Assessment of biological nitrogen fixation, agricultural management, and aboveground and belowground biomass of maize (Zea mays L.) landraces from Oaxaca, Mexico

Maize (Zea mays L.) is the most highly produced crop in the world, requiring more N fertilizer than any other crop in the world. Because the use of N fertilizers contributes to negative environmental and agronomic impacts, a focus in maize research integrating alternative N sources to maize is important. One potentially useful approach is to find or develop cultivars of maize that have associations with N2-fixing (diazotrophic) bacteria that can produce their own N through biological nitrogen fixation (BNF). Separate experiments were conducted in a native maize agroecosystem in Oaxaca, Mexico and in a greenhouse study at the University of California, Davis to assess BNF, agricultural management, and aboveground and belowground biomass of maize (Zea mays L.) landraces from Oaxaca, Mexico. In 2004, the 15N enrichment and 15N natural abundance techniques were evaluated for their ability to estimate BNF in maize. The 15N natural abundance method demonstrated similar results to the 15N enrichment method yet had less variability and was cheaper and easier to use. Thus, subsequent experiments used the 15 N natural abundance technique. In one field experiment, the effects of herbicide/zero-tillage, no-herbicide/tillage, and no-herbicide/zero-tillage agriculture on biological nitrogen fixation (BNF) associated with maize, weed control, and maize plant growth in Oaxaca, Mexico were studied. In the greenhouse experiment, the shoot and root biomass production of 5 traditional Oaxacan maize varieties (landraces) and a US maize hybrid cultivar were compared. The 2004 field experiments showed BNF estimates associated with maize landraces ranging from values of -254.8 +/- 155.3 to 7.9 +/- 33.4%N derived from the atmosphere (%Ndfa). The greenhouse experiment showed BNF estimates of these landraces ranging from -20.5 +/- 14.0 to 14.9 +/- 10.2%Ndfa. The large variation in %Ndfa estimates is similar to previous studies of BNF associated with grass species. In 2006, it was discovered that agricultural management had a significant effect on BNF. Under no-herbicide/tillage agriculture, maize landraces had received up to 39.7 +/- 5.5%Ndfa regardless of which reference plant was used in estimating BNF. In a separate 2006 experiment, maize plants randomly sampled from the farmers fields yielded 84.6 +/- 28.2%Ndfa or 95.7 +/- 24.1%Ndfa depending on the species of reference plant used in making the estimation. The maize landraces were found to accumulate more biomass both in the roots and shoots and have greater dry root density of fine roots (mg/cm3) than the US maize. Agricultural management and the selection of reference plant in calculating BNF were shown to both affect the resulting %Ndfa of maize, which had only been demonstrated previously for grain legumes. The upper range of BNF values reported here is higher than has been previously reported for maize.