Richard Lowrance1 and Adam Davis2

1 Southeast Watershed Research Laboratory, USDA Agricultural Research Service, U. S.A.

2Global Change and Photosynthesis Research Unit, USDA Agricultural Research Service, U. S.A.

16.1 Introduction

All forms of energy extraction/production/use have environmental footprints, most of which have not been thoroughly analyzed. The U. S. Energy Independence and Sustainability Act (EISA) and the European Union Renewable Energy directive establish goals for bioenergy but the EISA is unique in establishing specific sustainability goals with the regulations. The national United States bioenergy system goals, or RFS2 goals, focus on transport fuels and have numerous requirements related to sustainability. Because of the thorough environmental analysis of RFS2 that has been done by the USEPA, there is a clearer understanding of how cellulosic bioenergy crops can meet RFS2 goals. This analysis will also be relevant to other types of energy systems (non-liquid fuels), such as the use of cellulosic biomass for heating pellets. This chapter focuses on the types of systems described in Chapters 1-12 and includes an examination of woody crops and perennial herbaceous crops. Issues related to environmental sustainability of crop residue removal for cellulosic feedstocks have been covered in Chapter 7, Crop Residues. Also not addressed is the environmental sustainability of annual winter cover crops grown for cellulosic feedstocks because the expectation is that those crops would be grown on existing croplands and with only minor inputs of water, nutrients, and pesticides.

Cellulosic Energy Cropping Systems, First Edition. Edited by Douglas L. Karlen. © 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd.

Future United States bioenergy production as mandated by the RFS2 provisions of the EISA is an unique enterprise in that regulations have been developed that define the required greenhouse gas (GHG) reductions relative to gasoline and diesel fuel from fossil fuels. In the United States, this is the first national regulation to evaluate direct and indirect GHG for any product — GHG life cycle emissions. This is done through an accounting that has been developed by the USEPA and has undergone rigorous peer review [1]. For the first time, biofuels must meet new GHG reduction thresholds to qualify as “Renewable Fuel” or other EISA specified categories of fuels. In addition, only feedstocks produced from certain types of lands will be considered renewable biomass, and thus eligible to be used in production of a “renewable fuel”. In general, the feedstock/conversion/fuel systems that meet RFS2 standards will be considered environmentally sustainable, largely because of the factors taken into account in the greenhouse gas reduction analysis. To qualify as a “renewable fuel” [1], the fuel must:

“Be produced from ‘renewable biomass’ as defined in the rule and demonstrated by reporting and recordkeeping requirements of the rules and qualify based on fuel type, feedstock, and pro­duction processes specified in Section 80.1426(f) [or qualify byway of an alternative pathway petition]; “Renewable Biomass means:

1) Planted crops and crop residue harvested from existing agricultural land cleared or culti­vated prior to December 19, 2007 and that was nonforested and either actively managed or fallow on December 19, 2007.

2) Planted trees and tree residue from a tree plantation located on non-federal land (including land belonging to an Indian tribe or an Indian individual that is held in trust by the U. S. or subject to a restriction against alienation imposed by the U. S.) that was cleared at any time prior to December 19, 2007 and actively managed on December 19, 2007.

These provisions of the RFS2 rules are generally aimed at environmental sustainability and recognize that if renewable fuels are produced on lands that are cleared of forest or other native vegetation, there are impacts due to both loss of habitat and loss of ecosystem functions as well as the need to account for a large carbon debt (the carbon stored in biomass and soil, [2]).

Environmental sustainability analysis of cellulosic bioenergy necessarily models indus­tries that do not yet exist (e. g., cellulosic ethanol and renewable diesel) [1,3]. The cellulosic feedstocks assumed to meet total EISA goals of 16 Bgal (ethanol equivalent liquid fuel) by 2022 are: dedicated energy crops 7.9 Bgal; agricultural residues 5.7 Bgal; corn (Zea mays) stover 4.9 Bgal; urban waste 2.3 Bgal; sugarcane (Saccharum officinarum) bagasse 0.6 Bgal; and other sources (wheat (Triticum aestivum) residue, sweet sorghum (Sorghum bicolor) pulp, forestry biomass) 0.3 Bgal. Assuming adequate biomass availability, these projections are consistent with the “Billion Ton Study” conducted by the USDA and USDOE [3].

The environmental sustainability of feedstock production systems will depend on sev­eral factors, including their effects on GHG emissions, soil and water resources, wildlife, and whether the feedstock is a potentially invasive species or can harbor them. These direct effects are generally determined based on direct land use change from annual crops, managed perennial vegetation (e. g., perennial pastures, industrial or non-industrial for­est plantations), or other types of perennial vegetation (i. e., natural forest or grassland). Although conversion of natural grasslands or forests to bioenergy crops may occur, both the USDA regulations concerning conversion of natural grasslands and stated policies for biomass sources are meant to avoid conversion of native vegetation. Most of the direct environmental effects of cellulosic biomass production will come from existing agri­cultural land (pasture and row crop), existing forest plantations, or previously harvested forest land.

International land use change is often a large part of the GHG life cycle analysis when food crops such as corn or soybean [Glycine max (L.) Merr.] are used for biofuel. For cellulosic bioenergy crops such as switchgrass (Panicum virgatum) that are not likely to directly displace food crops, it will generally be a smaller component of the GHG life cycle analysis (LCA) because less international land conversion is forecast per unit of bioenergy produced [3]. Although indirect international land use change effects are considered to be an aspect of sustainability for bioenergy feedstocks, it will not be covered in this chapter. Rather, readers are referred to others [4-6] for different perspectives on this issue.

Direct land use effects are based on our understanding of the changes in field, landscape and watershed attributes when dedicated feedstock crops replace other land uses or covers. Direct effects (all of which can be either positive or negative) include GHGs, soil quality (including but not limited to soil carbon), water quantity and quality, invasive species, and wildlife habitat. Some of these factors can lead to direct effects on adjacent ecosystems and some can lead to larger effects at the watershed or landscape scale. The effects will also be dependent on where bioenergy crops are produced. For instance, there may be net environmental benefits (especially GHG and soil/water benefits) from conversion of cropland to warm season grasses (WSG) or to short-rotation woody crops (SRWC) but for conversion from forest, managed for saw timber production, to biomass there may not be any net benefits. The scale of benefits also depends on the land converted with the general idea being that the less productive and more marginal the land, the greater the benefit from conversion to perennial cellulosic biomass crops [7,8].

The global potential to produce bioenergy [9] indicates that as much as 59 million ha of abandoned agricultural land may be available nationwide in the United States alone [3]. There are abandoned and under-used croplands in many parts of the United States and in other agricultural regions. The western United States has areas with the largest amount of available abandoned agricultural land but in many cases these areas are not irrigated [1]. Midwestern states, including typical Corn/Soybean Belt states such as Iowa, Illinois, and Ohio, have approximately 1-2 million ha of abandoned land, all likely to be more productive than more arid areas in the West [1]. In a study of marginal lands in 10 Midwestern states, Gelfand et al. [10] estimated that about 11 million ha of marginal lands would be available for cellulosic biofuel feedstocks.