Ability of the SUNDIAL model to simulate the short-term dynamics of N-15 applied to winter wheat and oilseed rape

Labelling using the stable 15 isotope of nitrogen allows a close monitoring of the fate of the fertilizer applied to arable crops. Because N-15 data g ive direct estimates of nitrogen transformation rates, they also provide mo re stringent tests for N models than those based on bulk inorganic N dynami cs. They may therefore point at flaws in models that had previously gone un noticed, especially if N-15 was monitored on short time steps which capture even rapid processes like nitrification. Here we tested the simple, process-based model SUNDIAL on two such data set s obtained in Northern France under winter wheat and winter rape crops rece iving various doses and forms of fertilizer N. In both experiments, micropl ots (approximate to 1 ml in size) within larger blocks were dressed with 2. 0 atom-% enriched labelled N-15, as urea, or ammonium-nitrate as NH4+-(NO3- )-N-15 or (NH4+)-N-15-NO3-. Replicate micro-plots were subsequently sampled on four occasions after fertilizer application, and N-15 enrichment was mo nitored in plant roots and tops, and at several depths in the soil in inorg anic and organic forms. Comparison between observed and simulated data showed that, shortly after a pplication, SUNDIAL either underestimated (rapeseed) or overestimated (whea t) the rates of crop uptake, Also, the gradual incorporation of N-15 into s oil organic matter was too quick in autumn and too slow in spring under the rapeseed crop. The simulation of the rapid depletion of the labelled soil inorganic N pool was correct under wheat, whereas under rape, SUNDIAL predi cted an accumulation of nitrate which was not observed. After a longer time interval (1-2 months), the simulated and observed amoun ts of fertilizer-derived N in the crop and in the soil became more comparab le. However, SUNDIAL only accounted for part of the unrecovered labelled N. Additional measurements indicated that denitrification and ammonia volatil ization were responsible for most of the losses, with discrepancies occurri ng because SUNDIAL failed to volatilize ammonia after fertilizer spreading. The other major source of error lay in the simulation of crop demand for n itrogen, which may be improved based on sounder ecophysiological concepts, such as that of a regulation of plant uptake by shoot biomass.