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 We synthesized novel and legacy soil data from published and unpublished studies to better understand the drivers of soil property distributions and soil diversity in the West Usambaras, and to serve as a foundation for improved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.
Bibliographical noteFunding Information:
This work was supported in part by a U.S. Borlaug Global Food Security Graduate Research Grant to N.A. Jelinski through the Borlaug Institute for Global Food Security, Purdue University /USAID. Data collection was funded in part by the Belgian Vlir-UOS: Own Initiatives, “Landscape-ecological clarification of bubonic plague distribution and outbreaks in the western Usambara Mountains, Tanzania (LEPUS)”, and was also implemented in through the CGIAR Research Program on Climate Change, Agriculture and Food Security (CCAFS), and Forest Trees and Agroforestry (FTA), which is carried out with support from CGIAR Fund Donors and through bilateral funding agreements. For details on CCAFS please visit https://ccafs.cgiar.org/donors . The views expressed in this document cannot be taken to reflect the official opinions of these organizations. The authors wish to thank numerous farmers and community members in the West Usambara mountains (particularly Mr. Swadakati and Mr. Musa) for participating in soil sampling and description efforts and generously granting access to their land. The authors also wish to thank two anonymous reviewers for their detailed review and comments on the original manuscript which significantly improved its final form. Finally, the authors would like to express sincere appreciation to Geoderma Regional Editor-in-Chief Dr. Alfred Hartemink for constructive review of several versions of the manuscript, which initiated important conversations among the authors and greatly contributed to the final manuscript.
© 2017 Elsevier B.V.
- Cation exchange capacity
- Land use
- Landscape position
- West Usambara mountains