Rates of nitrogen application required to achieve maximum energy efficiency for various crops: Results of a long-term experiment

A long-term experiment with four rates of mineral nitrogen (N) application (averaged across all the crops in a crop rotation: 0, 50, 100, 150 kg ha^-1 per year) was conducted on a fertile loess-derived soil in central Germany. The objectives of this study were to (i) determine the rates of mineral nitrogen N application required for maximum net energy output (energy output minus energy input), maximum energy output/input ratio, and minimum energy intensity (energy input per unit grain equivalent) for various crops in a realistic crop rotation (potatoes [Solanum tuberosum L.], winter wheat [Triticum aestivum L.], winter barley [Hordeum vulgare L.], sugar beets [Beta vulgaris L.], spring barley [Hordeum vulgare L.]): (ii) identify long-term trends (from 1968 to 2000) in the rates of mineral N application necessary to achieve the most efficient use of energy in the production of winter wheat and (iii) assess the effects of changing the system boundaries and the energy equivalents assigned to selected inputs on the energy balance by means of a sensitivity analysis. In the last two crop rotations (1989-1993 and 1994-1999), the amount of N fertilizer required to maximize net energy output of the main products (cereal grains, beet roots) increased in the order sugar beets-winter wheat-winter barley. At optimum N fertilization, the net energy output increased in the order winter barley-winter wheat-sugar beets. Averaged across the two rotations, the N fertilizer demand for a maximum output/input ratio and minimum energy intensity increased in the order sugar beets-winter wheat-winter barley. There was no clear-cut time trend in the rate of N application required to maximize grain yield and net energy output of wheat; maximum grain yield, maximum net energy output, and output/input ratio increased significantly with time, whereas the minimum energy intensity decreased over the experimental period. For all the crops, the rate of N application required for the maximum net energy output was much higher than that required for the maximum output/input ratio and minimum energy intensity.