Low-Input Cropping Systems and Nitrogen Fertilizer Effects on Crop Production: Soil Nitrogen Dynamics and Efficiency of Nitrogen Use in Maize Crop

Soil N is a major factor limiting crop production in maize-based agriculture in the tropics. In this study, soil N dynamics and efficiency of N use by maize was investigated in five cropping systems (CSs) over two annual cropping phases. The CSs comprised cereal-legume (CS1, CS2, CS3), cereal-cereal (CS4) and cereal-bare fallow (CS5) rotations with three rates of N fertiliser application to maize (0, 60 and 120 kg ha ). Legumes 1 (cowpea, field pea and lucerne) were either grown in rotation or intercropped with the cereals. All CSs with legumes in the rotation had greater soil total N and soil mineral N at planting of maize. Net and cumulative N mineralization during maize growth in both phases was always highest for CS1, where winter field pea was incorporated as a green manure. Soil total N and mineral N increased with application of inorganic N fertilizer, but the efficiency of N use by maize decreased with increasing N application rate. N use efficiency was highest where N was present in the form of legume residues. The estimated soil N balance after harvest showed significant N gains for CSs that had legumes in the rotation at all N fertilizer rates. In contrast, CSs without legumes had lower soil N gains and addition of N fertilizer at 120 kg N ha resulted in a net loss of N from the 1 soil profile. This study shows that the inclusion of legumes, supplemented by inorganic N fertilizer where the legume crop is removed, has the potential to maintain adequate crop production and soil N quality in maizebased cropping systems. Where legumes are absent from rotations and inorganic N fertilizer is the sole N source, soil N quality is likely to deteriorate over time.