Total nitrogen determination in plant material by persulfate digestion

Determination of total plant N concentration is important in understanding crop nutritional value, nutritional requirements, and physiological responses to the environment. Nitrogen analysis by either the Kjeldahl or Dumas method is expensive and requires specialized equipment. An alternative procedure, developed primarily for use on water samples, was optimized for digestion of vegetative soybean [Glycine max (L.) Merr.] tissue and compared with the Dumas method. Finely ground plant material (0.425 mm) was digested in alkaline potassium persulfate (K2S2O8) at 120 degrees C (0.1 MPa) for 1.5 h in an autoclave, converting organic N to NO3-. Nitrate in the digest was reduced to NO2- and quantified by the sulfanilamide-alpha-naphthyl-ethylenediamine dihydrochloride procedure. Persulfate digests of soybean material agreed closely with determinations made by the Dumas method (r = 0.99), Digestion of NIST standard tomato (Lycopersicon esculentum Mill.) leaves (4.8 g N 100 g(-1)) indicated complete N recovery (4.9 +/- 0.1 g N 100 g(-1)). Persulfate digestion of total plant N was accurate and precise, and required little initial investment.