Crop residues as soil amendments and feedstock for bioethanol production

Traditional solid fuels account for more than 90% of the energy supply for 3 billion people in developing countries. However, liquid biofuels (e.g., ethanol) are perceived as an important alternative to fossil fuel. Global crop residue production is estimated at about 4 billion Mg for all crops and 3 billion Mg per annum for lignocellulosic residues of cereals. One Mg of corn stover can produce 280 L of ethanol, compared with 400 L from 1 Mg of corn grains; 1 Mg of biomass is also equivalent to 18.5 GJ of energy. Thus, 3 billion Mg of residues are equivalent to 840 billion L of ethanol or 56 × 109 GJ of energy. However, removal of crop residues exacerbates soil degradation, increases net emission of CO2, and aggravates food insecurity. Increasing the SOC pool by 1 Mg C ha−1 yr−1 through residue retention on soil can increase world food grain production by 24–40 million Mg yr−1, and root/tuber production by 6–11 million Mg yr−1. Thus, identifying alternate sources of biofuel feedstock (e.g., biofuel plantations, animal waste, municipal sold waste) is a high priority. Establishing biofuel plantations on agriculturally marginal or degraded lands can off-set 3.5–4 Pg C yr−1.