Biofuel crops and the nitrogen problem in the Mississippi River basin

Background/Question/Methods

The primary focus of political policies to encourage the development of biofuel feedstock crops has been through corn and soybean row crops.  However, the traditional management of these crops in the upper Mississippi River basin (MRB) often results in large nitrate yields through tile drainage to receiving waters.  It is this riverine N that leads to hypoxia in the Gulf of Mexico. The development of cellulosic biofuel crops, such as switchgrass and miscanthus, have the potential for increased yields and reduced environmental consequences.  Consequently, more recent biofuel policies have sought to promote the establishment of cellulosic biofuel crops.  The goal of this study was to determine the most likely areas to replace corn-soybean rotations with either switchgrass or miscanthus in response to current biofuel policies and then to determine the subsequent effect of crop type on nitrate leaching in the MRB.  For this study we combined empirical data from two side by side field trials (both in central Illinois) of switchgrass, miscanthus, and traditionally managed corn-soybean rotation with a nonlinear model of county level N inputs and landscape factors to predict riverine nitrate yields.  These results were combined with a multi-market nonlinear model to simulate the U.S. agricultural and fuel sectors and the formation of market equilibrium in the commodity markets.

Results/Conclusions

In both field trials, switchgrass and miscanthus had much less nitrate loss than the traditionally managed corn/soybean rotation, once the perennial crops were established.  Nitrate loss from corn/soybean was typically 20 – 40 kg N ha^-1 yr^-1 compared to 1-8 kg N ha^-1 yr^-1 for both switchgrass and miscanthus. Depending on annual precipitation rates, the Gulf of Mexico typically receives about 1.6 million tons of nitrate-N from the Mississippi River.  Considering the effect of federal policies and subsidies on market incentives, nitrate yield to the Gulf of Mexico could be substantially reduced or barely affected depending on the location of cellulosic acreage.   While there are many different factors to consider before replacing traditional row crop agriculture with a perennial biofuel crop like switchgrass or miscanthus, these results highlight an important environmental and economic benefit.