PHOSPHORUS REGULATION OF NITROGEN-FIXATION IN A TRADITIONAL MEXICAN AGROECOSYSTEM

Although nitrogen is considered to be the nutrient that most commonly limits production of natural and managed terrestrial ecosystems, I pro pose that phosphorus may regulate productivity in many continuously cu ltivated agroecosystems that do not receive applications of synthetic fertilizers. One way P may limit agroecosystem productivity is by cont rolling nitrogen fixation of legume crops, thus affecting nitrogen ava ilability in the overall agroecosystem. I tested this hypothesis in tw o studies by examining the effect of phosphorus nutrition on nitrogen fixation of alfalfa in traditional Mexican agroecosystems. All farms u sed in the research relied on alfalfa as the primary nitrogen source f or maize cultivation and other crops, and had minimal or no reliance o n synthetic fertilizers. In one study, I used the natural abundance of N-15 to estimate nitrogen fixation in five alfalfa plots with soils r epresenting a wide range of P fertility. I found a correlation of r = 0.85 between foliage P concentrations and nitrogen fixation in the alf alfa plots. Mean nitrogen fixation in alfalfa plots ranged between 232 -555 kg ha-1 yr-1 as estimated by the N-15-natural abundance method. I n a second study, I sampled soils from alfalfa plots on traditional fa rms located in 5 different physiographic regions of Mexico. Half of ea ch soil sample was augmented with phosphorus in a greenhouse experimen t. I grew alfalfa on the fertilized and unfertilized soils from each s ite and then determined nitrogenase activity (acetylene reduction) of the Rhizobium on the plant roots. Nitrogenase activity increased in th e alfalfa grown on all soils with added phosphorus, with two of the fi ve differences being statistically significant at P < 0.01, and one at P < 0.05. Foliage P concentrations and nitrogenase activity were posi tively correlated (r = 0.81, P < 0.01).