Data of a 32-year field experiment (1967-1998), conducted on a fertile sandy loess in the Hercynian dry region of central Germany, were used to (i) demonstrate the methodological basis of energy balancing in a long-term trial and (ii) identify time trends of various energetic parameters, as affected by the rate and form of nitrogen (N) application. Of the 16 fertilizer regimes, 5 were selected to represent the broad range of N supply conditions: no N, mineral N only, farmyard manure N only, high input of both mineral and farmyard manure N, and moderate input of the two forms of N. The crop rotation included various cereals, sugar beets, and potatoes. In calculating the energy balances, changes in soil and crop husbandry practices over the past decades were taken into account. The input of energy associated with mineral N fertilization declined markedly with duration of the experiment, whereas the input of energy associated with chemical plant protection increased. The input of energy was highly variable; it ranged from 8.9 to 36.9 GJ ha(-1) per year in the last crop rotation, depending on the N regime and the crop. Because of the high soil fertility, the average biomass yield of all the crops grown within a rotation was as high per year. On the fertilized plots, net as 13.5 t dry matter (DM) ha(-1) per year; the output of energy was as high as 215 GJ ha(-1) energy output (energy content of the grains minus energy input) and energy utilization improved with time. Winter wheat, fertilized with moderate amounts of mineral and farmyard manure N, showed an increase in net energy output of 86% from 1972 to 1995. During the same period, the energy intensity (input of fossil energy per grain equivalent) declined by 45%, and the output/input ratio increased by 67%. In part, these trends can be attributed to the higher grain yield (+59%). Energy output and net energy output are the crucial parameters when the demand for plant products cannot be met because of the limited area for growing crops. Energy intensity and energy output/input ratio are integrative indicators of the environmental effects of crop production, which can be used to formulate recommendations for fertilization, which are optimum as far as the environment is concerned. (C) 2001 Elsevier Science B.V. All rights reserved.
