Nitrogen Fertilization Does Significantly Increase Yields of Stands of Miscanthus x giganteus and Panicum virgatum in Multiyear Trials in Illinois

The C4 perennial grasses Miscanthus x giganteus and Panicum virgatum (switchgrass) are emerging bioenergy crops. They are attractive because they are productive and recycle nutrients to the overwintering belowground rhizomes, before the dry shoots are harvested. They are therefore expected to require minimal fertilizer inputs, a desirable trait for any crop. Until now, Europe has had the only long-term trials of M. x giganteus, and these have either shown no or a small effect of nitrogen fertilization, while trials of P. virgatum in the USA have shown a clear positive effect of N fertilization. This study exploited the first long-term trials of M. x giganteus in the USA, and first side-by-side comparison with P. virgatum, to test the hypothesis that N fertilization would only improve yields of the latter. A split-plot N fertility treatment (0, 67, 134, and 202 kg(N) ha(-1)) was added to > 5-year-old replicated stands of the two crops at seven locations on contrasting soils in the US Midwest. Averaged across all locations, M. x giganteus yields increased significantly from 23.4 Mg ha(-1) with no N fertilization to 28.9 Mg ha(-1) (+25 %) at a N application rate of 202 kg ha(-1). P. virgatum also showed significant yield increases from 10.33 Mg ha(-1) at 0 kg(N) ha(-1) to 13.6 Mg ha(-1) (+32 %) at 202 kg(N) ha(-1). Both species therefore responded to N fertilization and to a similar extent. The increase per unit of added N was small compared to crops such as Zea mays and unlikely to be economically worthwhile. Nitrogen fertilization arrested most of the long-term yield decline that would otherwise have occurred in P. virgatum, but eliminated only about 40 % of the decline observed in M. x giganteus, suggesting additional causal factors for long-term yield decline in this crop. While the crops responded to nitrogen addition at some locations, they did not at others. Therefore a one-case-fits-all optimum fertilization rate cannot be prescribed.