SOIL ORGANIC-MATTER AS A POTENTIAL NET NITROGEN SINK IN A FERTILIZED CORNFIELD, SOUTH DEERFIELD, MASSACHUSETTS, USA

Net nitrogen (N) mineralization in situ and N mineralization potential (N0) over one complete year (1986-1987) were examined for a conventionally managed silage cornfield that received at least 235 kg fertilizer N ha-1. Net N mineralization at the site, measured by sequential in situ polyethylene-bag incubations, totaled -54 kg N ha-1 yr-1, and -31 kg N ha-1 over the May-to-August growing season. Nitrogen mineralization potential of the soil organic matter (SOM) measured by laboratory anaerobic incubations, was positive uniformly and varied with month of sample collection. The soil gained 72 kg inorganic N ha-1 from April to October, principally because of a fall manuring, only 7 kg N ha-1 from April to September. The in situ incubations, likely more representative of the balance between N mineralization and immobilization under N-fertilized conditions, suggest that SOM at the site is accumulating N.