Abstract
BackgroundOld-growth forests are irreplaceable with respect to climate change mitigation and have considerable carbon (C) sink potential in soils. However, the relationship between the soil organic carbon (SOC) turnover rate and forest development is poorly understood, which hinders our ability to assess the C sequestration capacity of soil in old-growth forests.MethodsIn this study, we evaluated the SOC turnover rate by calculating the isotopic enrichment factor β (defined as the slope of the regression between 13C natural abundance and log-transformed C concentrations) along 0–30 cm soil profiles in three successional forests in subtropical China. A lower β (steeper slope) is associated with a higher turnover rate. The three forests were a 60-year-old P. massoniana forest (PF), a 100-year-old coniferous and broadleaved mixed forest (MF), and a 400-year-old monsoon evergreen broadleaved forest (BF). We also analyzed the soil physicochemical properties in these forests to examine the dynamics of SOC turnover during forest succession and the main regulators.ResultsThe β value for the upper 30-cm soils in the BF was significantly (p < 0.05) higher than that in the PF, in addition to the SOC stock, although there were nonsignificant differences between the BF and MF. The β value was significantly (p < 0.05) positively correlated with the soil recalcitrance index, total nitrogen, and available nitrogen contents but was significantly (p < 0.01) negatively correlated with soil pH.ConclusionsOur results demonstrate that SOC has lower turnover rates in old-growth forests, accompanied by higher soil chemical recalcitrance, nitrogen status, and lower soil pH. This finding helps to elucidate the mechanism underlying C sequestration in old-growth forest soils, and emphasizes the important value of old-growth forests among global C sinks.
Highlights
Old-growth forests are irreplaceable with respect to climate change mitigation and have considerable carbon (C) sink potential in soils
The soil 13C natural abundance (δ13C) value at each depth was lowest in the broadleaved forest (BF), and there was no significant difference between the P. massoniana forest (PF) and mixed forest (MF) (p > 0.05; Table 1)
Our results showed that the β value was positively correlated with soil recalcitrance index’ (RI) (Fig. 3a), which is in accordance with the conventional opinion that reduced soil organic carbon (SOC) turnover is accompanied by enhanced soil chemical recalcitrance (Sollins et al 1996)
Summary
Old-growth forests are irreplaceable with respect to climate change mitigation and have considerable carbon (C) sink potential in soils. The relationship between the soil organic carbon (SOC) turnover rate and forest development is poorly understood, which hinders our ability to assess the C sequestration capacity of soil in old-growth forests. Increasing evidence shows that old-growth forests can continue to accumulate carbon (C) in soils (Zhou et al 2006a; Luyssaert et al 2008), contrary to the longstanding view that they are C neutral (Odum 1969; Jarvis 1989). The mechanism of the soil C sink in old-growth forests remains uncertain. Plant C input and the soil organic carbon (SOC) turnover rate are commonly considered two key factors in determining the C sequestration capacity of soil (Paul 2016; Nyawira et al 2017). Accurately quantifying the soil C turnover rate and its relationship with forest development is crucial to understanding the mechanisms of soil C sinks and predicting future C budget dynamics in the world’s forests
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