Among nutrient inputs, nitrogen is the most critical for maintaining the productivity of established switchgrass stands. Nitrogen management and the feedbacks associated with harvest management have significant consequences for biomass yield, feedstock quality, environmental impact, and system economics. Consensus regarding appropriate recommendations for nitrogen management may be harder to find, however (Parrish and Fike

2005) . The broad range of responses to applied N is a function of inherent demand and capacity for recycling, soil type and N status, precipitation and atmospheric deposition, and harvest timing. For example, in a summary of yield responses to added N (vs. a 0-N control), Brejda (2000) found biomass yield increased from 0 to 6.2 Mg ha-1.

Long-term stand sustainability will be best supported by fertility management that replaces similar amounts of N in the harvest biomass (Lemus, Parrish, and Abaye 2008). Greater N inputs are required for

biomass (or especially forage) systems that collect multiple harvests (Parrish and Fike 2005; Fike et al. 2006a, b; Guretzky et al. 2011). Given the added costs associated with such management, one end-of-season harvest is the prevailing recommendation for bioenergy cropping. However, in some cases, multiple harvests may provide value to the system as a whole (Cundiff 1996; Fike et al. 2007; Cundiff et al. 2009) by reducing logistic constraints, and we will consider this further in a subsequent section.

Compared with many other potential energy crops, switchgrass has low nutrient demand. Although N needs during the growing season may be relatively high on a mass basis, plant N concentrations decline during the growing season (Waramit et al. 2011), and N is returned to roots and rhizomes at the end of the growing season (Beaty et al. 1978; Lemus et al. 2008; Garten et al. 2011). This ability to translocate nutrients to belowground storage structures is a major component of the apparent thriftiness of many perennial, warm season grasses (e. g., see Hargrave and Seastedt 1994). End — of-season nitrogen concentrations often are in the range of 5 to 8 g kg1 for plants harvested after senescence (Madakadze et al. 1999; Fike et al. 2006a, b; Guretzky et al. 2011). Fertility practices also affect switchgrass morphology, as plants grown at a higher level of N fertility apparently conduct a greater proportion of nutrients to shoots (vs. roots) than plants grown at a lower plane of nutrition (Heggenstaller et al. 2009; Garten et al. 2011). Nitrogen- fertilized switchgrass may also have fewer tillers, particularly under one-cut management (Fike et al. 2006a; Muir et al. 2001). These changes in plant morphology may not affect biomass yields (Muir et al. 2001) but may have consequences for carbon sequestration and greenhouse gas emissions (Garten et al. 2010). The relationships of N fertility to overall system sustainability in terms of increased biomass vs. reduced soil organic carbon stocks bears further investigation (Jung and Lal 2011).

Across regions, the data regarding switchgrass N requirements—and consequent recommendations—may seem rather disparate. Some of the greatest responses to applied N have occurred in sandy soils with little nutrient retention capacity (Ma et al. 2001; Muir et al. 2001). In contrast, Stout and Jung (1995) reported little response to N for switchgrass grown on soils with high levels of N in the soil organic pool. Along with inherent soil fertility, there is increasing evidence that bacterial-based biological nitrogen fixation and plant growth stimulation occurs with switchgrass in some settings (Tjepkema 1975; Riggs et al. 2002; Ker et al. 2010; Ker et al. 2012). Such reports help to further explain the negligible responses to N often reported for switchgrass (Parrish and Fike 2005) and increase the appeal of a plant that already gets high marks for its ability to capture, sequester, and recycle N from soils.

As input costs increase, the economics of applying fertilizer nutrients may be marginal in low-value, high-volume biomass production systems. Under such circumstances, developing management strategies with alternative nutrient sources may provide an important route to the production and economic sustainability of these systems. Several researchers have reported that animal manures can support switchgrass production (Sanderson et al. 2001; Lee et al. 2007, 2009) and Lee et al. (2007) suggest they may improve stand composition, but long-term increases in soil phosphorus and other nutrients will require monitoring. Adding legumes to these systems may be another approach for reducing N input costs. However, finding compatible species that do not reduce biomass production may be a challenge in some locations (El Hadj 2000; Springer et al. 2001; Bow et al. 2008) although some researchers have reported success with this strategy (Springer et al. 2001; Bow et al. 2008).