A Soil Organic Nitrogen Fraction that Reduces the Need for Nitrogen Fertilization

The need to estimate mineralization has long been recognized in making N fertilizer recommendations, but little progress has thus far been made in identifying a specific fraction of soil organic N that affects crop responsiveness to N fertilization. After eliminating major defects in the methodology employed to fractionate the N in soil hydrolysates, a study was conducted to compare N‐distribution analyses for soils differing in N‐fertilizer responsiveness by corn ( Zea mays L.). Hydrolyses with 6 M HCl were performed on composite soil samples (0–30 cm) that had been collected in late March or early April of 1990, 1991, or 1992, from 18 sites in a N‐response study involving 75 site–years throughout Illinois with different soil types, crop rotations, and N management practices. Concentrations of amino sugar N were 33 to 1000% greater ( P < 0.001) for 11 nonresponsive than for seven responsive soils, whereas no consistent difference was observed in their content of total hydrolyzable N, hydrolyzable NH 4 –N, or amino acid N. Upon aerobic incubation for 3 mo with biweekly leaching, production of (NH 4 + NO 3 + NO 2 )‐N averaged 260% greater for three nonresponsive soils than for two responsive soils, and was accompanied by a net decrease in amino sugar N but not in amino acid N. Soil concentrations of amino sugar N were very highly correlated with check‐plot yield ( r = 0.79***) and fertilizer‐N response ( r = −0.82***). On the basis of amino sugar N, all 18 soils were classified correctly as responsive (<200 mg kg −1 ) or nonresponsive (>250 mg kg −1 ) to N fertilization.