Soil organic carbon accumulation modes between pioneer and old‐growth forest ecosystems

Abstract Increasing evidence suggests that high biomass and litterfall do not necessarily bring about soil organic carbon (SOC) sinks, contrary to the assumption that higher litterfall implies higher SOC when designing carbon models. The underlying mechanism is related to the quality of litter. We conducted 15 years (2000–2015) of consecutive field measurements of δ13C values in SOC and plants in a pioneer forest (Pinus massoniana forest, PF) and an old‐growth forest (monsoon evergreen broadleaved forest, BF), using an isotope mixing model based on mass balance to quantify the effects of vegetation on SOC stock and soil characteristics. The carbon to nitrogen (C/N) ratio of litter in BF was lower than that in PF. The proportion of organic carbon yield input to the soil (Cinput) to the total litter carbon loss during decomposition was 38.7 ± 3.3% and 28.0 ± 2.1% in BF and PF respectively. New carbon input was higher in BF (148.7 ± 8.8 g C m−2 year−1) than PF (99.7 ± 4.5 g C m−2 year−1), though there was a non‐significant difference in annual litterfall between the two forests. Moreover, the Cinput was concentrated in the topsoil layer in PF but distributed in a more dispersed state across the whole soil profile in BF. Consequently, only the δ13C values of SOC decreased in the topsoil layer of PF, whereas these decreased at both soil depths in BF from 2000 to 2015. Compared with PF, BF exhibited higher carbon input and a more favourable soil environment for carbon storage. It was the amount of intermediate product (i.e. Cinput) of litter decomposition, not the amount of litterfall itself, that drove the contrasting differences in SOC status. Synthesis and applications. Litter quality controls soil organic carbon (SOC) accumulation by regulating the fate of decomposing litter, which may explain why old‐growth forests can sustainably accumulate carbon in soil. This finding questions the carbon models that predict the dependence of SOC accumulation on biomass and litter yield and suggests that litter quality should be valued in future carbon cycling models.