Isotopic model (NISOTOP) used to investigate N-15-Urea transformations in the presence of phenanthrene, chrysene and benzo(a)pyrene in a soil-plant system

An isotopic model (NISOTOP) has been developed to investigate the effect of the addition to soil of xenobiotics on urea hydrolysis, N mineralization a nd immobilization, nitrification and plant uptake of nitrogen in a soil-pla nt system, after addition of N-15 enriched compounds. The rationale of the model follows from the errors in % N-15 abundance (N-15(D)) and N concentra tion (C-N) determinations which cause high variability coefficients in the calculation of the amount of nitrogen present in the different compounds de rived from the added N-15 enriched urea. The extent of these errors, beside s depending on C-N and N-15(D) errors, will also depend on natural N-15 and N-15 of the added compound, and therefore on the experimental conditions. The model is described by 18 first-order differential equations and is nume rically solved by Euler's method with a time increment of 0.01 day. As an i llustration, the model is applied to the effect of phenanthrene, chrysene a nd benzo(a)pyrene to a soil-plant system, following the addition of N-15-ur ea. These compounds have been chosen as examples of molecules having 3, 4 a nd 5 fused aromatic rings and are hereafter collectively referred to as PAH s. PAHs at the rate of 2 mg kg(-1) soil and N-15-urea at the rate of 166.7 mg N kg(-1) soil were added to wheat pots. At harvesting (after 14 days fro m plantation) the dry matter yield, the total N content and the N concentra tion of the wheat seedlings were not statistically affected by addition of the PAHs (P = 0.05). The efficiency of N uptake, that is the percentage of fertilizer taken up by the plants at harvesting in the absence of PAHs was 47.3%, while it was 11.7, 15.2 and 14.8% in the presence of phenanthrene, c hrysene and benzo(a)pyrene, respectively. The computation of the first-orde r rate constants of the N transformations showed that N mineralization, nit rification and N-uptake were affected by the addition of phenanthrene, chry sene and benzo(a)pyrene, whilst benzo(a)pyrene inhibited urea hydrolysis mo re than phenanthrene and chrysene.