Soil organic carbon is buffered by grass inputs regardless of woody cover or fire frequency in an African savanna

Abstract

Woody plant encroachment (WPE) is a global trend that occurs in many biomes, including savannas, and accelerates with fire suppression. Since WPE can result in increased storage of soil organic carbon (SOC), fire management, which may include fire suppression, can improve ecosystem carbon (C) sequestration in savannas. At our study site in Kruger National Park, South Africa, we used a long‐term (~70 year) fire experiment to study the drivers and consequences of changes in woody cover (trees and shrubs) on SOC sequestration. We surveyed four fire manipulation treatments, replicated at eight locations within the park: annual high‐intensity burns, triennial high (dry season) and low‐intensity (wet season) burns, and fire exclusion, to capture the range of fire management scenarios under consideration. The changes in woody cover were calculated over a period similar to the experiment's duration (~80 years) using aerial photographs (1944–2018). Soils were analysed to 30 cm depth for SOC and δ13C, under and away from the tree canopy to isolate local‐ and landscape‐level effects of WPE on SOC. The largest increases in woody cover occurred with fire exclusion. We found that plots with higher increases in woody cover also had higher SOC. However, trees were not the only contributor to SOC gains, sustained high inputs of C4‐derived C (grasses), even under canopies in fire suppression plots, contributed significantly to SOC. We observed little difference in SOC sequestration between cooler triennial (wet season) burns and fire suppression. Synthesis. Grass input to soil organic carbon (SOC) remained high across the full range of woody cover created by varying burning regimes. The total SOC stocks stored from tree input only matched grass‐derived SOC stocks after almost 70 years of fire exclusion. Our results point to C4 grasses as a resilient contributor to SOC under altered fire regimes and further challenge the assumption that increasing tree cover, either through afforestation schemes or fire suppression, will result in large gains in C sequestration in savanna soils, even after 70 years.

Framework for a savanna burning emissions abatement methodology applicable to fire-prone miombo woodlands in southern Africa

“…In contrast to woody sequestration potential, this applies also to cultivated sites given very slow rates of identifiable SOC accumulation following agricultural abandonment, especially when associated with ongoing fire disturbance (Chidumayo and Kwibisa 2003;Walker and Desanker 2004;Williams et al 2008). Based on evidence from a multi-decadal study of SOC sequestration potential under different experimental fire regimes, Coetsee et al (2023) conclude that frequent low-severity fires offer a conservative management approach -at least in habitat conditions not undergoing detrimental woody encroachment.…”

“…As Syampungani et al (2020) observe, such vigorous regeneration potential on degraded sites offers substantially enhanced carbon sequestration opportunities compared with essentially moribund old-growth stands. Sequestration of SOC also has been shown to have possible, if variable, potential in southern African savanna systems under conditions of: (a) longer-term (multi-decadal) absence of, or infrequent, low-intensity burning; (b) commensurate maintenance of tree canopy cover to provide sustained litter inputs in addition to inputs from grasses; (c) in soils with higher clay contents (Bird et al 2000;Williams et al 2008;Holdo et al 2012;Pellegrini et al 2018Pellegrini et al , 2020Zhou et al 2022;Coetsee et al 2023). SOC sequestration potential in sandy miombo soils with characteristically very low organic content and base-exchange capacity is further limited given the complementary impacts of recurrent fires and intense harvesting on leaf litter inputs (Trapnell et al 1976;Frost 1996).…”