Soil carbon turnover and sequestration in native subtropical tree plantations

Approximately 30% of global soil organic carbon (SOC) is stored in subtropical and tropical ecosystems but it is being rapidly lost due to continuous deforestation. Tree plantations are advocated as a C sink, however, little is known about rates of C turnover and sequestration into soil organic matter under subtropical and tropical tree plantations. We studied changes in SOC in a chronosequence of hoop pine (Araucaria cunninghamii) plantations established on former rainforest sites in seasonally dry subtropical Australia. SOC, δ13C, and light fraction organic C (LF C<1.6 g cm−3) were determined in plantations, secondary rainforest and pasture. We calculated loss of rainforest SOC after clearing for pasture using an isotope mixing model, and used the decay rate of rainforest-derived C to predict input of hoop pine-derived C into the soil. Total SOC stocks to 100 cm depth were significantly (P<0.01) higher under rainforest (241 t ha−1) and pasture (254 t ha−1) compared to hoop pine (176–211 t ha−1). We calculated that SOC derived from hoop pine inputs ranged from 32% (25 year plantation) to 61% (63 year plantation) of total SOC in the 0–30 cm soil layer, but below 30 cm all C originated from rainforest. These results were compared to simulations made by the Century soil organic matter model. The Century model simulations showed that lower C stocks under hoop pine plantations were due to reduced C inputs to the slow turnover C pool, such that this pool only recovers to within 45% of the original rainforest C pool after 63 years. This may indicate differences in soil C stabilization mechanisms under hoop pine plantations compared with rainforest and pasture. These results demonstrate that subtropical hoop pine plantations do not rapidly sequester SOC into long-term storage pools, and that alternative plantation systems may need to be investigated to achieve greater soil C sequestration.