AbstractDepending on the applied concentration, nitrate can either stimulate or inhibit germination of dormant seeds ofSinapis arvensisL. (wild mustard). Seed NO−3levels that corresponded to the maximal germination frequencies recorded, ranged from 0.3 to 4.4 nmol seed−1for applied NO−3concentrations between 2.5 and 20 mol m−3. Germination was significantly lower in seeds containing more than 5 nmol NO−3. Although the presence of NO−3within the seed was required to promote germination, seed NO−3levels were 5 to 15 times less than levels calculated from the volume of solution taken up by the seeds. Seed NO−3levels also responded in a passive fashion to the external NO3−concentration when the seeds were incubated on filter paper or in soil. In a representative soil containing 26 mg NO−3‐N per kg dry weight and 8 to 16% water by weight, the amount of NO−3taken up by the seeds reached inhibitory levels. Our results suggest that levels of inorganic nitrogen (NO−3, NH+4) in managed soils may play an important role in regulating germination o