PLANT-RESPONSE TO ORGANIC AMENDMENTS AND DECREASING INORGANIC NITROGEN RATES IN SOILS FROM A LONG-TERM EXPERIMENT

To decrease the potential for adverse environmental impacts of N in ag roecosystems, there is increasing emphasis on improving N efficiency o f organic and inorganic sources. Greater reliance on organic N sources can cause short-term crop yield decreases. Consequently, a study was conducted to determine the effect of decreasing rates of inorganic N d uring a transition from inorganic to organic N sources. The approach w as to grow four crops of maize (Zea mays L.) during a period of 306 d in the greenhouse on soils that varied widely in organic matter and bi ological activity because of long-term residue and fertilizer manageme nt. Treatments were arranged as a complete factorial with the followin g factors: four soils obtained from the Residue Utilization Plots (RUP ), Columbia Basin Research Center, Pendleton, OR (beef manure or pea [ Pisum sativum L.] vine residue, 0 or 90 kg N ha-1, applied biennially since 1931); four greenhouse amendments (pea vine, composted beef or p oultry manure, or control); and four rates of N fertilizer (0-1600 mg N [2 kg]-1 soil as NH4NO3). In the absence of organic residue or N fer tilizer, soil from the manure RUP produced greater dry matter yield (D MY) and plant N uptake than soil from the other RUP treatments. There was no net N mineralization from beef manure added in the greenhouse. Between 10 and 30% of the total poultry manure and pea vine N added wa s taken up by the four maize crops. Results showed that, by the fourth crop, when inorganic N applications had decreased to zero, plant prod uctivity by organic residues was best maintained by pea vine residue o r poultry manure.