Fire frequency differentially affects carbon pools and pyrogenic organic matter in a South African montane grassland

Abstract

Aims Grasslands are globally important for biodiversity and ecosystem services, including carbon storage, and have coevolved with disturbances such as fire. However, the effects of fire on soil carbon storage in southern African grasslands remain poorly understood. We investigated how fire frequency (annual, biennial, quinquennial, and infrequent) influences the distribution of soil organic carbon (SOC) pools in a 40-year burning experiment in a South African montane grassland. Methods Surface soils (0–15 cm) were sampled and separated into free particulate (fPOC), occluded particulate (oPOC), and mineral-associated organic carbon (MAOC) using density fractionation. Pyrogenic organic matter (PyOM) was quantified within each pool across fire treatments. Results Most SOC was mineral-associated, with 88.2% stored as MAOC and 10.1% as particulate organic carbon (free and occluded). Higher fire frequency increased SOC contents, particularly in subsurface soil (5–15 cm), where fPOC and MAOC showed the strongest responses. PyOM increased with depth across all treatments. Annual and biennial fires were less effective at increasing PyOM than infrequent fires, although this pattern depended on the temperature range used in thermogravimetric analysis. Conclusions This study advances our understanding of SOC dynamics in fire-adapted montane grasslands in South Africa by showing that frequent fire can lead to higher fPOC and MAOC pools in subsurface soil. This suggests an important role of fire for the soil carbon storage potential of these grasslands, both through the formation of more stable carbon in pools that are better protected against decomposition and through the contribution of recalcitrant pyrogenic organic material.

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