006) reported no benefit with N application rates above 56 kg ha^–1 on CRP lands in South Dakota.

Harvest timing and frequency for sustainable biomass production vary with cultivars and locations. Sanderson et al. (1999) suggested that a single fall harvest (mid-September) worked best for maximum ‘Alamo’ switchgrass biomass in the southcentral USA, whereas Vogel et al. (2002) indicated that for the midwestern USA, the optimal time to harvest ‘Cave-In-Rock’ switchgrass was at full panicle emergence to postanthesis developmental stages (late August). Maximum biomass yield of ‘Dacotah’ and ‘Cave-In-Rock’ switchgrass was obtained during late July through early August and September, respectively, in the northern Great Plains (Lee and Boe, 2005). They also found that there was a negative impact on switchgrass spring vigor when harvested during midsummer for 3 yr. In South Dakota, the percentage of switchgrass decreased significantly when switchgrass-dominated CRP land was harvested annually at anthesis, but stands were well maintained when harvest was delayed until a killing frost (Mulkey et al., 2006).

Research has demonstrated that livestock manure could be a good source of N fertilizer for perennial grasses (Sanderson and Jones, 1997; Sanderson et al., 2001; Cherney et al., 2002; McLaughlin et al., 2004). Because much of the switchgrass enrolled in CRP in South Dakota is relatively close to livestock operations, manure could be used to supply at least part of the recommended N. However, inappropriate application of manure may cause environmental contamination of water, air, and land (Eghball and Power, 1994). Manure could replace mineral N fertilizer in a switchgrass biomass production system and minimize possible environmental problems associated with manure application to traditional row crop systems (Sanderson et al., 2001).

Land enrolled in CRP and planted to perennial grasses improves C sequestration compared with annual row crops. Gebhart et al. (1994) observed that maximum C sequestration was 1.1 Mg C ha^–1 on CRP land planted to perennial grass during the first 5 yr of the program. Other studies have demonstrated that switchgrass grown for biomass feedstock production has the potential to substantially increase soil C levels (Garten and Wullschleger, 2000; Zan et al., 2001; Frank et al., 2004). Soil C sequestration in switchgrass managed for biomass production was comparable to or higher than that observed under CRP (Garten and Wullschleger, 2000; McLaughlin et al., 2002). Ma et al. (2000) reported that N fertilizer and harvest frequency during switchgrass establishment did not change soil organic carbon (SOC) content over a 2- to 3-yr period. However, after 10 yr, switchgrass sequestered more SOC than fallow soil, with an increase of 44.8 and 28.2% at depths of 0 to 15 cm and 15 to 30 cm, respectively.

Little is known about harvest and N management of long-term CRP switchgrass stands for biomass production and the effects of these management techniques on C sequestration. The objectives of this study were (i) to determine switchgrass biomass yield, biomass characteristics, and the proportion switchgrass in managed stands in response to N fertility (NH₄NO₃–N and manure-N) and harvest frequency and (ii) to estimate soil C sequestration during 4 yr of biomass feedstock production on long-established switchgrass-dominated CRP land in South Dakota.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The experiment was conducted on switchgrass-dominated CRP land located in Moody County, South Dakota (96°41' W, 44°10' N; 470 m elev.). The average (30-yr) annual temperature is 6.3°C, and the average annual precipitation is 602 mm. The average daily temperature during the growing season (April–September) is 10.5°C, and the maximum daily temperature is 22.2°C in July. Precipitation during the growing season (April–September) is about 75% of annual precipitation. Monthly precipitation during 2001 through 2004 is shown in Table 1. Soil at the site is an Egan silty clay loam (fine-silty, mixed, superactive, mesic Udic Haplustolls); selected soil chemical and physical properties at initiation of the research are shown in Table 2. Switchgrass (cultivar unknown) was planted at this site in 1975 and enrolled in CRP since 1990. Detailed records for years in which these stands were harvested for hay (emergency CRP release) or burned were unavailable; however, to our knowledge none of the herbage was routinely removed during the life of the stand. Existing switchgrass was mowed at a 10- to 15-cm height and removed from the site during fall 2000, and treatments were initiated in spring 2001. Phosphorus (25 kg ha^–1) was broadcast across the entire experimental area during spring 2001 to bring soil P to recommended levels of 12 to 15 mg P kg^–1 (Gerwing and Gelderman, 2002).

