Fate of 15N fertilizer applied to maize in rotation with tropical forage grasses

Abstract Tropical forage grasses of the genus Megathyrsus and Urochloa can suppress soil–nitrification by releasing inhibitory substances, reducing N losses and increasing fertilizer N recovery of the cash crop in rotation. In contrast, ruzigrass ( Urochloa ruziziensis ) has been reported to decrease the yield and N accumulation of the subsequent crop and hence can affect N use efficiency and the fate of applied N. We investigated the effects of Guinea grass ( M. maximum ), palisade grass ( U. brizantha ), and ruzigrass on succeeding crop yield, N accumulation, and the fate of 15 N–labeled fertilizer applied to maize ( Zea mays L.) in a 2–year field experiment in Brazil. Maize was fertilized with 140 kg ha –1 N as ( 15 NH 4 ) 2 SO 4 or not fertilized, and recovery of residual 15 N was quantified in the second season. Net nitrification rates through an incubation study had no differences among grasses. Nitrogen application increased maize yield and N accumulation in both seasons, whereas maize yield decreased by 9.5% following ruzigrass compared with the other forages. The grasses had no effect on 15 N recovery by maize or in the system. On average, the recovery of 15 N in maize and soil was 34% and 46% in the first growing season and 2.9% and 20% in the second season, respectively. Our results indicated that tropical perennial grasses had no differential effects on nitrification rates and the fate of 15 N–labeled fertilizer in the plant–litter–soil system in the season of application nor in the subsequent crop (residual effect).