9 In this paper, a new spectrofluorimetric method for the determination of nitrate in aqueous 10 compost extracts is presented. The microplate procedure is based on the reduction of nitrate 11 to ammonium by means of Zn powder under acidic conditions and the following derivatization 12 of ammonium with o-phtaladehyde (OPA) and N-acetylcysteine (NAC) to give a fluorescent 13 derivative. Optimization work allowed performing the reduction in 30 min on a small sample 14 volume (150 µL) and using a little quantity of metal (20 mg). The use of strong acids is also 15 avoided. Excellent limit of detection was achieved (1.3 µM = 0.08 mg.L-1 NO3-) along with good 16 precision levels (4.2 and 14.3 % for 50 and 5 µM, respectively) and a satisfactory linear 17 dynamic range (4-100 µM). A good agreement between data coming from our method and 18 from ion-exchange chromatography was found on real compost samples, indicating a good 19 level of accuracy. 20 21 22