Soil organic carbon responses to cessation of annual prescribed burning in a semi-natural grassland in Aso, Kumamoto, Japan

In the Aso region of Kumamoto Prefecture, Japan, approximately 23,000 ha of semi-natural grasslands have been traditionally maintained through annual prescribed burning. However, the response of soil organic carbon (SOC) and humic substances (HS) when prescribed burning is ceased, and vegetation succession begins, is not fully understood. We compared grasslands subjected to continued burning management with those in which burning ceased for 10 and 40 years to assess the impact on SOC stocks, and on the carbon stocks and the chemical properties of HS. The SOC stocks did not change significantly after 10 years without burning. However, a significant decline was observed after 40 years. The C4-derived SOC fraction decreased within 10 years of burning cessation. Although the carbon content of humic acid (HA) increased, the proportion of C4-derived carbon within HA decreased. HA also showed reduced blackness accompanied by decreased H/C and O/C ratios, suggesting a structural transition from aromatic and unsaturated bonds to more saturated structures. After 10 years, the stable SOC stock compensated for the decline in C4-derived carbon by increasing C3-derived inputs. However, after 40 years, the increase in C3-derived carbon was insufficient to offset the loss of C4-derived carbon, resulting in a net reduction in SOC. These findings show how long-term cessation of burning has reduced SOC stocks and decreased the chemical stability of HS, in the context of the replacement of grasslands (C4 plants) by C3 species in a classic model of shrub encroachment.