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

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. Previous studies on carbon input might miss some key processes which constrain SOC accumulation.

To address these uncertainties related to SOC accumulation, Dr. XIONG Xin, under the joint guidance of Prof. ZHOU Guoyi and ZHANG Deqiang, conducted 15 years (2000–2015) of consecutive field measurements of carbon isotope abundance 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.

Results showed that the carbon to nitrogen 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 higher in BF than PF. Thus, new carbon input was higher in BF than PF, 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.

The researchers proposed that the amount of intermediate product (i.e., Cinput) of litter decomposition, not the amount of litterfall itself, drove the contrasting differences in SOC status between the two forests. Litter quality controlled 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.

This research entitled by "Soil organic carbon accumulation modes between pioneer and old-growth forest ecosystems" was published in Journal of Applied Ecology.

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