Mineral changes in the rhizospheres of conifer plantations for a weathered subtropical soil

Abstract

Tree species can have great impacts on soil. Since the rhizosphere is more responsive to external inputs than the surrounding bulk soil, we investigated the rhizosphere effect of the exotic slash pine and the native araucaria in long-term conifer monocultures in southern Brazil. Araucaria trees in a natural section of a nearby mixed araucaria forest were taken as the control. We assessed physical, chemical, and mineralogical properties of the underlying subtropical highly weathered soil. We did not find rhizosphere effects for most chemical and physical soil attributes, yet principal component analysis clearly distinguished the effects of the exotic and native conifers on rhizosphere properties by separating slash pine cluster from reforested and native araucaria clusters. Reductions in silt and sand contents under slash pine reforestation led to increases in clay and well-crystallized iron oxides contents. The clay mineralogy comprised kaolinite, mica/illite, and hydroxy-interlayered minerals, with kaolinite enrichment in the rhizospheres of both araucaria sites, mica/illite depletion in the rhizospheres of both araucaria and pine monocultures, and prevalence of well-crystallized iron oxyhydroxides in the slash pine rhizosphere. This study demonstrates a tendency toward increasing soil weathering in the conifer monocultures, potential negative impacts of the faster growing slash pine reforestation on soil organic carbon, available P, and total K possibly by mica/illite depletion, while araucaria had lower impacts on soil properties. These results show long-term potential loss of soil fertility and quality, which should be considered when monitoring soil changes in human modified ecosystems.