MODELING NITROGEN CYCLING IN AN ECOLOGICAL CROP-ROTATION AN EXPLORATIVE TRIAL

A crop rotation representative of a typical Danish dairy farm was esta blished at the Taastrup field station in 1988. Detailed information of the manuring, nitrogen fixation, harvested dry matter and harvested n itrogen was available for the period 1988-1992, indicating that the ha rvested nitrogen almost exactly countered the input, including the bio logically fixed nitrogen. As no measurements of N-losses were availabl e a deterministic model (DAISY) was applied in order to obtain an esti mate. Dry matter harvest (R=0.9) and N harvest (R=0.93) were simulated adequately, and only in the case of spring barley the model slightly overpredicted in a consistent manner, indicating that the use of N as delimiter of crop production in this ecological crop rotation was real istic. Considerable leaching losses of N (as high as 100-180 kg ha(-1) yr(-1)) were predicted in connection with the application of 50 tonne s of solid manure to bare soil in winter time preceeding the sowing of fodderbeet. Thus the model interpretation of the available data indic ated that the apparent balance between harvested nitrogen and input ni trogen was nonsustainable and achieved by a depletion of soil organic nitrogen in the order of 40 kg N ha(-1) yr(-1). Strategies for achievi ng sustainability are discussed in the light of the model results.