Accelerated soil carbon turnover under tree plantations limits soil carbon storage
The replacement of native forests by tree plantations is increasingly common globally, especially in tropical and subtropical areas. As soils store significantly more carbon (C) than both the atmosphere and plants combined, improving our understanding of the long-term effects of this replacement on soil organic carbon (SOC) content remains paramount for effectively managing ecosystems to mitigate anthropogenic C emissions. Current predictions of long-term forest C storage are based on meta-analyses which provide only indirect and ambiguous information about C turnover mechanisms. In this study, a long-term (c. 100 yr) chronosequence was used during which areas of subtropical native forest were replaced by Chinese fir.
We show by direct measurement that plantations significantly accelerate SOC turnover compared with native forest, an effect that persists for almost a century. The immediate stimulation of SOC decomposition was caused by warmer soil before the closure of the plantation’s canopy, but the long-term changes in mean residence time (tSOC) were coupled to litter inputs. Faster SOC decomposition (shorter tSOC) was associated with smaller litter inputs and correspondingly reduced nutrient availabilities which necessitated increased microbial mining of organic matter. Our results indicate a previously unelucidated mechanism of SOC accumulation and persistence associated with nutrient cycling in native forests and demonstrate a strong long-term potentially negative effect on climate mitigation when native forests are replaced by tree plantations.