Conversion of cropland to WSG such as switchgrass can increase infiltration and improve soil structure over time with measurable changes generally taking place over 6-15 years [13,14]. Changes in soil hydraulic properties under warm season bioenergy grasses will depend on grass species, soil type, climate, and management [7]. In a study of both hybrid poplar plantations and switchgrass plantings in north-Central Minnesota, Coleman et al. [15] showed that a combination of variable native soil organic carbon (SOC) levels and slow rates of SOC change made it difficult to verify soil carbon sequestration in the first 12 years of short-rotation poplar plantations. Conversion of cropland to short-rotation poplar only led to increases in soil carbon on poorer soils that were marginal for agriculture [15]. It is likely that SRWC plantations will only change SOC over multiple rotations as the influence of larger structural roots on SOC becomes more important [16].

The use of buffers can enhance environmental sustainability of row-crop systems [17,18], and may be a source of bioenergy feedstock because of the perennial characteristics of the species generally recommended for these buffers. Furthermore, while streamside and other riparian buffers may not be appropriate places for bioenergy crops because of the frequency of harvest, bioenergy plantations would likely require smaller buffers than annual crops [19].

Concern about soil erosion in biomass production systems dates back at least 25 years and is likely the greatest threat to sustainability of soil resources on which cellulosic bioenergy feedstocks generally depend [20]. Soil erosion may be reduced by as much as an order of magnitude in SRWC, compared to annual crops, although the establishment phase of the woody crops may leave considerable bare ground until a canopy and leaf litter layer are established [21]. Establishing SRWC with cover crops reduced erosion by 35­64% compared to SRWC without cover crops [22], with better erosion control for winter annual ryegrass (Lolium multiflorum) than for perennials fescue (Festuca arundinacea) or lespedeza (Lespedeza cuneata). During the first year of establishing SRWC, a poplar planting without a winter cover had higher erosion than either poplar with a fescue cover or no-till corn [21]. Nyakatawa et al. [23] found similar high erosion rates early in the establishment of sweetgum (Liquidambar styraciflua) plantations. Even in the first year of establishment, sites in the southern United States showed lower erosion from poplar plantations than from conventional tilled cotton (Gossypium hirsutum) and corn [21, 24]. After the establishment phase, well-managed WSG and SRWC species can be managed to provide year-round cover and reduce soil erosion compared to annual crops [7, 25]. As with woody crops, the establishment phase of WSG is critical to soil erosion. Management is also critical for WSG in that they should not be harvested at heights below 0.1 m in order to retain erosion control and soil building benefits [7].

There is a general consensus that conversion of cropland to perennial bioenergy crops (SRWC or perennial WSG) will result in an increase in soil carbon sequestration, but the conversion of grassland may not be as beneficial [26,27]. Soil carbon concentrations will not increase indefinitely, as eventually a new, higher carbon equilibrium will be achieved, although it is not clear how long this process will take [28]. Assessment of SOC changes under perennial bioenergy crops for a range of soils, climates, and management practices should be a research priority because of the importance of soil carbon accumulation on the potential net greenhouse gas measures of bioenergy crops [7].