Perennial biofuel crops reduce nitrogen losses during establishment in central Illinois

Background/Question/Methods

Current biofuel feedstock crops such as corn lead to large environmental losses of N through nitrate leaching and N₂O emissions, and require large inputs of N fertilizer. Second generation cellulosic crops have the potential to reduce these N losses, and provide even greater biomass for conversion to fuel or combustion. Our objective was to measure N losses and cycling in establishing perennial biofuel crops in central Illinois. Miscanthus, switchgrass (fertilized with 56 kg N ha^-1 yr^-1), and mixed prairie were established on typical row crop fields (Mollisols, formerly in corn, soybeans, and alfalfa) in a randomized complete block design with five replicates (one 4 ha plot with individual tile drainage systems, and four 0.7 ha plots), along with a corn-corn-soybean rotation (corn fertilized at 168 to 202 kg N ha^-1). Most measurements were conducted during the first four years of establishment and growth. Initial soil C and N pools were determined to 1 m, soil N mineralization measured using buried soil-bags, nitrate leaching at 50 cm was measured in all plots using resin lysimeters, and tile flow and nitrate concentrations measured to determine overall nitrate losses to a nearby stream. Nitrous oxide emissions were measured using vented static chambers. Harvested biomass N was measured for each crop.

Results/Conclusions

We found that perennial crops quickly reduced nitrate leaching at 50 cm as well as concentrations and loads from the tile systems (year 1 tile nitrate concentrations of 10 to 15 mg N L^-1 declined by year 4 in all three perennial crops to < 0.6 mg N L^-1, and loads of < 0.8 kg N ha^-1 yr^-1). A lag in Miscanthus establishment demonstrated that lack of plant uptake can lead to large losses of nitrate, even without fertilization. Nitrous oxides emissions were 2.6 to 7.6 kg N ha^-1 yr^-1 in corn and soybeans, but were < 1 kg N ha^-1 yr^-1 by year 4 in the perennial crops, even with fertilization of switchgrass. Overall N balances (atmospheric deposition + fertilization + soybean N₂ fixation – harvest, leaching losses, and N₂O emissions) were near zero for corn and soybeans as well as switchgrass, but were about -30 kg N ha^-1 yr^-1 for prairie and Miscanthus. This N could be from fixation or soil organic N, and needs further study. Our results demonstrate the rapid tightening of the N cycle as perennial biofuel crops establish on a rich Mollisol soil.