Root dynamics of native savanna and introduced pastures in the Eastern Plains of Colombia

Understanding of the process of soil C accumulation under introduced pastures on the savannas of South America is limited by lack of information on production, turnover and decomposition of roots. We estimated below-ground net primary productivity (BNPP), assessed the rate of root decomposition, and calculated the annual input of soil organic carbon (SOC) under native savanna vegetation (NS), a degraded pasture of the introduced grass Brachiaria humidicola (Bh), and well-managed pastures of the introduced grass B. dictyoneura alone (Bd) and in mixture with the legume Arachis pintoi (Bd + Ap). Responses were estimated using an in-growth mesh tube method and a flow model for C accumulation. The BNPP of the well-managed pastures was twice that of either NS or Bh. Turnover losses of the well-managed pastures were 1–2 times greater than NS and Bh. Decomposition constant of roots of Bd was lower than that of either NS or Bd + Ap, which resulted in higher residence time for Bd roots. The amount of BNPP remaining in the soil after 1 year of decomposition was about 2.3 times higher under well-managed pastures compared to that under NS. Higher C accumulation under well managed introduced pastures resulted, to some extent, from the high BNPP of the introduced species, possibly stimulated by fertilizer applied at establishment and regularly for maintenance and coupled with lower litter quality as indicated by high C:N and C:P ratios and decomposition dynamics of the below-ground residues.