Nitrogen balance, transfer and competition in a Macroptilium atropurpureum - Panicum coloratum mixture

Abstract

Nitrogen is the nutrient that most frequently limits growth of grasses in rangelands. Symbiotic nitrogen fixation through establishment of legume-grass mixtures is a means of providing N inputs to rangeland systems. Siratro (Macroptilium atropurpureum(DC.) Urb. cv. siratro) and Kleingrass (Panicum coloratum L.) were grown in pots, in pure stand and mixtures to investigate competition for mineral N, N transfer from legume to grass, and N2 fixation by siratro as influenced by kleingrass. The replacement series approach incorporating the grass and legume, utilizing biomass and plant density data were used to investigate plant competition and mixture stability. The dilution, difference, and modeling techniques were utilized and evaluated in an investigation of the N balance of the siratro-kleingrass mixture. Natural 15N abundance and 15N enriched fertilizer N techniques were used. Data was collected from biomass of shoots, roots, and crowns in five experiments. The space assigned to individual plants influenced legume density only while densities of the grass plants remained constant. Both the grass and legume successfully competed for the low levels of soil N available. The amount of soil N absorbed by the legume exceeded that of the grass during the initial 6 weeks, then declined to levels lower than for the grass in the latter portion of the experiments.Nitrogen fixation of siratro increased with time in all experiments on a per plant basis and as the proportion of the grass in the pot increased. It was apparent that a portion of the N absorbed by the grass was the result of associative N2 fixation. Transfer of N from the legume to the grass paralleled the N fixing capacity of siratro and increased with time. Although N transfer only accounted for 1 to 3 percent of the total N fixed by the legume, it was significant to the production of kleingrass (24% of the N in the grass) when grown in mixture with siratro.It was concluded that: (1) the space arrangement of plants affected the legume stand only, (2) under all N levels the legume did compete with the grass for soil N, (3) natural abundance did not show N transfer from legume to grass, (4) the model is an effective tool to investigate N balance and stability in grass-legume mixtures, (5) legume mineral N uptake decreased with time, and (6) the sensitivity of the difference method depends on the level of N in the soil.