Row spacing effects on N2-fixation, N-yield and soil N uptake of intercropped cowpea and maize

In the tropics, cowpea is often intercropped with maize. Little is known about the effect of the intercropped maize on N2-fixation by cowpea or how intercropping affects nitrogen fertilizer use efficiency or soil N-uptake of both crops. Cowpea and maize were grown as a monocrop at row spacings of 40, 50, 60, 80, and 120 cm and intercropped at row spacing of 40, 50, and 60 cm. Plots were fertilized with 50 kg N as (NH4)2SO4; microplots within each plot received the same amount of N-depleted (NH4)2SO4. Using the N-dilution method, the percentage of N derived from N2-fixation by cowpea and the recovery of N fertilizer and soil N-uptake was measured for both crops at 50 and 80 days after planting. Significant differences in yield and total N for cowpea and maize at both harvest periods were dependent on row spacing and cropping systems. Maize grown at the closer row spacing accumulated most of its N during the first 50 days after planting, whereas maize grown at the widest row spacing accumulated a significant portion of its N during the last 30 days before the final harvest, 80 days after planting. Overall, no significant differences in the percentage of N derived from N2-fixation for monocropped or intercropped cowpea was observed and between 30 and 50% of its N was derived from N2. At 50 DAP, fertilizer and soil N uptake was dependent on row spacing with maize grown at the narrowest row spacing having a higher fertilizer and soil N recovery than maize grown at wider spacings. At 50 and 80 DAP, intercropped maize/cowpea did not have a higher fertilizer and soil N uptake than monocropped cowpea or maize at the same row spacing. Monocropped maize and cowpea at the same row spacing took up about the same amount of fertilizer or soil N. When intercropped, maize took up twice as much soil and fertilizer N as cowpea. Apparently intercropped cowpea was not able to maintain its yield potential. Whereas significant differences in total N for maize was observed at 50 and 80 DAP, no significant differences in the atom % 'N excess were observed. Therefore, in this study, the atom % 'N excess of the reference crop was yield independent. Furthermore, the similarity in the atom % 'N excess for intercropped and monocropped maize indicated that transfer of N from the legume to the non-legume was small or not detectable.