The relation between mode of legume nitrogen nutrition, yield determinants and N assimilation efficiency

Abstract

Results of the effect of nitrogen (N) on legume dry matter (DM) and N accumulation are mixed. This study measured whole plant OM and N accumulation by legumes grown at different levels of soil N. Two field experiments tested whether reducing N2 fixation by increasing soil mineral N results in: greater OM accumulation due to increased allocation of DM to leaves; increased N assimilation due to greater N uptake efficiency; and similarity between three species of plant response to N. CROPGRO V3. 0 predictions of legume growth under different levels of soil N were also tested with observed data. In 1991 soybeans [Glycine max Merr. (L.) cv. Clark] were grown with 0 (NO), 256 (N1) and 430 (N2) kg applied N ha^-1. Leaf area and weight and N concentration of leaves, stems, and pods were determined at 10, 22, 30, 39 and 51 days after emergence (DAE) . At 22 and 51 DAE root and nodule weight and root length were also measured. In 1993 soybean, peanut (Arachis hypogaea cv. Pronto) and common bean (Phaseolus Vulgaris cv. Porillo Sintetico) were grown with either 0 (NO) or 275 (375 for peanut) (N1) kg applied N ha^-1. Leaf area and weight and N concentration of above and below ground components were measured at 10, 23 and 44 DAE. Shoot and seed weight were measured at harvest maturity. On average nitrogen reduced N2 fixation to approximately 20% of NO plants. Nitrogen application resulted in greater grain yield and DM accumulation but harvest index was unaffected. with N application there was an average increase in above ground DM and leaf area index of 23 and 20%, respectively. This was due primarily to greater allocation of DM to leaves in N1 than NO during early vegetative growth. Nitrogen assimilation was 17% more in N1 than NO plants due to increased root weight (17%) and root surface area (27%) and greater N assimilated per unit below ground in N1 plants compared to NO. CROPGRO simulations of total N also increased with application of N. Simulated above ground DM, however, was either unresponsive or lower with N application. This indicates a need for further development of CROPGRO's simulation of DM accumulation with an increase in available soil N.