Tropical Grasslands (1994) Volume 28, 4352
Nitrogen cycling in a pure grass pasture and a grass-legume mixture on a red latosol in Brazil
G. CADISCH1, R.M. SCHUNKE2 and K.E. GILLER1
1Department of Biological Sciences, Wye College, University of London, Wye, Ashford, United Kingdom
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.