Yields of Annual and Perennial Energy Crops in a 12-year Field Trial

To find an energy cropping system with low input and high productivity, a 12-yr field trial was conducted in Southwest Germany with perennial crops, and monocropping or rotation of annual crops. The perennials were willow (Salix schwerinii E. Wolf x viminalis L.) short rotation coppice, miscanthus (Miscanthus x giganteus Greef et Deu.), and switchgrass (Panicum virgatum L.). The monocropped annual was maize (Zea mays L.), and the rotation was oilseed rape (Brassica napus L. ssp. oleifera)-wheat (Triticum aestivum L.)-triticale (Triticale x triticosecale Wittmack). The rotation was split into tillage with moldboard plow or no-till. These systems were implemented with three N fertilization levels. Annual yield trend in years, accumulated yields of biomass, and gross energy were compared across N levels between perennials and annuals, and between tillage managements. Among these cropping systems, maize (18.5 Mg ha(-1) yr(-1)) and miscanthus (18.3 Mg ha(-1) yr(-1)) were most productive. Without N fertilization, miscanthus was most productive (13.6 Mg ha(-1) yr(-1)). In the long run, N fertilization significantly increased the yields of all systems. The long-term yield trends of the perennials were relatively stable, while the annuals without N fertilization showed a prevailing trend of yield decrease. No-till did not significantly lower the yield compared to plowing (8.7 vs. 9.3 Mg ha(-1) yr(-1)). Generally, the perennial systems produced higher gross energy yield compared to the annual systems under comparable N fertilizations. Particularly miscanthus gained high yields even with moderate to no N inputs.