Nitrogen, energy and land use efficiencies of miscanthus, reed canary grass and triticale as determined by the boundary line approach

The quantities and use efficiencies of nitrogen (N) fertilizer and energy input are seen as important indicators for the environmental impact of the production of energy crops. On the other hand, the high targets set in Europe for the production of biofuels will require high energy yields and efficient use of available agricultural land. The aim of this study is to describe the N, energy and land use efficiencies in relation to the N supply, for the energy crops triticale (Triticosecale Wittmack) - harvested as whole crop - reed canary grass (Phalaris arundinacea L.) and miscanthus (Miscanthus x giganteus). Field trials in Southwest Germany (48-49 degrees N latitude) were performed to measure the biomass and bioenergy yields at different N fertilizer levels. The nitrogen use efficiency (NUE), defined as the ratio of biomass yield to N supply (sum of soil NO3--N and N fertilized) and the energy use efficiency (EUE) (net energy yield/energy input), were derived from data on biomass and bioenergy yields by the boundary line approach. For all three crops, NUE and EUE decreased with increasing N fertilizer rates. NUE and EUE were at all N and energy inputs highest for miscanthus and lowest for reed canary grass. At an N supply of 100 kg ha(-1) a(-1), the NUEs of miscanthus, triticale and reed canary grass were 0.35, 0.14 and 0.11 t dry biomass/kg N, respectively. At an energy input of 10 GJ ha(-1) the EUEs for miscanthus, triticale and reed canary grass were 54, 26 and 13 GJ bioenergy per GJ energy input, respectively. The highest net energy yields (here used as indicator for the land use efficiency) of triticale and reed canary grass were harvested at the highest N fertilizer level of 140 kg N, with maximum values of 281 and 129 GJ ha(-1) a(-1), respectively. These results show that for triticale and reed canary grass, the maximization of NUE, EUE and land use efficiency are conflicting. Only for miscanthus, the N, energy and land use efficiencies were simultaneously highest at the lowest N supply level. A maximum net energy yield of 590 GJ ha(-1) a(-1) was harvested from miscanthus. It was concluded that the best way to maximize resource use efficiency in biomass production is to choose for the production of the perennial C4 crop miscanthus, at those locations that are suitable for miscanthus production. (c) 2005 Elsevier B.V. All rights reserved.