Biomass quality analysis was determined by the method described by Mulkey et al. (2006). Concentrations of neutral detergent fiber (NDF), acid detergent fiber (ADF), acid detergent lignin (ADL), and TN were predicted for all forage samples using near infrared reflectance spectroscopy (NIRS) (NIRS Model 5000; Foss NIRSystems, Silver Springs, MD) based on a calibration data set of 174 samples representing all harvest years (Garcia-Ciudad et al., 1993). A set of 30 samples was used for cross-validation. Calibration and validation statistics were generated using WinISI (Version 1.5) system software (Infrasoft International LLC., State College, PA). For calibration and validation samples, NDF and ADF were determined using an Ankom²⁰⁰ Fiber Analyzer (ANKOM Technology Corp., Fairport, NY), ADL was determined with a Daisy Incubator II Digestor (ANKOM Technology Corp., Fairport, NY), and TN was quantified using a Vario Max CNS elemental analyzer (Elementar Instrument, Mt. Laurel, NJ). Ash concentrations were determined using the methods described by Undersander et al. (1993).

A hydraulic soil probe (6.6 cm internal diameter) was used to collect soil samples at initiation and completion of the research in November 2000 and October 2004, respectively. Four random cores were collected from each plot to a depth of 90 cm. Each core was subdivided into depth increments of 0 to 5, 5 to 10, 10 to 20, 20 to 30, 30 to 60, and 60 to 90 cm, after which soil from each of the four cores at each depth was composited for analysis. Surface residue was removed before sampling. Soil samples were initially sieved to pass an 8-mm screen and dried in a forced-air oven at 40°C until consistent mass was attained. Visible plant residue and roots were removed before drying. Dried soil samples were ground to pass a 2-mm screen for chemical analysis. A portion of the moist soil sample was dried at 105°C for 24 h to measure field moisture content and bulk density.

Total soil C and N were determined by dry combustion method in a Vario Max CNS analyzer (Elementar Instrument, Mt. Laurel, NJ). Soil was analyzed for inorganic C using a modified pressure-calcimeter method (Sherrod et al., 2002) when soil pH was higher than 6.5. Soil organic C was calculated by subtraction of inorganic C from total C. Soil organic C concentration (g C kg^–1) was converted to SOC content with depth (Mg C ha^–1 within specified depth) using measured soil bulk density.

Statistical Analysis
Data were analyzed using JMP software (SAS Institute, Cary, NC). Biomass yield, chemical characteristics, and species composition data were analyzed separately by year and subjected to ANOVA for a randomized complete-block design. Nitrogen fertility and harvest frequency were considered fixed. Biomass production data from EY plots were combined across 2-yr cycles (2001–2002 and 2003–2004) to compare with data from AY plots (harvested 2002 and 2004 only). In 2001 and 2003, species composition and quality data were analyzed for EY plots. In 2002 and 2004, species composition and quality data were analyzed for EY and AY plots. Species composition data were arcsine transformed before analysis to ensure a normal distribution (Gomez and Gomez, 1984). A LSD was used to separate means among N fertility and harvest timing treatments when the appropriate F test was significant (P = 0.05). Orthogonal contrast statements were used to compare means of selected N treatments.

Changes in SOC (SOC) content during 4 yr of biomass production were calculated based on the difference between initial and post-treatment values. Soil C sequestration was determined using a t test by testing SOC compared with zero. For treatment effects, SOC data were analyzed separately by depth and subjected to ANOVA. A LSD was used to separate means among N fertility treatments when the appropriate F test was significant (P = 0.05). Orthogonal contrast statements were used to compare means of selected N treatments.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Species Composition
Source of N affected the proportion of switchgrass in the stand in three of 4 yr and grassy weeds in two of 4 yr. From 2002 to 2004, the percentage switchgrass was higher with manure-N (61.6–70.4%) than with NH₄NO₃–N (35.9–54.4%) (Table 4). By 2004, the proportion of switchgrass in all treatments except the unfertilized control were numerically lower than in 2001; however, plots to which manure was applied maintained switchgrass proportions similar to the control. Mulkey et al. (2006) demonstrated at this same location that harvesting at anthesis was much more detrimental to switchgrass than harvesting after a killing frost. Applying manure-N rather than NH₄NO₃–N seemed to mitigate the negative effects of an annual anthesis harvest.

