Nitrogen transfer from Phaseolus bean to intercropped maize measured using 15N-enrichment and 15N-isotope dilution methods

Abstract

Transfer of N from Phaseolus bean to intercropped maize was studied in glasshouse experiments using 15N-foliar feeding and 15N-isotope dilution methods. Nodulated and non-nodulating Phaseolus genotypes were included in separate treatments to help distinguish between benefits due to transfer of fixed N and competition for N in the growth medium. When intercropped with bean (bliarly fed with ( 15NH 4) 2S0 4, maize was enriched with 15N, showing that N had been transferred. The amounts of N transferred were small, and always <5% of the N in the N 2-fixing beans. There was a decrease in shoot-N in maize intercropped with N 2-fixing bean compared to maize intercropped with the non-nodulating beans. Non-nodulating bean transferred comparable amounts of N to intercropped maize plants although their total N content was less than a quarter of that in the N 2-fixing beans. For the isotope dilution experiments, 15N-fertilizcr was incorporated into a soil-based compost together with sucrose to stabilise the 15N-cnrichmcnt of available N. When plants grew vigorously no transfer of N from bean to maize was detected by isotope dilution, and again shoot N of maize intercropped with N 2-fixing beans was less than that of maize with non-nodulating beans. In a further experiment, growth of maize and bean plants was reduced by severe insect attack and up to 15% (between 9 and 15mgN pot −1) of the N in N 2-fixing beans was estimated by isotope dilution to have been transferred. Small (5–10 mg N pot −1) but significant increases in total N yield were found in the maize intercropped with N 2-fixing bean compared to maize intercropped with non-nodulaling bean. In this experiment treatments with or without vesicular -arbuscular mycorrhiza were established but showed no significant differences in N-transfer from uninoculatcd plants. As transfer of N from the beans to intercropped cereals showed such little benefit under conditions of severe N limilation. our results indicate that many careful field experiments arc required before we can conclude that N-transfcr from Phaseolus to intercropped cereals is significant in agriculture.