The effects of harvest regime, irrigation, and salinity on stem lignocellulose concentrations in alfalfa (Medicago sativa L.)

Alfalfa (Medicago sativa L.) is a potential candidate for cellulosic ethanol production due to its high biomass, perennial-habit, relationship with nitrogen-fixing bacteria, and other co-products. We examined the effects of harvest regime, irrigation, and salinity on stem lignocellulose concentrations in alfalfa during the 2010 and 2011 growing seasons in Southern Minnesota. Stem cellulose, hemicellulose and lignin concentrations, and theoretical ethanol yields were examined in eight alfalfa cultivars with full-bud and 50%-flower harvest regimes, irrigation, and salinity as applied treatments. Plants received weekly applications of (1) 1.27 cm of well water ("fresh water;" 0.75 dS m−1), (2) 1.27 cm of saline water (NaCl; "brackish water"; 5.0 dS m−1) or (3) ambient precipitation ("rainfed"). Holocellulose concentrations reached the highest values during the full bud (2010) and 50% flower (2011) harvest regimes with concentrations averaging 45%. Theoretical ethanol yields were generally higher for the 50%-flower harvest regime, suggesting the longer growth period increased holocellulose concentrations while not being hindered by more lignin in older stems. Alfalfa growing under brackish-water treatments had 1.3–6.1% more holocellulose than those receiving irrigation or ambient precipitation over two growing seasons. Lignin concentrations across all treatments were almost 23% lower during the second growing season. Interestingly, plants growing under brackish-water treatments had higher holocellulose to lignin ratios and higher theoretical ethanol yields during both field seasons suggesting that moderate levels of salt may stimulate holocellulose concentrations.