The effect of grazing intensity and the presence of a forage legume on nitrogen dynamics in Brachiaria pastures in the Atlantic forest region of the south of Bahia, Brazil

• Published in: Nutrient cycling in agroecosystems Vol. 64; no. 3; pp. 257 - 271
• Main Authors: Cantarutti, R, Tarre, R, Macedo, R, Cadisch, G, Rezende, C D, Pereira, J, Braga, J, Gomide, J, Ferreira, E, Alves, B, Urquiaga, S, Boddey, R
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Nature B.V 01-11-2002
• Subjects: Agricultural practices; Beef cattle; Biomass; Brachiaria; Desmodium ovalifolium; Fertilizers; Forage; Forests; Grasses; Grazing; Grazing intensity; Legumes; Litter; Microorganisms; Mineralization; Nitrogen; Nitrogen fixation; Nitrogenation; Pasture; Pasture management; Regrowth; Stocking; Stocking rates; Sward; Brazil; ASW, Brazil
• ISSN: 1385-1314; 1573-0867
• DOI: 10.1023/A:1021415915804

It has been shown that with careful grazing management and addition of P and K, but not N, fertilisers Brachiaria pastures are able to maintain sustainable live weight gains over many years. However, standard on-farm practice, which generally involves high stocking rates, leads after a few years to pasture decline due mainly to N deficiency for grass regrowth. To generate an understanding of the mechanism of pasture decline and possible management options to mitigate this process, a study was performed in the Atlantic forest region of the south of Bahia state to study the N dynamics in pastures of Brachiaria humidicola subject to three different stocking rates of beef cattle, with and without the presence of the forage legume Desmodium ovalifolium. Despite the fact that the C:N ratio of the deposited litter was high (60 to 70) the rate of decomposition was very rapid (k similar to -0.07 g g super(-1) day super(-1)) and annual rates of N turnover through the litter pathway were between 105 and 170 kg N ha super(-1) year super(-1). In the grass-only pastures as stocking rate increased from 2 to 3 head ha super(-1), N recycled in the litter decreased by 11%, but a further increase to 4 head ha super(-1) decreased N recycling by 30% suggesting that beyond a certain critical level higher grazing stocking rates would lead to pasture decline if there was no N addition. High stocking rates decreased the proportion of the legume in the sward, but at all rates the concentration of N in both the green and dead grass in the forage on offer and in the litter was higher in the mixed sward. The presence of the legume caused a decrease in the C:N ratio of the microbial biomass while both soil N mineralisation and nitrification increased. This increased rate of turnover of the microbial biomass and the contribution of N sub(2) fixation to the legume resulted in large increases in the N recycled via litter deposition ranging from 42 to 155 kg N ha super(-1) year super(-1).