Carbon and nitrogen mineralization in soils under agro-pastoral systems in subtropical Central Brazil

Agro-pastoral systems that combine soybean and grasses in crop rotation have been recommended to sustain grassland productivity in the subtropical soils of Central Brazil (Cerrados forest area) with a low-fertility. In 1999, soil samples were taken during a field experiment on agro-pastoral system initiated in 1993 in a Purple Red Latosol in Campo Grande, MS, Brazil, from 1) a field with continuous soybean cropping; 2) a field with soy-bean cultivation after 4 years of use as a grassland (rotational soybean); 3) continuous grassland; 4) grassland planted after 4 years of soybean cultivation (rotational grassland), 5) degraded grassland; and 6) original Cerrados forest. Total carbon (C) and nitrogen (N) contents and mineralization potentials were determined. The results indicated that replacement of the Cerrados forest by a pure grassland and exploitation without proper cattle management and fertilizer application led to a decrease in both total C and N contents. Continuous cultivation of soybean on such a degraded grassland area induced a further decrease, which could be alleviated by rotation with a grassland. On the other hand, soil C and N mineralization potentials were not appreciably affected, indicating that grassland degradation did not affect these parameters. C and N mineralization showed a seasonal pattern, being higher in the rainy season (November) than in the dry season (April). Overall, it can be concluded that a grass-soybean rotation (agro-pastoral system) may benefit agricultural exploitation in the Brazilian Cerrados area, as both complement each other in maintaining a suitable soil C balance.