There was a harvest frequency effect on switchgrass percentage in 2002 and 2004 and on grassy weeds in 2004 (Table 4). In both years, AY-harvested plots maintained higher switchgrass percentages (65.1 and 69.1% in 2002 and 2004, respectively) than EY-harvested plots (54.1 and 39.6% in 2002 and 2004, respectively). In South Dakota, it is recommended that switchgrass be harvested for biomass after or near a killing frost in the fall to help maintain stands (Lee and Boe, 2005; Mulkey et al., 2006). If an earlier harvest (e.g., at anthesis) is desired for biomass production, it is apparent from these results that harvesting in alternate years would be better for stand longevity.

Yield
There was a N fertility effect on total production during both 2-yr cycles and on switchgrass production during the 2001–2002 cycle. Compared with the unfertilized control, total biomass, which included all desirable and undesirable species, increased with N application during both 2-yr cycles (Table 5). Differences in total yield during either 2-yr cycle were not different among plots to which N fertilizer or manure had been applied; however, switchgrass biomass tended to be higher with manure than with NH₄NO₃ during both 2-yr cycles (P = 0.079 in 2001–2002 and P = 0.063 in 2003–2004). The apparent inconsistency between total and switchgrass biomass response to N source is likely related to the increased proportion of grassy weeds in NH₄NO₃–fertilized compared with manure-fertilized plots (Table 4). Because nitrogen from manure becomes available for plant growth more gradually than from NH₄NO₃–N (Eghball and Power, 1999a), it may be more readily used by switchgrass during hot summer months when it is rapidly growing. By comparison, rapidly available NH₄NO₃ may favor growth of early season grassy weeds. Eghball and Power (1999a) determined that approximately 38% of the N in cattle manure became available for corn growth in the year of application. Other researchers have demonstrated the benefits of manure as a N source for perennial grass production (Sanderson and Jones, 1997; Sanderson et al., 2001; Cherney et al., 2002; McLaughlin et al., 2004).

Chemical Characteristics
Nitrogen application did not consistently affect biomass chemical constituents. There was a N fertility effect for NDF and ADF in 2003 (EY plots only) and for ADF in 2004 (EY and AY plots), but there was no effect on ADL in any year. Values for NDF, ADF, and ADL ranged from 641 to 724 g kg^–1 DM, 346 to 435 g kg^–1 DM, and 29.0 to 48.7 g kg^–1 DM, respectively, across years and harvest frequency (Table 6). Average NDF, ADF, and ADL concentrations were lower than the values of 755, 434, and 63 g kg^–1, respectively, reported by Lemus et al. (2002) for switchgrass harvested in late autumn in Iowa. The lower concentration of lignocellulose was likely a result of harvesting at a less advanced stage of maturity (anthesis). Sanderson et al. (1999) observed an increase in NDF concentration of 29 to 34 g kg^–1 in switchgrass between September and November.

With the exception of 2001, TN was higher in biomass to which both rates of N were applied (Table 7). The concentration of TN in harvested biomass was lower in manure- than in NH₄NO₃–treated plots in 2003 (P < 0.05) and 2004 (P < 0.05) and tended to be lower in 2001 (P = 0.055). There are probably two major reasons for the reduced plant N concentrations in biomass to which manure was applied. First, N from manure is less readily available (Eghball and Power, 1999a); thus, N uptake may have been lower in plots to which manure had been applied. Second, grassy weeds, primarily Setaria spp. and Bromus inermis L., generally represented a higher proportion of the total biomass in NH₄NO₃–treated than manure-treated plots, which may have resulted in increased TN concentrations in harvested biomass. Cherney et al. (2002) found that shoots of green foxtail [Setaria viridis (L.) Beauv.] had lower TN concentrations when fertilized with cattle manure than with equivalent amounts of N fertilizer.

