Root distribution and production in native and introduced pastures in the South American savannas

Abstract

The performance of root systems of introduced tropical pastures under grazing was compared with that of native savannah pasture. The site, located in the Eastern Plains (Llanos) of Colombia, was on a clay loam Oxisol. The pastures were an introduced grass-alone (Brachiaria dictyoneura) and B. dictyoneura + a legume (Centrosema acutifolium). The introduced pastures were fertilised at establishment with either low or high amounts. Together with the native pasture, they were measured for the distribution of root biomass and root length over four consecutive growing seasons, that is, at 10, 15, 19,22, 27, 32, 35, 39 and 47 months after sowing. The average standing live root biomass (and root length) at 0-80 cm soil depth of the grass-alone pasture was about 5.7 Mg ha-1 (21.5 km m-2); of the grass + legume pasture, 3.8 Mg ha-1 (13.8 km m-2); and of the native pasture, 1.4 Mg ha-1 (9.5 km m-2). Although both native and introduced pastures were deep rooted, the native pasture was relatively the most deep rooted and had the finest root system. The amount of N in the roots of the introduced pastures was as high as 18 kg ha-1 whereas in native pasture it was about 6 kg ha-1. At 32 months after sowing, the grass-alone pasture was estimated to have 31.6 Mg ha-1 more soil organic carbon than had the native pasture. These results indicate that nutrient cycling and carbon sequestration in soil occur at significant levels via root turnover of introduced tropical pastures under grazing.