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Bioavailable inorganic forms of nitrogen and phosphorus in extracts of herbs, flowers and bark of medicinal plants

Abstract The concentrations of total nitrogen and phosphorus, and their water-extractable inorganic chemical forms – nitrate nitrogen and – phosphate phosphorus, were analyzed in a set of 58 samples of Polish medicinal plant materials: herbs, flowers and bark. Total and extractable inorganic phosphorus were determined spectrophotometrically by the molybdenum blue method, the total nitrogen was based on Nessler reaction, and the nitrate nitrogen on the nitration method. On a dry weight basis, the total nitrogen covered the values from 4.69 to 27.42 mg/g in herbs, from 11.10 to 40.67 mg/g in flowers and from 5.92 to 9.77 mg/g in bark samples. Total phosphorus was found from 2.70 to 10.19 mg/g in herbs, from 2.15 to 8.82 mg/g in flowers and from 0.46 to 1.30 mg/g in bark samples. On average, the nitrate nitrogen represented the fraction from 2.6% (flowers) to 15.1% (bark) of the total amount of nitrogen, whereas inorganic phosphate phosphorus was from 28.3% (herbs) to 60.2% (bark) of the total amount of this element. Correlation analysis revealed that the total N is not correlated with N–NO3 except the bark samples, but P–PO4 was positively related to the total P level, with the exception of herbs (a<0.05). Student’s t-test confirmed a statistically significant difference between the level of N, P and their water-extractable inorganic forms determined in bark with the comparison with the values found in herbs and flowers. Potentially bioavailable inorganic forms of nitrogen and phosphorus were compared with the nutritional norms of WHO/FAO and RDA, indicating that they can deliver significant amounts of N and P to humans.