Fate and efficiency of ¹⁵N-labelled slow- and controlled-release fertilizers

Slow- and controlled-release N fertilizers are designed to increase efficiency and reduce N losses by better synchronizing N availability with plant demand. This paper reviews the use of ¹⁵N with these fertilizers to collect quantitative data on the efficiency, residual value and N losses for which relatively few data are available compared with conventional labelled urea and ammonium-based fertilizers. In general, studies of slow-release forms (isobutylidene diurea, oxamide, ureaform) with rice and upland crops show one or more benefits, including improved N uptake efficiency, and reduced N losses via leaching or NH₃ volatilization under conditions which favor such losses (coarse textured soils, alkaline pH, respectively). Benefits from residual ¹⁵N may accrue in the year following application. Studies with controlled-release ¹⁵N-labelled sulfur coated urea (SCU) show benefits in situations such as paddy soils where losses from broadcast urea are a substantial problem. In experiments with ¹⁵N polyolefin-coated urea (POCU), rice plant recovery of broadcast conventional urea or ammonium salts ranged from 24 % with losses of 50–45 % with losses of 33 %. Where ¹⁵N labelled SCU or POCU was used, the rice recovery ranged from 26 % with losses of 14–72 % with losses of 10 %. Experiments using POCU with corn, barley and potato show similar results. The paucity of published data obtained using ¹⁵N points to the need for further studies that will provide concrete evidence for the development of innovative fertilizers with enhanced efficiency and an evidence-based set of recommendations for their selection and use.