Comparative Nutrient Use Efficiency by Candidate Biofuel Crops

Current U.S. plans for energy security rely on the conversion of large acreages from food crop production to the production of cellulosic biomass in order to produce 86 billion gallons of biofuels, thereby reducing U.S. dependence on imported oil by 25% by 2025. Additionally, lands currently considered too marginal for intensive food production may be considered suitable for biofuel production, bringing highly erodible, nutrient-poor soils currently in conservation reserve programs back into intensive agriculture. In the U.S. Midwest, cropping systems may shift from the predominant corn-soybean base to a more varied array of species, including novel perennial grasses for which little agronomic and environmental impact data exist. Sustainable biofuels production with the concomitant protection and improvement of air, soil, and water resources requires a concerted effort by the scientific community to gain knowledge regarding the comparative production potentials and environmental impacts of candidate biofuel systems.

This multi-disciplinary team has initiated a study of the most promising biofuel crop species and management systems at Purdue University’s Water Quality Field Station (WQFS) Project team expertise combined with the unique WQFS capabilities for quantifying agro-ecosystem carbon, N, and water balance are permitting a quantitative assessment of candidate system net energy balance. Our overall goal is to develop a cropping system-level analysis of the potential for miscanthus, switchgrass, maize-based, and native prairie production systems to provide renewable fuel while protecting natural resources. Our hypothesis is that biofuel cropping systems differ in total yield and yield of structural and non-structural carbohydrate pools relevant to system profitability. In addition, we hypothesize that tangible differences in the water, N, and C economies of candidate systems exist and these differences will drive changes in soil and water quality. IN-25



Justification

Current U.S. plans for energy security rely on the conversion of large acreages from food crop production to the production of cellulosic biomass (Perlack et al., 2005). To ensure the long-term sustainable production of biofuel crops, we propose to conduct comparative analyses of the productivity potential and the environmental impacts of the most promising biofuels crop species and management systems at Purdue University’s Water Quality Field Station (WQFS). Project team expertise combined with the unique WQFS capabilities for quantifying agro-ecosystem carbon (C), nitrogen (N) and water balance will permit a quantitative assessment of candidate system net energy balance.

The proposed research addressed two of seven strategic goals for Indiana agriculture. Goal 2 aims to maximize Indiana’s competitive advantage in agriculturally derived biofuels, while Goal 5 focuses on identifying diversification strategies that enhance the economic viability of producers of different sizes and areas of production. In the near term, maize will be the feedstock used for ethanol production. However, as competition increases the value of maize, and the need to use maize for human food rather than fuel grows with our every-increasing world population, the biofuels industry will evolve to alternative, non-food feedstocks including herbaceous crops grown on marginal landscapes like switchgrass and Miscanthus. This will impact Goal 5 by diversifying cropping systems and on-farm income opportunities because herbaceous plants have multiple uses in agro-ecosystems including use as biofuel feedstocks, forage for ruminant livestock and horses, and wildlife habitat.


Objectives

Our overall goal is to develop a cropping system-level analysis of the potential for Miscanthus, switchgrass, maize-based and low-input native prairie production systems to provide renewable fuel while protecting natural resources. Our hypothesis is that candidate biofuel cropping systems differ in total yield and yield of structural and non-structural carbohydrate pools that determine profitability, but that candidate systems also differ in water, N, and C economies; these differences will drive changes in soil, water and air quality that, in turn, determine the scope and nature of environmental impacts and sustainability. Specific objectives are to:

1. determine the comparative environmental impacts of switchgrass, Miscanthus, maize (grain and grain plus stover removal), and low-input big bluestem including the assessment of: a) cropping system impact on soil storage (sequestration) of C and N; b) cropping system impact on nitrate and dissolved organic carbon edge-of-field loss to water as facilitated by artificial tile drainage, and c) cropping system impact on greenhouse gas emissions.
2. determine the comparative biofuels feedstock potential of switchgrass, Miscanthus, maize (grain and grain plus stover removal), and low-input big bluestem including the assessment of: a) the quantity and quality (chemical composition) of feedstocks; b) the crop-specific nutrient use efficiency including uptake and physiological efficiency, and c) the crop-specific water balance.

