G. CADISCH¹, R.M. SCHUNKE² and K.E. GILLER¹

¹Department of Biological Sciences, Wye College, University of London, Wye, Ashford, United Kingdom
²Centro Nacional de Pesquisa de Gado de Corte, Campo Grande, Brazil

Abstract

A nitrogen-balance method and a process orientated approach, comparing a pure Brachiaria decumbens sward with a B. decumbens-Calopogonium mucunoides mixture, were used to discuss the major pathways of nitrogen in improved tropical grazing systems in the Cerrados of Brazil. Both methods predicted a drain of soil N with the pure grass pasture which could be reversed by introducing a legume into the pasture. The impact of legumes on the system was demonstrated both in terms of achieving a positive nitrogen balance by adding symbiotically fixed N₂ and by increasing plant available soil N through improved net mineralisation of litter and root materials. The magnitude of the latter effect depended on the palatability and decomposability of the legume which govern the animal-litter pathway and the addition to soil organic-mineral N. A legume N₂ fixation of 31–46% (depending on legume utilisation) of total sward above-ground nitrogen accumulation (equivalent to 60–117 kg fixed N₂/ha for this system) was considered to be sufficient to sustain the productivity of the system. This corresponded to a proportion of about 13–23% on a dry matter basis (2600–5200 kg DM/ha) provided that a high proportion of the legume N is derived from N₂ fixation.
Management options that increase N₂ fixation and efficient N recycling include optimising grazing management (short rest periods to improve legume persistence), adequate stocking rate (to improve litter recycling), use of less palatable legumes (to improve persistence) which are easily decomposed, and alleviation of nutrient deficiencies (especially P).