Vegetation shift from annually prescribed burned Miscanthus sinensis semi-natural grassland to C3 plant caused depth-dependent soil organic matter turnover

In semi-natural grasslands in temperate regions such as Japan, the cessation of prescribed burning leads to a shift in vegetation toward broad-leaved forests and reduces soil sequestrated carbon. The objective of this study is to evaluate the effects of vegetation transition from grass species to broad-leaved forest on soil organic carbon (SOC) and soil organic matter (SOM) fractions in Miscanthus sinensis semi-natural grasslands. We collected biomass and soil at 5 or 10 cm intervals down to a depth of 30 cm from semi-natural grasslands where prescribed burning had been conducted continuously for at least 20 years, and from former semi-natural grassland where prescribed burning had ceased 10 years earlier and which are currently undergoing succession to broad-leaved forest. SOM was separated into free particulate organic matter (fPOM), occluded particulate organic matter (oPOM), and mineral-associated organic matter (MAOM). The change of vegetation by cessation of burning led to decreases δ13C and C/N of biomass. The rhizomes of M. sinensis were distributed particularly abundant at 5–10 cm depth, while the roots of C3 plants were restricted to the 0–5 cm layer in the semi-natural grassland. In contrast, M. sinensis declined and the roots of C3 plants were distributed more deeply, when prescribed burning was ceased. No significant decrease in SOC stock was observed following the cessation of burning. On the other hand, δ13C of the SOM fractions changed, with the magnitude of change increasing in the order of fPOM, oPOM, and MAOM, and the largest changes occurring at a depth of 5–10 cm. This suggests that, following burning cessation, C3 plant roots extended into deeper soil layers as vegetation succession progressed, resulting in the greatest shift in SOM δ13C at 5–10 cm depth. Furthermore, the results suggest char generated by prescribed burning may be directly stabilized as MAOM within the 0–5 cm layer. These results suggest that cessation of prescribed burning alters belowground biomass distribution and induces depth-dependent turnover of SOM fractions before a clear decline in SOC becomes apparent.