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

Background/Question/Methods

Rapid consumption of crude oil reserves have made it necessary to find methods of processing a renewable and sustainable feedstock for conversion into ethanol.  Lignocellulosic feedstocks are promising because they are typically environmentally friendly and can meet the high-yield potential necessary for ethanol production.  Alfalfa (Medicago sativa L.) has promise as a feedstock for ethanol production because of its high biomass yields, perennial-habit, relationship with nitrogen-fixing bacteria, and other co-products.  This study focused on the effects of harvest regime, irrigation, and salinity on stem lignocellulosic concentrations in alfalfa for ethanol production during the 2010 and 2011 growing seasons in Southern Minnesota.  Stem cellulose, hemicellulose, and lignin (lignocellulosic) concentrations were examined in eight alfalfa varieties 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 (0.75 dS m^-1), (2) 1.27 cm of saline (NaCl) water (5.0 dS m^-1) or (3) ambient precipitation. A fiber analyzer was used to estimate lignocellulosic concentrations by acid-detergent fiber (ADF), neutral-detergent fiber (NDF), and acid-detergent lignin (ADL) methods.  Theoretical ethanol yields were determined using cellulose and hemicelluloses (holocellulose) concentrations and fermentation assumptions based on 1000 kg of dried biomass.   

Results/Conclusions

Holocellulose concentrations were greatest during the full bud (2010) and 50% flower (2011) harvest regimes with concentrations averaging 45.50 and 45.23%, respectively.  Holocellulose to lignin ratios increased from 2010 to 2011 and averaged 2.3 to 3.1. 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 the increased lignin typical with increased growth periods of alfalfa. 

Alfalfa that received saline treatments in 2010 had 3.2 and 3.5% more holocellulose than those that were irrigated or received ambient precipitation (control), respectively.  Holocellulose concentrations between the control and irrigated treatments were not different in 2010, which was a wet year and irrigation added no supplementary benefit.  However, in 2011 plants growing in saline treatments had 1.3 and 6.1% more holocellulose than irrigated and control treatments, respectively.  The addition of irrigation or saline water increased concentrations of holocellulose during the drier 2011 season.  Lignin concentrations across all treatments were almost 23% lower during the second year of growth.  Interestingly, plants growing under saline treatments had higher holocellulose to lignin ratios (and higher theoretical ethanol yields) during both field seasons and warrants further investigation.