DEEP SOIL HETEROGENEITY AND FINE-ROOT DISTRIBUTION IN FORESTS AND PASTURES OF EASTERN AMAZONIA

Little is known about deep soil heterogeneity, or its relationship wit h fine root distribution. Beneath a mature, closed-canopy forest of ea stern Amazonia, and the pastures and secondary forests that are derive d from this forest, soil soft spots and hollow chambers occur to at le ast 9 meters depth. We measured the vertical distribution of these soi l patches, and compared chemical characteristics, mycorrhizal infectio n, and root density of soil soft spots with the surrounding matrix of more homogeneous soil. Soil soft spots and chambers varied little with depth, but occupied the greatest soil volume (0.8 to 1.2%) from 4 to 6 m depth in the mature forest. Soft spots had lower pH, P availabilit y and arbuscular mycorrhizal infection, and higher K availability than surrounding soil. Root length density was 2 to 15 times higher in sof t spots than in surrounding soil. In the pastures, roots were found on ly in soil soft spots at depths of > 3 m. Pastures and secondary fores t had more soil chambers in the upper meter of soil than mature forest , but were otherwise indistinguishable in their patterns of deep soil heterogeneity. Soil soft spots may be vestiges of cutter ant nest cham bers, while hollow chambers are cutter ant chambers and root channels. Chambers may act as conduits for root penetration and water penetrati on to deep soil.