Soil Carbon Pools in Adjacent Natural and Plantation Forests of Subtropical Australia

Soil C dynamics are not only important to both productivity and sustainability of terrestrial ecosystems, but also contribute significantly to global C cycling. Adjacent natural forest (NF), and first (1R) and second rotation (2R) hoop pine (Araucaria cunninghamii Aiton ex A. Cunn.) plantations in southeast Queensland, Australia, were selected to investigate the effects of conversion of NF to hoop pine plantations and forest management (harvesting and site preparation of plantation) on the size and the nature of C pools in surface (0–10 cm) soils using chemical extraction, laboratory incubation and 13C cross‐polarization with magic‐angle‐spinning nuclear magnetic resonance spectroscopy (13C CPMAS NMR). Conversion from NF to hoop pine plantations not only led to the reduction of soil total C (by 19.8%), water‐soluble organic C (WSOC) (by 17.7%), CaCl2–extractable organic C (by 38.8%), and hot water‐extractable organic C (HWEOC) (by 30.9%) and bioavailability of soil C (as determined by CO2 evolved in the incubation), but also to a change in chemical composition of soil C with lower O‐alkyl C and higher alkyl C under the 1R plantation compared with NF. Harvesting and site preparation did not significantly affect total soil C and most labile C pools (except for a decrease in WSOC), but led to a lower signal intensity in the alkyl C spectral region and a decreased alkyl C/O‐alkyl C (A/O‐A) ratio in the soil under the 2R compared with the 1R plantation. The shifts in the amount and nature of soil C following forest conversion may be attributed to changes in litter inputs, microbial diversity and activity, and the disturbance of soil during harvesting and site preparation.