Chemical and biological indicators of decline/degradation of Brachiaria pastures in the Brazilian Cerrado

Some years after their installation, areas of pasture planted to Brachiaria and other African grasses in the Brazilian savanna ("Cerrado") region show a continuous decline in forage and animal productivity. If no remedial measures are taken, plant cover decreases, the areas become invaded by weeds and the soil becomes compacted. In Brazil these pastures are described as degraded ("degradada") and this phenomenon has become the major limiting factor to extensive beef production. However, this process is not yet well understood. A series of evaluations were implemented in representative beef production areas in the Cerrado region in order to determine which chemical and biological parameters could best be used as indicators of degradation. Chronosequences of Brachiaria brizantha and Brachiaria decumbens pastures were used in this study. In degraded pastures the lowest quantities of plant litter and organic matter light fraction were observed, which are important pools in nutrient cycling. Soil microbial biomass carbon decreased as pastures age increased. Soil fertility and plant tissue analyses, and potentially mineralisable nitrogen were not suitable indicators of degradation. Physical fractionation by flotation of soil organic matter (SOM) macro-aggregates (>150 mum) differentiated very clearly more recently reformed B. brizantha pastures from a 20-year-old degraded B. decumbens pasture but this technique requires a lot of time and effort and did not show any very significant changes with age of the reformed pastures. The results of this study suggest that the most reliable and simple indicator of pasture decline was found to be the rate of litter deposition, followed by microbial biomass C and pasture regrowth after cutting. (C) 2004 Elsevier B.V. All rights reserved.