Biological Survey of a Low-Productivity Pasture in Rondônia State, Brazil

Abstract

Some years after their establishment, areas of pasture planted to Brachiaria and other African grasses in the Brazilian Amazon 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. However, this process is not yet well understood. A series of evaluations was implemented in a 20-year-old pasture located on a representative cattle ranch in Rondônia state to estimate the botanical composition, standing phytomass (including leaf litter), macrofauna species richness and microbial biomass to characterize the soil biological conditions of a pasture site about to degrade and before rehabilitation practices were adopted. The aim was to characterize the type and degree of degradation already completed to record the baseline against which the results obtained by restoration practices could be compared. Soil C, N and available P stocks were 20.4 Mg C ha−1, 1.5 Mg N ha−1 and 8.3 kg P ha−1 respectively, higher than under natural forest and close to those found in productive pasture sites. Physical and chemical analyses did not indicate effective degradation. Woody species dominated by trees and shrubs such as Tabebuia spp., Erisma uncinatum and Vismia guianensis occupied 15–18% of the total area. Babaçu palm (Orbignya phalerata Mart) covered 12%, herbaceous weeds 4–11%, while the rest (mean 63.5%) contained a mixture of Brachiaria brizantha and Pannicum maximum. The high biomass accrual and the diversity of species suggest that the soil is still able to support a productive forage grass cover. Biomass above ground, including litter layer, totalled 131 Mg DM ha−1, or 42% of original forest. High standing biomass in the experimental area can be attributed to insufficient and/or inapropriate weed control since the early stage of pasture installation. Reformation of the study site would implicate a potential above-ground input of about 130 Mg C ha−1 (C/N = 49; C/P = 744), besides another 6.4 Mg C ha−1 derived from root biomass decomposition. The most frequently found invertebrates were the termites (368 ind m−2), restricted to the 0–10 cm layer, while earthworms (204 ind m−2), ants (152 ind m−2) and Coleoptera (146 ind m−2) were found to a depth of 30 cm. The presence of so-called ecosystem engineers indicates that this population, probably comprising anecics and rhizophagous species, is related to the high root and litter density. The average number of representatives is more comparable with sites undergoing regrowth of secondary vegetation than with degraded pastures. Microbiological evidence indicates that the pasture studied constitutes an unstressed system about to accumulate organic matter in the soil.