Nitrogen recovery and loss in a fertilized elephant grass pasture G. B. Martha Jr*, M. Corsi†, P. C. O. Trivelin‡ and M. C. Alves§ *Embrapa Cerrados, Planaltina, Distrito Federal, Brazil, †Departamento de Produc ¸a ˜o Animal, Universidade de Sa ˜o Paulo, Escola Superior de Agricultura ‘Luiz de Queiroz ’, Piracicaba, Sa ˜o Paulo, Brazil, ‡Laborato ´rio de Iso ´topos Esta ´veis, Universidade de Sa ˜o Paulo, Centro de Energia Nuclear na Agricultura, Piracicaba, Sa ˜o Paulo, Brazil, and §Centro de Informa ´tica na Agricultura, Universidade de Sa ˜o Paulo, Escola Superior de Agricultura ‘Luiz de Queiroz’, Piracicaba, Sa ˜o Paulo, Brazil Abstract Limited information is available regarding the recovery and loss of fertilizer nitrogen (N) applied to intensively managed tropical grass pastures. An experiment was carried out in Brazil to determine the fertilizer-N recovery and ammonia volatilization loss in an elephant grass (Pennisetum purpureum, Schum.) pasture fertilized with 100 kg N ha )1 as urea or ammonium sulphate, labelled with 15 N, in late summer (LS) or in mid- autumn (MA). Herbage mass was highest and litter mass was lowest in LS (P <0®05). The N concentration of herbage was highest in autumn (P <0®05) and the total N content in soil was lower in LS than in MA (P <0®05), reflecting the high N uptake capacity of the grass. Proportionately higher 15 N recovery in litter mass (P <0®05) was observed in autumn (0®094) than in LS (0®0397) and the 15 N recovery in herbage was 0®046 higher for ammonium sulphate-fertilized pastures (P <0®05; proportionately 0®243 for ammonium sul- phate and 0®197 for urea). Around 0®60 of the fertilizer- 15 N recovered was retained in soil and in non-harvestable fractions of the plant. The NH 3 vola- tilization loss was higher in LS and most of the N loss occurred soon after fertilizer application. Urea and ammonium sulphate fertilizers were equally effective in sustaining herbage dry matter yield in the short term. However, the use of ammonium sulphate, rather than urea, would be preferable for LS applications when the objective is to reduce NH 3 volatilization losses. Keywords: 15 N, N fertilizer, N balance, N cycling, Pennisetum purpureum, tropical pasture Introduction Grasslands are important to Brazilian agriculture and represent the main source of nutrients for cattle. Almost 0®90 of all beef produced in Brazil comes from herds fed exclusively on pastures (Arruda, 1997) and estimates suggest that most of the 20 billion litres of milk produced annually (Jank et al., 1999) comes from tropical pasture-based enterprises (Faria et al., 1997). Grasslands are also important in geographical terms and recent surveys have shown that pastures occupy 0®73 of the agricultural land in Brazil (c. 180 m ha), which roughly corresponds to 0®20 of the total area of the country (IBGE, 2002). Some farmers in Brazil use high-yielding tropical grasses. In such production systems high dry-matter (DM) yields of pasture require a large mineral nutrient supply, and N has been claimed as the most important nutrient controlling grass productivity (Jarvis et al., 1995; Whitehead, 1995; Oliveira et al., 2001). However, additions of N through the mineralization of soil organic matter, atmospheric (wet and dry) deposition and biological-fixing activities in soils are uncertain and generally contribute inadequate amounts to sustain high pasture productivity. In this context, intensively managed grassland systems require considerable amounts of N fertilizers to sustain high pasture DM yields because N fertilizer provides N for pasture plants to achieve the potential growth allowed by the amount of energy intercepted (Lemaire and Gastal, 1997). Pastures fertilized with N attain animal production levels higher than those expected from tropical grass-legume mixtures and are more robust to various forms of management (Teitzel, 1991; Teitzel et al., 1991). For tropical grass species a Correspondence to: G. B. Martha Jr, Embrapa Cerrados, BR 020, km 18, CP 08223, CEP 73301-970, Planaltina, Distrito Federal, Brazil. E-mail: gbmartha@cpac.embrapa.br Received 25 January 2002; revised 1 October 2003 Ó 2004 Blackwell Publishing Ltd. Grass and Forage Science, 59, 80–90 80