Soil carbon dynamics in Afromontane grasslands, Cathedral Peak, Drakensberg, South Africa

Lindokuhle Dlamini soutiendra sa thèse mardi 19 décembre en visioconférence depuis l’Afrique du sud à 8 h 30 (heure française).

Résumé

Despite the importance of South Africa’s Afromontane grasslands for ecosystem services (water supply and biodiversity), soil organic carbon (SOC) research remains limited. These ecosystems evolved with fire, and fire exclusion leads to native plant afforestation. There is a global aversion to fire use, as evidenced by historical fire suppression initiatives. Recently, tree planting advocated as a “nature-based” solution to global warming prioritises carbon (C) sequestration through tree biomass but overlooks the impact on biogeochemical cycling and the fate/storage of C in soils in the form of SOC. Using the Cathedral Peak Research Catchments, initiated in the 1940s, this study sought to investigate the impact of fire-exclusion-driven afforestation, aspect, post-afforestation grassland degradation, and climate variability on SOC dynamics at a catchment scale. The study focused on three catchments: the degraded north-facing Catchment III which was planted with Pinus patula in 1958 but burned down by wildfires in 1981, the natural north-facing Catchment VI maintained through prescribed biennial burns, and the south-facing Catchment IX with a fire exclusion treatment. Accidental fires still occurred every 2 to 5 years, maintaining periodically burnt grassland and an afforested patch in Catchment IX. Soil samples were collected at set increments down to 1 m depth across a slope gradient (top, middle, bottom) to determine soil bulk density, SOC stocks, δ¹³C signature, active C, total microbial activity, and water-stable aggregates. Measurements of Rs were performed using an 8-chamber LI-8100A automated system (C-VI) and a monthly static chamber-based manual technique (all sites, approximately 3 years dataset). Despite global concerns about fire use, results revealed that excluding fire reduced topsoil SOC stocks, macro-aggregates and SOC protected within these fractions, and increased CO₂ emissions as well as doubled the Q10 values. Cooler south-facing slopes stored more SOC and emitted less CO₂ than north-facing slopes. Post-afforestation degradation led to accelerated soil erosion, SOC redistribution, and SOC loss while creating localized SOC sinks at depositional zones. The legacy effect explained the observed δ¹³C values, with more SOC derived from C4 grasses than C3 trees. This study cautions against afforestation in these grasslands, though vulnerable, Afromontane grasslands serve as valuable SOC reservoirs with potential stability amidst rising droughts and wildfires.

Mots clés

active carbon; afforestation; fire exclusion; microbial activity; soil organic carbon stocks; soil respiration; temperate grasslands; water-stable aggregates; wildfires; δ¹³C signature

Jury

Heike Knicker, Instituto de la Grasa-CSIC – présidente
Ailsa Hardie, Stellenbosch University – examinatrice
Johan Van Tol, University of The Free State – examinateur
Elmarie Kotze, University of The Free State – directeur de thèse
Jean Lévêque, université de Bourgogne – directeur de thèse
Marco Panettieri, Instituto de Ciencias Agrarias – examinateur
Olivier Mathieu, université de Bourgogne – codirecteur de thèse
Mathieu Thevenot, université de Bourgogne – codirecteur de thèse
Gregor Feig Efteon, SAEON – rapporteur
Philippe Amiotte-Suchet, université de Bourgogne – invité

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Soil carbon dynamics in Afromontane grasslands, Cathedral Peak, Drakensberg, South Africa

Lindokuhle Dlamini soutiendra sa thèse mardi 19 décembre en visioconférence depuis l’Afrique du sud à 8 h 30 (heure française).

 

Résumé

Despite the importance of South Africa's Afromontane grasslands for ecosystem services (water supply and biodiversity), soil organic carbon (SOC) research remains limited. These ecosystems evolved with fire, and fire exclusion leads to native plant afforestation. There is a global aversion to fire use, as evidenced by historical fire suppression initiatives. Recently, tree planting advocated as a “nature-based” solution to global warming prioritises carbon (C) sequestration through tree biomass but overlooks the impact on biogeochemical cycling and the fate/storage of C in soils in the form of SOC. Using the Cathedral Peak Research Catchments, initiated in the 1940s, this study sought to investigate the impact of fire-exclusion-driven afforestation, aspect, post-afforestation grassland degradation, and climate variability on SOC dynamics at a catchment scale. The study focused on three catchments: the degraded north-facing Catchment III which was planted with Pinus patula in 1958 but burned down by wildfires in 1981, the natural north-facing Catchment VI maintained through prescribed biennial burns, and the south-facing Catchment IX with a fire exclusion treatment. Accidental fires still occurred every 2 to 5 years, maintaining periodically burnt grassland and an afforested patch in Catchment IX. Soil samples were collected at set increments down to 1 m depth across a slope gradient (top, middle, bottom) to determine soil bulk density, SOC stocks, δ¹³C signature, active C, total microbial activity, and water-stable aggregates. Measurements of Rs were performed using an 8-chamber LI-8100A automated system (C-VI) and a monthly static chamber-based manual technique (all sites, approximately 3 years dataset). Despite global concerns about fire use, results revealed that excluding fire reduced topsoil SOC stocks, macro-aggregates and SOC protected within these fractions, and increased CO₂ emissions as well as doubled the Q10 values. Cooler south-facing slopes stored more SOC and emitted less CO₂ than north-facing slopes. Post-afforestation degradation led to accelerated soil erosion, SOC redistribution, and SOC loss while creating localized SOC sinks at depositional zones. The legacy effect explained the observed δ¹³C values, with more SOC derived from C4 grasses than C3 trees. This study cautions against afforestation in these grasslands, though vulnerable, Afromontane grasslands serve as valuable SOC reservoirs with potential stability amidst rising droughts and wildfires.

 

Mots clés

active carbon; afforestation; fire exclusion; microbial activity; soil organic carbon stocks; soil respiration; temperate grasslands; water-stable aggregates; wildfires; δ¹³C signature

 

Jury

Heike Knicker, Instituto de la Grasa-CSIC - présidente
Ailsa Hardie, Stellenbosch University - examinatrice
Johan Van Tol, University of The Free State - examinateur
Elmarie Kotze, University of The Free State - directeur de thèse
Jean Lévêque, université de Bourgogne - directeur de thèse
Marco Panettieri, Instituto de Ciencias Agrarias - examinateur
Olivier Mathieu, université de Bourgogne - codirecteur de thèse
Mathieu Thevenot, université de Bourgogne - codirecteur de thèse
Gregor Feig Efteon, SAEON - rapporteur
Philippe Amiotte-Suchet, université de Bourgogne - invité