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Improved soil information in tropical montane regions is critical for conservation, sustainable agricultural
management, and land use planning, but is often challenged by topographic and land-use heterogeneity. The
West Usambara mountains are a part of the Eastern Arc chain of mountains of Tanzania and Kenya, a globally
important tropical montane ecoregion made up of isolated fault-block mountain complexes characterized by
high biological endemism, population density, and agronomic productivity. We synthesized novel and legacy
soil data from published and unpublished studies to better understand the drivers of soil property distributions
and soil diversity in the West Usambaras, and to serve as a foundation for improved soil mapping efforts across
the Eastern Arc. Analysis of the resulting dataset of 468 sites (ranging in elevation from 1040 to 2230 m.a.s.l.)
revealed that soil properties varied more significantly by land use and topography than by soil type, suggesting
that future mapping efforts in the region should focus primarily on soil property prediction and secondarily on
soil classification. Sites under cultivated land uses had the lowest topsoil soil organic carbon (SOC) concentrations
and highest pH values, and SOC generally increased with increasing elevation. Valley soils had
significantly lower surface SOC concentrations but higher exchangeable bases and pH values than all other
landscape positions. Soil pH decreased by an average of 3.5 units across the entire elevation gradient and decreased
by 1 unit with elevation even after SOC, land use and landscape position were included in multiple
regression models. The relationship of cation exchange capacity (CEC) to SOC and clay content varied by
landscape position. Therefore, particularly in montane regions where soils can vary significantly over short
distances, multiple functions may be necessary to produce improved estimates of parameters such as CEC. Soil
classification was driven most strongly by topography, with Acrisols (WRB Reference Group) and Ultisols (U.S.
Soil Taxonomy (ST)) as the dominant soil types, located primarily on mid slope, upper slope and crest landscape
positions, making up 47% and 75% of observed profiles, respectively. However, five ST Orders and seven WRB
Reference Groups were present in the dataset, with the highest soil diversity occurring at lower slope landscape
positions. Conclusions drawn from this large dataset support previous work in the West Usambaras and provide a
conceptual foundation from which to build improved soil maps across the Eastern Arc and in other tropical
montane systems throughout the world. |
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