Grasses continue to trump trees at soil carbon sequestration following herbivore exclusion in a semiarid African savanna

Abstract

2020. Grasses continue to trump trees at soil carbon sequestration following herbivore exclusion in a semiarid African savanna.Abstract. Although studies have shown that mammalian herbivores often limit aboveground carbon storage in savannas, their effects on belowground soil carbon storage remain unclear. Using three sets of long-term, large herbivore exclosures with paired controls, we asked how almost two decades of herbivore removal from a semiarid savanna in Laikipia, Kenya affected aboveground (woody and grass) and belowground soil carbon sequestration, and determined the major source (C 3 vs. C 4 ) of belowground carbon sequestered in soils with and without herbivores present. Large herbivore exclusion, which included a diverse community of grazers, browsers, and mixed-feeding ungulates, resulted in significant increases in grass cover (~22%), woody basal area (~8 m 2 /ha), and woody canopy cover (31%), translating to ã 8.5 t/ha increase in aboveground carbon over two decades. Herbivore exclusion also led to a 54% increase (20.5 t/ha) in total soil carbon to 30-cm depth, with~71% of this derived from C 4 grasses (vs.~76% with herbivores present) despite substantial increases in woody cover. We attribute this continued high contribution of C 4 grasses to soil C sequestration to the reduced offtake of grass biomass with herbivore exclusion together with the facilitative influence of open sparse woody canopies (e.g., Acacia spp.) on grass cover and productivity in this semiarid system.

Three Decades of Divergent Land Use and Plant Community Change Alters Soil C and N Content in Tallgrass Prairie

“…However, in the annually burned and ungrazed treatment, we observed the opposite pattern—greater soil C content when the isotopic signature indicated more C 4 ‐derived organic matter. These conflicting patterns indicate that soil C content is higher when land management selects for a particular plant growth form (i.e., C 3 woody shrubs or C 4 grasses; Pellegrini et al, 2020; Wigley et al, 2020), especially if that growth form contributes proportionally more C belowground. The one exception to this was in the annually burned, grazed treatment (G1) where the plant community became increasingly more dominated by C 3 forbs over time with no subsequent increase in soil C content.…”

With Power Comes Responsibility – A Rangelands Perspective on Forest Landscape Restoration

“…Restoration of natural and semi-natural terrestrial ecosystems has important potential to deliver climate change mitigation and other ecosystem services (Morecroft et al, 2019). Restoring savannas and grasslands improves carbon storage in soils, protects water resources, and reduces the risk of catastrophic fires (Archibald et al, 2013;Buisson et al, 2019;Morecroft et al, 2019;Wigley et al, 2020). To regain ecological functionality and ecosystem services in degraded grassy biomes requires restoring native grass cover, the removal of woody plants and the application (and often re-introduction) of appropriate fire and herbivory regimes (Buisson et al, 2019).…”

A network to understand the changing socio‐ecology of the southern African woodlands (SEOSAW): Challenges, benefits, and methods

“…Carbon projects (Fisher, 2012; Grace et al., 2010; Khatun et al., 2015), national programs to reduce emissions from deforestation and degradation (Bond et al., 2010; Burgess et al., 2010), as well as other interventions that seek to reduce land use carbon emissions (Berry et al., 2013) all need information on changes in carbon stocks over time, both to predict carbon sequestration rates and to report emissions. PSP data can provide such information, and particularly help to quantify processes such as degradation, which is hard to measure with remote sensing (Chidumayo, 2013), and the role of grasses and soil in carbon sequestration, which can exceed sequestration by woody plants (Wigley et al., 2020).…”