Soil carbon pool and rhizosphere carbon effects under naval orange orchards of different cultivation periods in hilly red soil region, southern China

The annual amount of carbon dioxide (CO₂) assimilated by orange orchards in subtropical region, China is more than 20% of total Chinese forest ecosystems. However, the studies on soil carbon pool in these orchards are rarely reported, especially in hilly red soil region, southern China. Carbon contents in bulk, rhizosphere and non-rhizosphere soils at 0-15 cm, 15-30 cm and 30-45 cm depths were measured in three typical orange orchards that varied in cultivation periods, but had similar fertilizer management practice. Compared with local forests, bulk soil carbon contents at 0-45 cm depth increased 16% and 66% in middle-aged (~20 year old) and old (~30 year old) orchards, respectively, while no significant difference with young orchard (~10 year old) was found (P<;0.05). All carbon contents were higher in rhizosphere soil than non-rhizosphere soil, which showed rhizosphere carbon effects were positive under different orchards of each depth. The rhizoshpere carbon effects at 0 15 cm decreased with increasing cultivation periods, while those at 15-30 cm and 30-45 cm depths were no significant differences among the three typical orange orchards. Additionally, carbon contents in bulk, rhizoshpere and non-rhizosphere soils decreased with increasing depths in our study orchards. Therefore, naval orange orchards would become the potential of carbon sink due to soil carbon accumulation with the increasing cultivation periods. We suggest that developing naval orange industry would be an unheeded practice to decrease atmosphere CO₂ concentration and construct low carbon agriculture in hilly red soil region, southern China.