Abstract
Soil organic matter (OM) is a source of fertility for food provisioning and a means for climate mitigation. A high pedodiversity observed over very short distances due to both past and present land use, such as in Western Africa (Northern Togo), makes this a crucial issue. In particular, some small spots covered by sacred forests (forêts sacrées) of high cultural value in this area are extremely biodiverse, calcareous and high in OM in contrast with the non-calcareous, poor OM, overgrazed woody savannah and agricultural surroundings. The objectives of our work were to understand this pedodiversity and the process of CaCO3 formation in the soil under the highly biodiverse forest using morphological, micromorphological, chemical and mineralogical soil analyses. The results show a high degree of weathering of the gneiss parent material, presence of plinthite gravels, dominance of 1:1 clays, formation of swelling clays in the imperfectly drained soils, and low OM content. The presence of palygorskite, although in traces, suggests drier intervals and is consistent in this context with the extreme wet and dry climate conditions in the region. In the OM rich soils under forest, bio-calcification takes place in the form of CaCO3 needles, micrite hypocoatings around biopores and calcified cells. Oribatid excrements are associated with calcite and organic matter in the sacred forest soil, indicating that litter recycling has played an important role in the formation of calcite. We hypothesize that the high biological activity releasing CO2, formation of HCO3– and precipitation of CaCO3 due to the Ca2+ released by the recycled organic matter and weathering of plagioclases, lead to different forms of secondary calcium carbonate in the sacred forest soils. The high oxalate content of the vegetation in the sacred forest suggests that calcium carbonate formation, possibly via the oxalate-carbonate pathway, may also have played a role in calcite precipitation in these in organic matter rich soils. The parent material of these soils is not calcareous, meaning that these are not lithogenic carbonates, thus making them an important carbon sink. The soil characteristics indicate a high potential for development of the soils of the area in both agricultural yields and in potential carbon sequestration relevant to global change policies.
Highlights
Across Benin and Togo (Dahomey Gap, West Africa, Fig. 1), sacred forests represent islands of pedo- and biodiversity in the middle of overgrazed woody semiarid savannahs and croplands (Kokou and Sokpon, 2006; Poch and Ubalde, 2006)
The cultural or religious significance of sacred forests in West Africa has contributed significantly to the conservation of these forest resources (Bhaghwat et al, 2005; Ceperley et al, 2010; Fraser et al, 2016; Irakiza et al, 2016; Kibet, 2011; Kokou and Dzifa, 2007; Upadhaya et al, 2003). Due to their tendency to be maintained over long periods of time, sacred forests often contain ecological communities that have otherwise been removed from a re gion (Fig. 2) and these forests remnants play an important role in the conservation of biodiversity (Campbell, 2004; Sanou et al, 2013)
The high organic matter (OM) and clay contents in profile 5 are consistent with higher cation exchange capacity (CEC), which indicates a high potential fertility and consistent with the higher topsoil N content in the sacred forest (208 mg N-NO3- /kg t for profile 5 compared to 9 and 10 mg N-NO3- /kg for profile 1 and 9)
Summary
Across Benin and Togo (Dahomey Gap, West Africa, Fig. 1), sacred forests (forets sacrees) represent islands of pedo- and biodiversity in the middle of overgrazed woody semiarid savannahs and croplands (Kokou and Sokpon, 2006; Poch and Ubalde, 2006). The cultural or religious significance of sacred forests in West Africa has contributed significantly to the conservation of these forest resources (Bhaghwat et al, 2005; Ceperley et al, 2010; Fraser et al, 2016; Irakiza et al, 2016; Kibet, 2011; Kokou and Dzifa, 2007; Upadhaya et al, 2003) Due to their tendency to be maintained over long periods of time, sacred forests often contain ecological communities that have otherwise been removed from a re gion (Fig. 2) and these forests remnants play an important role in the conservation of biodiversity (Campbell, 2004; Sanou et al, 2013). The importance of a sustainable forest management to maintain ecosystem functions and preserve biological diversity has grown since 1992, after the United Nations Conference on Environment and Development (UNCED), promoting criteria and indicators which will help to prevent a further disappearance of some plant species from these modern relics of old vegetation cover and biodiversity (Brand, 1997)
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