A study was conducted in 40 farmers' fields in Zomba Rural Development Project, southern Malawi to measure nitrate-N dynamics in different landscape positions as influenced by N source. The landscape positions were dambo margin, valley bottom and steep slopes. Nitrogen sources consisted of an inorganic fertilizer, organic fertilizer and a control. The inorganic fertilizer plot received 120 kg N ha-1. The organic fertilizer plot was a relay intercropping system of Sesbania sesban biomass applied at the rate of 500-3000 kg ha-1, depending on tree growth the previous year.|Nitrate availability varied across the maize growing season. The highest nitrate levels (16 mg kg-1) were observed at 85 days after maize planting (DAP) and decreased markedly (7 mg kg-1) towards the end of the growing season. There were no significant differences in topsoil nitrate among the three landscape positions. However, nitrate in the subsoil was consistently highest at the valley bottom sites and lowest at the steep slope sites. This was expected due to higher soil organic C and total N at lower landscape position sites compared to eroded slopes. There was greater nitrate accumulation in the N input plots (both inorganic and organic) than the zero N control. Similarities in nitrate dynamics in the topsoil over the season suggested similar release patterns from organic and inorganic sources. In the subsoil, by contrast, nitrate at the end of the growing season accumulated 2-fold higher in the inorganic N treatment than any other treatment (12 mg kg-1 compared to about 5 mg kg-1). This was in part due to higher N inputs in the fertilizer treatment. Subsoil nitrate accumulation indicated the potential for nitrate leaching with use of inorganic N fertilizer, particularly for valley bottom sites.|

English

9906|AGRIS 9902|R98-99ANALY

Jose Juan Caballero

CIMMYT Publications Collection