Ash concentrations were lower in biomass fertilized with manure compared with NH₄NO₃ in 2001 and 2003 when only EY plots were harvested (Table 7). Furthermore, by 2004, ash was higher in EY- than in AY-harvested biomass. The reason for these differences is not clear. However, it may be related to the higher proportion of annual grassy weeds in NH₄NO₃–fertilized plots and in those harvested EY. Because a serrated knife was used to clip quadrat samples and because annual grassy weed roots are shallow, it is possible that soil particles contaminated EY more than AY herbage. Because ash represents an anti-quality factor in some conversion processes (Wiselogel et al., 1996), the use of cattle manure as a N source should be further explored to verify if it has a consistent effect on ash concentration.

Management Effects on Changes in Soil Organic Carbon and Carbon Sequestration
Initial SOC contents in the soil profile of switchgrass CRP land are shown in Table 2. There was no N rate x harvest frequency interaction or harvest frequency effect on net SOC at 0- to 90-cm cumulative depth. Soil organic C increased with N application compared with the control at 0- to 90-cm cumulative depth; however, SOC was affected more by N source (P = 0.078 for NH₄NO₃–N versus manure-N) than by N rate (P > 0.475) (Table 8). At the 0- to 30-cm cumulative depth, manure application consistently resulted in higher positive SOC values than in plots receiving NH₄NO₃ or the 0N control. Within this depth, SOC for the 224M treatment was approximately twice that of the 112M treatment. This indicated that increased SOC in the surface 30-cm was a result of additional C input from manure. Approximately 39% of the C from manure was retained in the top 30-cm. Similar results were observed by Gregorich et al. (1998) on corn to which manure was applied in the spring. They reported that approximately 50 to 60% of the C added in manure was lost during the growing season. Mikha and Rice (2004) reported a significant increase in SOC with manure compared with NH₄NO₃ after 10 yr of continuous corn. According to Grant et al. (2001), 23% of C applied in manure was stabilized as long-term C during 70 yr of manure application in a forage rotation system.

View this table: [in this window] [in a new window]   Table 8. Net changes in soil organic carbon content (SOC) in response to N fertility on switchgrass Conservation Reserve Program land from 2001 to 2004 at Moody Co., South Dakota. Values are averaged across harvest frequency.

Nitrogen application, as NH₄NO₃ or manure, had a greater impact on C sequestration from 0- to 5-cm and 30- to 90-cm depths than from 5- to 30-cm. The changes in SOC from 0- to 5-cm occurred only in plots to which manure was applied. The relatively small changes in SOC from 0- to 30-cm depths and large changes from 30- to 90-cm depths might be due to the fact that initial SOC from 0- to 30-cm depth (3.04 Mg ha^–1 cm^–1) was much higher compared with that from 30- to 90-cm (1.48 Mg ha^–1 cm^–1). Similar results were observed in switchgrass land managed for biomass production at Mandan, North Dakota (Frank et al., 2004). These results indicate that N fertilization is an important factor for C sequestration at the subsurface and for switchgrass biomass production.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Switchgrass grown on CRP land has potential as a bioenergy feedstock and as a source of C sequestration in South Dakota. Harvest and N management were important in ensuring sustainable biomass production and C sequestration. The importance of proper management was particularly apparent in this study because annual harvesting of plots receiving >112 kg NH₄NO₃–N ha^–1 resulted in a decrease in the proportion of switchgrass in the stand. However, manure exhibited good potential as a N source for maintaining switchgrass production and the proportion of switchgrass in the stand. If a summer harvest is desired, harvesting in alternate years would be the best option to maintain switchgrass stand health. Nitrogen fertilization with NH₄NO₃ or manure increased C sequestration, particularly at depths of 30 to 90 cm, and additional C input with manure application also increased C sequestration at the depth of 0 to 5 cm.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
This research was funded in part by the South Dakota Agric. Exp. Stn., the U.S. Dep. of Energy through contract DE-FC36-02G012028, A000 with the Great Plains Institute for Sustainable Development, Minneapolis, MN, and the U.S. Dep. of Energy through contract DE-A105-900R2194 with Oak Ridge National Laboratory (ORNL). ORNL is managed by UT-Battelle, LLC, for the U.S. Dep. of Energy under contract DE-AC05-00OR22725. South Dakota Agric. Exp. Stn. Journal Series no. 3586.

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