Microbial Biomass Carbon and Basal Respiration in a Sub-Humid Tropical Tree Legume-Grass Pasture of Brazil.

Brazil´s beef industry is based fundamentally in grass pastures, usually not fertilized with nitrogen due to economic reasons. These tend to degrade over time, due to N loss of the system, with economic and ecological consequences. One of the possible approaches to solve the degradation is the introduction of legumes into the pasture, including tree legumes, which have some management advantages. We describe the effect of shrub-tree legume introduction in a Brachiaria decumbens pasture on microbial biomass carbon (MBC) and basal respiration (BR) in a sub-humid tropical region, Pernambuco State, Northeastern Brazil. The experiment includes Brachiaria decumbens + Mimosa caesalpiniifolia (MC); Brachiaria decumbens + Gliricidia sepium (GS) and pure Brachiaria decumbens (BD). When included, the legumes are in double lines 1x0.5x15m. Each plot was sampled at the 0-0.10 and 0.10-0.20 m layers in a transect going from the middle of the legume row to the middle of the grass row. MBC and BR were determined through standard methods and the metabolic quotient (qCO₂) was calculated. The consortium plots had the highest MBC values with 299.05 e 215.72 μg g^-1 for MC and 374.20 e 269.32 μg g^-1 for GS at 0-0.10 m and 0.10- 0.20 m, but the effect was not significant. There was, though, a significantly higher BR and qCO2 for BD at the 0.10-0.20 layer. This is usually considered to indicate a higher stress level, since it denotes a higher respiration rate in relation to the current microbial biomass. If litter supply is not proportionally higher, this would lead to a decrease in microbial biomass over time, when compared to another system with lower qCO₂. Combined with the higher MBC for the legume mixtures, it may be surmised that the legume-grass mixture is probably more sustainable.