Strategies to optimise biological nitrogen fixation in legume/grass pastures in the southern region of Chile

Pastures in southern Chile are composed of forage grasses and associated legumes. Apart from native grass species, in many areas perennial ryegrass (Lolium perenne L.) and white clover (Trfolium repens L.) have been introduced. It is thought that most of the N in these pastures is derived from the symbiosis between rhizobium bacteria and the clover. However, various factors can limit the biological nitrogen fixation (BNF) potential. Soils of Chiloe (southern Chile) are derived from volcanic ash (Andisols), and show high acidity and phosphorus (P) retention capacity and there is often a low availability of exchangeable cations and, sometimes, micronutrients, which together with high aluminium (Al) concentrations can inhibit BNF potential by nodulated legumes. An experiment was carried out on an Andisol of Chiloe, on a permanent pasture, to evaluate the influence of these fertility factors grass and legume productivity and on the BNF contribution to white clover. Treatments included two rates of lime (0 and 4 Mg ha(-1)) and eight fertilisation treatments: 1. Complete (macronutrients in kg ha(-1): N-50, P2O5-180, K2O-100. MgO-70 S-50, and micronutrients); six other treatments with complete fertilisation but, respectively, without N, P, K, Mg, S and micronutrients, and a control without fertiliser application. The experiment was conducted over 2 years and soil fertility parameters, grass and clover biomass production were evaluated and the BNF contribution to the clover was estimated using N-15 isotope dilution technique. Potassium was found to be the most limiting element for dry matter production, especially for the clover. Liming increased soil pH values and Ca and Mg contents, strongly reduced Al saturation and increased dry matter production. The proportional contribution of BNF to the white clover was high, reaching 80% of total N accumulated by the plants and not significantly affected by addition of lime or fertiliser. However, the accumulation of dry matter and total N, and the total N derived from BNF by the clover were over three times higher in the complete fertilisation treatments than in the control in both years and in the limed and unlimed treatments. The results of this study indicate that pasture productivity can be improved by liming and fertilisation which favour growth of the legume and ryegrass to the detriment of the native grasses. The increase in the proportion of ryegrass and clover should result in improved quality of the available forage, and in the case of the clover, stimulates an increase in the N contribution from BNF thus dispensing with the need for N fertiliser.