A three year field experiment with zinc sulfate supplements was conducted in Moldova to study maize (Zea mays L.) nitrogen metabolism in years 13 through 15 of systematic application of mineral fertilizers ( no-fertilizer control, P60, N60K60, N60P60K60, N90P60K60, N60P90K60, kg ha-1) to carbonate chernozem soil. Zinc treatments for three years were P60Zn10, N60P60K60Zn5, N90P60K60Zn5 and N60P90K60Zn10 on half of each long-term treatment. The soil at the experiment site was a carbonate chernozem, containing: humus 4.3%, total nitrogen 0.29%, CaCO3 1.7%, plant available phosphorus and potassium averaging 0.88 and 34.5 mg/100g respectively, extractable cations Ca2+ and Mg2+ 31 and 2.9 meq/100g respectively, with pHH2O value of 7.9 at the 0-20 cm soil depth. Long term phosphorus (P60, N60P90K60) application increased available phosphorus from 0.88 to 2.7-3.22 mg/100g at the 0-20 cm soil depth, induced zinc deficiency and decreased protein synthesis in maize. The results revealed that the combined phosphorus and zinc fertilization increased total nitrogen in leaves from 201.4 to 526.6 and in stems from 91.3 to 316.2 mg plant-1 dry weight at the 8-10 leaves stages. In addition, zinc treatments compared to no zinc application increased total protein nitrogen in leaves from 131.3-166.3 to 316.9-385.9 and in stems from 42.1-73.8 to 143-203.5 mg plant-1 dry weight at the 8-10 leaves stage. Hence, the use of zinc sulfate following systematic application of phosphorus fertilizers to carbonate chernozem is essential to improving maze nitrogen metabolism and maize protein content. *The results were obtained in Moldavian Scientific-Research Institute of Soil Science and Agricultural Chemistry, Chisinau, Moldova.