Long-term yield potential of switchgrass-for-biofuel systems
Introduction
Switchgrass (Panicum virgatum L.) is a potential biofuel feedstock with promise for production across diverse climates in North America [1], [2], [3] and elsewhere [4], [5]. Management factors, such as fertility programs and harvest frequency, are of interest in biomass production systems because they have implications for stand survival, productivity, feedstock quality, and economic returns. Moreover, these factors can presumably interact with soil and climate to affect short- and long-term production potential.
Genotype×environment interactions play a large role in switchgrass production. Hopkins et al. [6] reported significant variation among switchgrass cultivars in date of heading and yield at heading, with significant genotype×environment interactions for these traits. They also noted that early heading was associated with lower yields. Subsequent research [7] has shown that the latitude of origin of a cultivar has a dramatic effect on its production at different latitudes.
Both soil type [1] and soil acidity [8] have been discounted as important factors for switchgrass production. However, soil moisture and water-holding capacity can be important factors in yield, with the species being more sensitive to moisture deficit than some other warm-season grasses [9]. Importance of moisture for switchgrass production in Texas was noted by Muir et al. [10], who reported that drought severely impacted yields and stands. Soil moisture effects may interact with other management factors such as cutting frequency or N fertility. Fike et al. [11] reported a positive response to moisture for early- season production. However, under a one-cut system, no response of yield to early- season rainfall was apparent. Stout et al. [12] reported that N was the primary factor limiting production of switchgrass in Pennsylvania and noted that, when rainfall was limiting, water-holding capacity of the soil controlled yield. Vogel et al. [3] tested responses to N rate and reported a 28% increase in yield as N application increased from 60 to 120 kg ha−1. However, the increase differed by site (about 12% at Mead, NE, vs. about 44% at Ames, IA), and the authors suggested the greater response to N at Ames, IA, was likely due to greater rainfall at that site.
Several other workers have also investigated biomass yield in response to N fertilization rate. In Texas, Muir et al. [10] reported increased yield of Alamo switchgrass (a lowland cultivar of southern origin) at N rates up to 224 kg ha−1. In a season of higher-than-normal rainfall, production was maximized at 168 kg N ha−1. Thomason et al. [13] applied N at up to 896 kg ha−1 for the northern lowland cultivar, Kanlow, but found only a minimal response. Yield maxima were typically attained at 448 kg N ha−1, but production was affected more by harvest frequency than by fertility. Vogel et al. [3] tested N at up to 300 kg ha−1 for the southern upland cultivar, Cave-in-Rock. They reported maximum yields at 120 kg N ha−1.
Because of the inconsistent reports on responses to N and cutting frequency, we decided to test the productivity of higher and lower management inputs within an existing, long-term field study. Specifically, we compared production of four switchgrass cultivars in response to a one-harvest system fertilized with 50 kg N ha−1 vs. a two-harvest system fertilized with 100 kg N ha−1. Further objectives were to determine impact of management on N removal by switchgrass, and, by looking at 6- to 9-yr-old stands, to get some sense of how truly “perennial” switchgrass plantings may be in a biomass production system.
Section snippets
Methods
A 10-yr study of switchgrass-for-biomass was begun in 1992 in the upper southeastern USA. Plots of four switchgrass cultivars were established in 1992 at eight sites located in Kentucky, North Carolina, Tennessee, Virginia, and West Virginia. (The Kentucky site was replanted in 1993.) The cultivars included two upland (Cave-in-Rock and Shelter) and two lowland (Alamo and Kanlow) cultivars. Subplots of each cultivar were harvested once (late fall) or twice (midsummer and late fall) per year
Results and discussion
Biomass yields (Fig. 1) in harvests from 1999 to 2001 were obtained from the two management systems—designated for simplicity as “higher” or “lower” input. Yields were affected by a year×site×management×cultivar interaction (). A year×management interaction was primarily due to differences in magnitude of response. Thus, responses within individual years will not be discussed unless atypical of production patterns over the course of the study. Average biomass yield across all factors
Conclusions
With proper management, switchgrass-for-biomass can be a truly perennial crop. Over a 10-yr study, we have seen no tendency for yields to decline in switchgrass managed for biomass production. Of the cultivars tested, Kanlow appeared the most robust across the region under lower inputs. In mature switchgrass stands, biomass yields should be maintainable at around 14 Mg ha−1 across the region. However, yields can be much greater with appropriate site, cultivar, and management combinations. Issues
Acknowledgments
This research was supported in part by the US Department of Energy's Biofuels Feedstock Development Program under contract ORNL/SUB-03-19XSY163/01 to Oak Ridge National Laboratory, managed at the time by University of Tennessee-Battelle LLC. Our thanks to Dr. Dan Ward, Statistical Consulting Services, VA-MD Regional College of Veterinary Medicine for help with statistical analysis. We also thank Dr. David Timothy of North Carolina State University for the provision of seeds from his breeder's
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