Methodology

Purdue University has unique capabilities with respect to the study of agro-ecology and the environmental costs and co-benefits of highly-productive, intensive agriculture. The Water Quality Field Station (WQFS) is a highly instrumented, field facility that includes 4 replicates of 12 cropping systems. At the core of each of the 48 treatment plots (10.8 x 48 m) is a 24 m x 9 m drainage lysimeter that is structured to permit a quantitative characterization of mass loss of soil constituents to surface water. Existing WQFS instrumentation also permits characterization of methane, carbon dioxide and nitrous oxide emissions from the soil surface. From 1995 to the present, 11 treatment plots have focused on maize and soybean production systems receiving different rates and sources (inorganic and manure) of fertilizer. One treatment plot has been continuously maintained as a native prairie. In spring 2007, several treatments were identified for conversion to biofuel production systems and establishment of these systems is ongoing. For the 2008 growing season, WQFS treatments will include:
1. Low-input big bluestem: a facsimile for the native prairie community with no fertilizer inputs
2. Maize grown in rotation with soybean and fertilized according to university recommendations
3. Continuous maize fertilized according to university recommendations with no residue removal
4. Continuous maize fertilized according to university recommendations with residue removal at harvest
5. Miscanthus production using best known management practices for establishment and maintenance
6. Switchgrass production using best known management practices for establishment and maintenance
Trts. 1, 4, 5, 6 represent candidate biofuel systems, while Trts. 2, 3, and 4 are traditional food/feed systems that can also provide maize grain and soybean seed for ethanol and biodiesel, respectively. Intensive plant tissue, soil, drainage water, and surface gas flux sampling campaigns will be conducted to quantify pools and fluxes of water, C and N throughout the growing season. Several calculations will permit us to compare the production efficiencies of these biomass cropping systems in the context of the water, N, and C economies, the contrasting composition of the biomass per se, and the mass losses of C and N to water and the atmosphere. Various multivariate statistical approaches will then be applied to measured and calculated parameters to determine significant differences among candidate biofuel systems. Calculations include:

Water Use Efficiency (WUE, kg biomass/kg water). We will calculate the quantity of above-ground biomass and key biomass components (cellulose and hemicellulose) produced per unit of water consumed in evapotranspiration (ET) as :
WUE_biomass=(biomass yield in kg/ha)/(kg ET water)
WUE_cellulose=(cellulose yield in kg/ha)/(kg ET water)
WUE_{hemicellulose}=(hemicellulose yield in kg/ha)/(kg ET water)

Nitrogen Use Efficiency (NUE, kg biomass/kg N): We will calculate the quantity of above-ground biomass and key biomass components (cellulose and hemicellulose) produced per unit of N assimilated into above-ground herbage as :
NUE_biomass=(biomass yield in kg/ha)/(kg N/ha in biomass)
NUE_cellulose=(cellulose yield in kg/ha)/(kg N/ha in biomass)
NUE_{hemicellulose}=(hemicellulose yield in kg/ha)/(kg N/ha in biomass)

Radiation Use Efficiency (RUE, kg/mole PAR): We will calculate the quantity of above-ground biomass and key biomass components (cellulose and hemicellulose) produced as a function of integrated, season-long accumulation of photosynthetically active radiation (PAR):
RUE_biomass=(biomass yield in kg/ha)/(mole PAR/ha)
RUE_cellulose=(cellulose yield in kg/ha)/(mole PAR/ha)
RUE_{hemicellulose}=(hemicellulose yield in kg/ha)/(mole PAR/ha)

Daily mass of dissolved organic carbon, NO₃-N and NH₄-N losses in drainage water will be calculated as the product of the 24 hr drainage volume and flow-proportional concentration of each analyte. Chamber gas flux measurements will be converted to daily gas flux estimates using accepted estimation algorithms that account for soil temperature at the time of measurement. Seasonal cumulative emission/uptake will be estimated by linear interpolation between gas sampling dates followed by numerical integration of the area under the curves based on the trapezoidal rule. Global warming potential (GWP-C) will be calculated by adding/subtracting the total seasonal emissions of CO₂-C, CH₄-C GWP equivalents and N₂O-N GWP equivalents.