Relative N Fertilizer Efficiency and Mineralization of Organic Amendments as Assessed by Tall Fescue (Festuca arundinacea L

A capability to determine the availability of nitrogen in organic amendments is necessary to ensure that such materials will provide sufficient nitrogen to the growing crop and cause minimal environmental degradation. Greenhouse bioassays were used to evaluate N availability of organic by-products, including several employed in the field study of this research. Tall fescue was grown in greenhouse pots amended with Huck’s Hen Blend yard waste compost, Panorama yard waste compost, Wolf Creek biosolids compost, Rivanna biosolids compost, and pelletized poultry litter at concentrations estimated to supply 100% agronomic N. Five inorganic N treatments applied at 0N (0% agronomic N), 0.5N (50% N), 1N (100% N), 1.5N (150% N), and 2N (200% N) were used to establish an N calibration curve. Yield, plant nitrogen uptake, and total Kjeldahl N, NO3 and NH4 concentrations of amended soils were compared against the inorganically fertilized treatments in order to determine organic N mineralization rates and N fertilizer values. Nitrogen mineralization rates were greatest in the poultry litter (21.4%) and Panorama yard waste compost (4.5%) amended plots. Nitrogen uptake (120 mg/pot, 133 mg/pot, respectively) in these treatments were greater than that achieved in the control (0N) (91.3 mg/pot) treatment. Wolf Creek biosolids compost and Huck’s Hen Blend yard waste compost induced N immobilization (-5.00% and 0.18%, respectively), and had N uptake values similar to the control (92.6 mg/pot and 95.7 mg/pot). Rivanna biosolids compost immobilized N (-14.8%) but N uptake (136 mg/pot) was greater than that in the control due to the relatively high inorganic N content in the amendment. The total N concentration and C:N values were less reliable variables in predicting N mineralization when a significant portion of the total N was in the inorganic form.