| Soil incubation and growth chamber studies were conducted to evaluate the effects of soil moisture (50, 70, and 90% of water holding capacity) and temperature (15/10, 20/15, and 25/20°C day/night) on nitrogen (N) availability from four organic N sources. In the soil incubation study, differential N release kinetics of the N sources was determined by measuring ammonium- and nitrate-N contents periodically over 12 weeks. Net N released, as a percentage of organic N, was greatest in the order: urea (91--96%) > blood meal (BM) (56--61%) > alfalfa pellets (AP) (41--52%) > partially composted chicken manure (CM) (37--45%). Increasing soil moisture increased net N released from AP and CM by 12 and 21%, respectively, but did not affect net N released from urea and BM. Increasing temperature increased net N released from AP, BM, and CM by 25, 10, and 13%, respectively, but did not affect net N released from urea. In the growth chamber study, kale (Brassica oleracea L.) was grown as an indicator of N availability. Regardless of soil moisture and temperature, N use efficiency by kale was greatest in the order: urea > BM > AP > CM. Soil moisture influenced N availability differently in the two studies, whereas temperature influenced N availability similarly in the two studies. Our results indicate that soil incubation data will be useful for evaluating variations in N availability among N sources and temperatures on a field scale. Increasing temperature improves N availability from natural organic materials, thereby contributing to better crop production. |
