Biofuel Cropping System Effects on Soil Moisture and Inorganic N Leaching
Research Specialist in Agriculture
Department of Natural Resources and Environmental Sciences
Production of corn and soybeans on tile drained land in the Midwest leads to large losses of nitrate-N from fields to streams and rivers. This is a drinking water concern and is the major source of N that leads to hypoxia in the Gulf of Mexico. Utilizing perennial crops for biofuel production in tile drained regions could greatly alter water, carbon, and nitrogen budgets in ways that might improve water quality, sequester carbon, and reduce spring time flooding. Established perennial plants such as switchgrass and Miscanthus are able to take advantage of growing conditions during early spring, because large below-ground root systems enable them to quickly initiate growth and take advantage of solar radiation earlier than annual crops such as corn or soybean. Large C to N ratios in the harvested biomass also should limit the need for fertilization, particularly on our best Mollisols. Our objective was to measure the response of soil moisture and nitrate leaching from Miscanthus and switchgrass production compared to corn and soybean cultivation. Corn and soybeans followed typical east-central Illinois production methods, with corn fertilized with 180 lb N per acre as ammonium-nitrate.
During the 2005 to 2007 growing seasons, soil water content was measured two to three times per week (from May through October) at 4 inch depth intervals to a depth of 40 inches. Inorganic N fluxes (nitrate and ammonium) were assessed using resin lysimeters buried at a depth of 20 inches. May through September precipitation was 57% of average in 2005, near average in 2006, and 52% of average in 2007.
Soil water content was less in the biomass plots compared to corn or soybeans during the early growing season. At the end of the growing season, soil moisture under Miscanthus was less than all other crops. Differences were relatively small in 2005 but were larger in 2006 and 2007.
Combined nitrate and ammonium leaching (measured at the 20 inch depth) from corn or soybeans was 30 to 45 lb N/ ac/yr compared to less than 9 lb N/ac/yr from the biomass crops. Most of this difference was due to very high nitrate leaching from corn and soybeans, and minimal nitrate leaching from the perennial crops. Higher inorganic N leaching was observed for the biomass crops during 2006 and 2007, compared to 2005, primarily as ammonium. These amounts of inorganic N are much less than from corn and soybeans, and may not indicate losses from the soil profile because root systems extend below 20 inches in the biofuel crops. They may reflect some movement of N within the soil profile as the organic matter content of the soils increase, and therefore mineralization increases. Further studies with leaching measured in tiles and at deeper depths are needed to fully assess this.
The results to date suggest that widespread conversion of tile drained land from corn-soybean production to switchgrass or Miscanthus would greatly decrease inorganic N leaching losses to surface waters. Additionally, either switchgrass or Miscanthus would reduce soil moisture during the spring which could reduce flood flows during that time. The increased water use by Miscanthus late in the growing season is more likely to extend the duration and severity of low flow periods in the late summer and fall, which could have negative consequences for water supplies and aquatic organisms.