Crop responses to 15N-labelled organic and inorganic nitrogen sources

A major challenge for low-external-input farming systems is to secure the N supply. Lack of synchrony between mineralization of organic N sources and plant N requirements is causing many growers to use different techniques to overcome this problem. One of these techniques is the application of soluble water extracts of different farm residues and plants. A field study was conducted to study the crop uptake of applied N-15-labeled alfalfa and clover extracts as compared to the N uptake from N-15-glycine and three levels of N-15-(NH4)(2)SO4. The results show that total N accumulation in the field crops, squash and lettuce was primarily affected by the amount of added N (P <= 0.05) and not by the form in which the N was applied (P <= 0.05). The utilization efficiencies of N (pNdff) from plant extracts and glycine increased (P <= 0.05) gradually from 10, 20, to 30 days after application in contrast to (NH4)(2)SO4 which peaked in utilization efficiencies of 56% around 20 days after application. The pNdff reached 60%, 40% and 36% of the applied glycine, alfalfa and clover extracts, respectively in lettuce. Squash showed the same pattern during at 10, 20 and 30 days; however, the proportion of N derived from most of the treatments was higher in squash than in lettuce (P <= 0.05). These results indicate that plant extracts of clover and alfalfa can be used as efficient N fertilisers in low-external-input agroecosystems.