Objectives The effects of different concentrations of Na₂SeO₃ on soil ammonium and nitrate nitrogen contents and spinach quality were studied. The possible pathways for Na₂SeO₃ to be absorbed, transported and transformed among soil-root-crop were comprehensive analyzed. It will provided reference for bio-enhanced production of selenium-rich agricultural products and regulation of selenium safety level.

Methods A pot experiment were conducted using spinach. Four treatments of control (without Na₂SeO₃) and 1, 10, 30 mg/kg Na₂SeO₃ were applied to potted soil. Soluble sugar, soluble protein, vitamin C and nitrate contents in spinach leaves were determined at 14, 28 and 42 days after planting, respectively. The contents of NO₃^–-N , NH₄⁺-N and urease activity in soil were determined.

Results At the low dosage (< 10 mg/kg) of Na₂SeO₃ treatments, the soil urease activity was activated first and then decreased to the control level during the spinach growing period, soil NO₃^–-N content showed the same trend with the enzyme activity, while NH₄⁺-N content showed the opposite trend. High dosage (30 mg/kg) Na₂SeO₃ treatment had no significant effect on soil urease activity, and all of them were at the control level. The NH₄⁺-N contents in soil decreased obviously in the later period, and the NO₃^–-N content increased obviously, which was more favorite of the N absorption by spinach. Under the same concentration, Na₂SeO₃ had different effects on the contents of nitrate, vitamin C, soluble protein and soluble sugar in spinach leaves. In the 1 mg/kg treatment groups, the nitrate content was increased significantly, the content of Vc increased first and then decreased, the content of soluble protein decreased first and then recovered, and the content of soluble sugar did not change. In the 10 mg/kg treatment groups, the content of nitrate and soluble sugar increased first and then decreased to the control level, while the content of Vc and soluble protein increased in varying degrees. In the 30 mg/kg treatment groups, the nitrate content of spinach leaves increased first and then decreased, the content of soluble protein and soluble sugar decreased, the content of Vc remained unchanged at the later stage.

Conclusions Under the experimental condition, low dosage of Na₂SeO₃ could stimulate the nitrogen supply in the early growing stage of plants, but not has significant effect on the later nutrient supply and leaf quality of spinach. Relatively high dosage could improve the availability of soil nitrogen and favorite the formation of good quality of spinach. The application rate of inorganic selenium fertilizer in selenium-enriched crop production should be determined by considering comprehensively the conversion rate of selenium in crop and the various factors affecting crop quality.