Influence of source, method of placement and simulated rainfall on the recovery of 15N-labelled fertilizers under zero tillage

Abstract

The availability of fertilizer N to crop under zero tillage (ZT) versus conventional tillage (CT) is affected by immobilization, ammonia volatilization, denitrification or leaching. This study examined the relative importance of those four factors in influencing the availability of N to the crop and the recovery of applied 15N-labelled N in soil and plants. Field experiments were conducted at two sites in central Alberta to determine the influence of tillage, N source, method of N placement and simulated rainfall on the recovery of 15N-labelled fertilizers in spring barley (Hordeum vulgare L.) plants and soil. A first experiment compared four soil surface broadcast N sources [urea, ammonium sulphate (AS), ammonium nitrate (AN) and potassium nitrate (KNO3)]. The N recovery in barley plants was KNO3> AN > AS > urea. With the exception of KNO3, plant N recovery was greater under CT than under ZT, particularly for urea. Immobilized N in soil was often less with KNO3 than the other N sources. There was apparently virtually no denitrification loss during the one crop season used in these experiments, and the recovery of KNO3 was complete. In a second experiment, methods of placement were evaluated. The recovery of N in plants was lowest for surface broadcast urea under ZT, particularly when no water was added after urea application. Banding urea beside and/or below the seedrow in comparison to broadcasting markedly increased N recovery in plants, especially under ZT. The differences in N recovery between ZT and CT were greater for broadcast application than for band placement. Method of placement had little effect on N immobilization under ZT, but under CT the amount of immobilized N was less with subsurface banding than incorporation. Simulated rainfall immediately after surface broadcast urea application under ZT increased the N recovery in plants. In conclusion, surface broadcast urea under ZT was less efficient than the other N sources and its recovery improved considerably when placed in bands below the soil surface near the seedrow. Key words: Ammonium nitrate, banding, fate of applied N, 15N-labelled fertilizers, N immobilization, N recovery, surface broadcasting, urea