Effect of application form of nitrogen on oxalate accumulation and mineral uptake by napiergrass (Pennisetum purpureum)

Oxalate occurs naturally in forage species, but excess levels of soluble oxalate adversely affect their nutritional quality as animal feed. Although induction of oxalate in plants has been associated with nitrate application, relatively little is known about oxalate in forage grasses. In the present study, we investigated oxalate accumulation and mineral uptake by napiergrass (Pennisetum purpureum, cv. dwarf-late) cultured in complete nutrient solution containing either solely nitrate or mainly ammonium as the main form of nitrogen. Total (shoot and root) yield of plants did not differ significantly between the nitrate and ammonium treatments. By contrast, the nitrogen form significantly affected oxalate accumulation and mineral uptake by plants. Nitrate application resulted in higher concentrations of soluble and insoluble oxalates in the shoot and soluble oxalate in the root than ammonium application. Nitrate application also resulted in higher concentrations of sodium and calcium in the shoot, and higher concentrations of sodium, calcium and magnesium in the root than ammonium application. The results suggest that the application of nitrate fertilizer as the sole source of nitrogen should be avoided in the cultivation of oxalate-rich forages, though further studies are needed under field conditions.