Long‐term (64 years) annual burning lessened soil organic carbon and nitrogen content in a humid subtropical grassland

Khatab Abdalla (1) , Pauline Chivenge (2) , Philippe Ciais (3, 4) , Vincent Chaplot (5, 6)

1 University of Bayreuth
2 UKZN - University of KwaZulu-Natal [Durban, Afrique du Sud]
3 LSCE - Laboratoire des Sciences du Climat et de l'Environnement [Gif-sur-Yvette]
4 IPSL (FR_636) - Institut Pierre-Simon-Laplace
5 LOCEAN - Laboratoire d'Océanographie et du Climat : Expérimentations et Approches Numériques
6 IRD - Institut de Recherche pour le Développement

Burning has commonly been used to increase forage production and nutrients cycling in grasslands. However, its long-term effects on soil organic carbon (SOC) and nitrogen (N) pools within the aggregates and the relation between aggregates-associated SOC and soil CO₂ emissions need further appraisal. This study evaluated the effects of 64 years of annual burning on SOC and N dynamics compared to annual mowing and undisturbed treatments in a grassland experiment established in 1950. Soils were sampled from four depths representing the upper 30 cm layer and fractionated into macroaggregates, microaggregates and silt + clay fractions. The macroaggregates were further fractionated into three occluded fractions. The SOC in the bulk soil and aggregates were correlated to soil CO₂ effluxes measured under field conditions. Compared to the undisturbed treatment, annual burning decreased aggregates stability, SOC and N in the upper 30 cm layer by 8%, 5% and 12%, respectively. Grassland mowing induced greater aggregates stability than burning only in the upper 5 cm. Burning also decreased SOC in the large macroaggregates (e.g., 0–5 cm) compared to mowing and the undisturbed grasslands but proportionally increased the microaggregates and their associated SOC. Soil N associated with aggregates decreased largely following grassland burning, for example, by 8.8-fold in the microaggregates within the large macroaggregates at 20–30 cm compared to the undisturbed grassland. Burning also increased soil CO₂ emissions by 33 and 16% compared to undisturbed and mowing, respectively. The combustion of fresh C and soil organic matter by fire is likely responsible for the low soil aggregation, high SOC and N losses under burned grassland. These results suggested a direct link between grass burning and SOC losses, a key component for escalating climate change severity. Therefore, less frequent burning or a rotation of burning and mowing should be investigated for sustainable grasslands management.

Mots clés: annual mowing; climate change; grassland sustainability; soil respiration; South Africa; Soil aggregates

Domaines: Géophysique [physics.geo-ph]; Géophysique [physics.geo-ph]

Khatab Abdalla, Pauline Chivenge, Philippe Ciais, Vincent Chaplot. Long‐term (64 years) annual burning lessened soil organic carbon and nitrogen content in a humid subtropical grassland. Global Change Biology, 2021, 27 (24), pp.6436-6453. ⟨10.1111/gcb.15918⟩. ⟨hal-03430263⟩