Life-Cycle Inputs for Production of Biomass
Nitrogen and phosphorus are the nutrients required in the largest amounts to sustain biomass-crop growth and development, and they have the greatest environmental impact. Nitrogen is leached below the crop root zone into subsurface tile drainage lines or groundwater, and phosphorus moves in runoff. Both nutrients cause eutrophication of water bodies, which contributes to such problems as hypoxia in the Mississippi River watershed and the hypoxic region (“dead zone”) of the Gulf of Mexico.
Nitrogen, phosphorus, and other crop-production inputs should be applied at economically optimal rates rather than at rates that will achieve the highest yields. Phosphorus and potassium rates are generally based on soil test levels that have been optimized for each state. For Iowa, application of phosphorus pentoxide (P2O5) fertilizer for corn grown in soils with optimal, low, or very low soil test ratings (26–35, 16–25, and 0–15 ppm, respectively) would be at 55, 75, or 100 lb/acre. Similarly, application of potassium oxide (K2O) rate in soils with optimal, low, or very low soil test ratings (131–170, 91–130, or less than 90 ppm, respectively) would be at 45, 90, or 130 lb/acre. For soils testing high or very high in phosphorus or potassium, fertilizer would not be recommended (Mallarino et al., 2002), assuming that corn residues are not removed.
The specific nitrogen fertilizer rate required to achieve the maximal economic net return to nitrogen (MRTN) will vary according to seasonal weather pattern, soil type, fertilizer price, management practices, and interactions among these factors. For Iowa, the MRTN for continuous corn or a corn–soybean rotation has been 175 or 125 lb/acre, respectively, for the last 10 years (Sawyer and Randall, 2008), assuming no harvest of crop residue.
Recent field studies (Karlen, 2007) indicated that nitrogen, phosphorus, and potassium removal with the cob and upper portion of the corn plant averaged 10, 2, and 13 lb/ton of dry stover, respectively. The baseline assessment is based on the assumptions that soils from which crop residues would be removed would have optimal phosphorus and potassium soil test levels and that nitrogen was being applied at the MRTN. Therefore, for a stover harvest rate of 1.5 tons/acre, the annual fertilizer requirements of nitrogen, phosphorus, and potassium would be 190, 27, and 57 lb/acre, respectively, for continuous corn or 140, 27, and 57 lb/acre for corn rotated with soybean. For 2020 projections (2.5 tons of dry stover per acre), the panel increased fertilizer input to account for nutrient removal and assumed only a slight increase (10 percent) in MRTN because of better efficiency of nitrogen use. That resulted in estimated fertilizer requirements of nitrogen, phosphorus, and potassium of 218, 29, and 69 lb/acre for continuous corn.
Inputs would be much lower for dedicated perennial biomass crops. Woody crops are rarely fertilized. Herbaceous perennial crops would be harvested when senescent, so loss of nitrogen, phosphorus, and other nutrients would be minimized because plants retranslocate such nutrients to roots in fall. The panel envisions a process in which biofuel processing facilities capture those nutrients, which can then be periodically returned to the soil. Given the ability of perennial grasses to maintain yields for many years with little or no fertilization, it might be feasible to return removed nutrients once every 3–5 years and thus reduce energy requirements for fertilizer transport and application. Commercial nitrogen fertilizer is energetically expensive but could be replaced, if needed, by growing one or more legume species with a biomass crop. Competition between the crop and the legume could be minimized, if competition occurs, by using a legume that has a different season of maximal growth from the biomass crop.
Karlen, D.L. 2007. Balancing bioenergy opportunities on your natural resources base. Paper read at Indiana Crop Advisors Conference Meeting Proceedings, December 18–19, 2007, Indianapolis.
Mallarino, A.P., D.J. Wittry, and P.A. Barbagelata. 2002. Iowa soil-test field calibration research update: Potassium and the Mehlich-3 ICP phosphorus test. North Central Extension–Industry Soil Fertility Conference 18:30-39.