Secondary forests are expanding and have become a dominant
feature of tropical landscapes, with approximately 60% of the total
tropical forest area classified as secondary or degraded (ITTO,
2002). These regrowth forests are therefore highly relevant for estimations
and predictions for the global carbon budget. Our work
provides a pioneer estimation of SOC stocks from active pastures,
secondary forests and mature forests and the comparison of soil
types in the Australian Wet Tropics. We have demonstrated that
soil organic C stocks in my secondary forests (61.4 ± 18.3 Mg
C ha1 for the 0–30 cm depth) are comparable to those reported
recently in a meta-analysis on plantations and natural regeneration:
77.6 ± 2.3 Mg ha1 (Marín-Spiotta and Sharma, 2013).
Despite assumptions that conversion of mature forests to agricultural
land reduces SOC stocks, our results indicate that secondary
forests (natural regeneration) can either maintain similar
levels of C in soil or recover it after around 30 years of pasture
abandonment, depending on the type of soil. As suggested by other
studies, soil type is a factor that should be considered in estimates
and predictions of SOC stock accrual. Additionally, our results indicate
that increases in species diversity can assist SOC stock recovery
during forest succession. Woody plant diversity was found in
our study to be an important factor affecting the accumulation of
SOC during secondary forest regeneration. We conclude that quantification
and predictions of SOC stocks are not trivial, especially
during the process of secondary forest regeneration, which itself
can be highly stochastic and influenced by multiple factors.