Evaluating a crop nitrogen simulation model, N_ABLE, using a field experiment with lettuce

A field experiment with lettuce was carried out to evaluate the simulation model, N_ABLE, which has been widely used to predict soil mineral nitrogen requirements and potential leaching hazards for vegetable and arable crops in England and parts of Western Europe. Plant and soil were sampled regular ly and dry weight (W), percent N in dry matter and soil mineral N (soil-N) were measured. Measured W and soil-N were compared with data simulated usin g N_ABLE both during growth and at final harvest. Dry weight followed an as ymmetrical S-shaped curve when the growth period was either 57 or 61 days f or all N levels. This implies that N_ABLE, which assumes a J-shaped growth curve, can only be used in the first three-quarters of the growing period. Simulated soil-N in the 0-30 cm layer corresponded well with measured value s throughout the experiment when parameters for the recovery of soil minera l N (REC) and mineralisation rate of soil organic-N (NR) were set at 0.70 ( i.e. 70%) and 0.86 kg ha(-1) d(-1) respectively, both calculated from field data, and were higher than default values. For longer periods of growth, t he best fit was obtained using a modified asymmetrical S-shaped growth curv e equation dW/dT = k(2)W G(f) G(k) /(1+W), where k(2) is a growth rate coef ficient, G (f) (less than or equal to 1) is a correction coefficient to all ow for any restriction in growth rate caused by sub-optimal%N in the crop a nd G(k)=(W/W)(n) is another correction coefficient to adjust the growth rat e which is decreased caused by genetic or other reasons in the later part o f the growth period. The S-shaped equation was examined by a lack of fit te st, and the results showed that the residual errors (SSR=Sigma( y-x)(2), wh ere x=simulated values, y=measured values) were not significantly different from experimental error, indicating that the S-shaped equation gave a good description of growth for the different N levels through the growth period s.