Soil carbon and nutrient pools, microbial properties and gross nitrogen transformations in adjacent natural forest and hoop pine plantations of subtropical Australia

Background, Aims, and Scope. An improved understanding of important soil carbon (C) and nutrient pools as well as microbial activities in forest ecosystems is required for developing effective forest management regimes underpinning forest productivity and sustainability. Forest types and management practices can have significant impacts on soil C and nutrient pools as well as biological properties in forest ecosystems. Soil C and nutrient pools were assessed for adjacent natural forest (NF), first rotation (1R) (50-year-old), and second rotation (2R) (1-year-old) hoop pine (Araucaria cunninghamii Ait. ex D. Don) plantations in southeast Queensland of subtropical Australia. Materials and Methods. Five transects spaced 3 m apart with 9 sampling points along each transect were selected (9.6 m x 12.0 m each site), with 45 soil cores (7.5 cm in diameter) collected and separated into 0-10 and 10-20 cm depths. These soils were analysed for total C, total nitrogen (N), C (delta C-13) and N (delta N-15) isotope composition. The 0-10 cm soils were analysed for pH, CEC, exchangeable cations, total P and total K, and assayed for microbial biomass C and N, respiration, metabolic quotient, potential mineralizable N (PMN), gross N mineralization (M) and immobilization (I). Results. Total C and N in 0-10 cm soils were higher under NF and 1R plantation than under 2R plantation, while they were highest in 10-20 cm soils under NF, followed by the 1R and then 2R plantation. delta C-13 was lower under NF than under the plantations, while delta N-15 was higher under NF than under the plantations. Total P was the highest under NF, followed by the 1R and then 2R plantation, while total K was higher under the 2R plantation. No significant differences were detected for pH, CEC, exchangeable cations, microbial C and N, respiration and metabolic quotient among the 3 sites. PMN and M were higher under NF, while 1 was the highest under the 2R plantation, followed by the NF and then 1R plantation. Discussion. Soil total C and N in 0-10 cm depth were significantly lower under 2R hoop pine plantation than those under NF and 1R hoop pine plantation. There were significant reductions in soil total C and N from NF to 1R and from 1R to 2R hoop pine plantations in 10-20 cm depth. This highlights potential N deficiency in the 2R hoop pine plantations, and application of N fertilizers may be required to improve the productivity of 2R hoop pine plantations. There were no significant differences in other soil chemical and physical properties in 0-10 cm depth among the 3 sites under NF, 1R and 2R hoop pine plantations, except for soil total P and K. Soil microbial biomass C, CO2 respiration and metabolic quotient did not differ among the 3 sites assessed, perhaps mainly due to these biological variables being too sensitive to variations in soil chemical and physical properties and thereby being associated with a larger variability in the soil biological properties. However, soil potential mineralizable N, gross N mineralization and immobilization were rather sensitive to the conversion of NF to hoop pine plantation and forest management practices. Conclusions. Total C and N in the top 20 cm soil were highest under NF, followed by 1R and then 2R hoop pine plantations, indicating that N deficiency may become a growth-limiting factor in the 2R hoop pine plantations and subsequent rotations of hoop pine plantation. The sample size for Soil delta C-13 Seems to be much smaller than those for soil total C and N as well as delta N-15. The significant reductions in soil total P from NF to 1R and then from 1R to 2R hoop pine plantations highlight that P deficiency might become another growth-limiting factor in the second and subsequent rotations of hoop pine plantations. Soil microbial properties may be associated with large spatial variations due to these biological properties being too sensitive to the variations in soil chemical and physical properties in these forest ecosystems. Recommendations and Perspectives. Soil potential mineralizable N, gross N mineralization and immobilization were useful indices of soil N availability in response to forest types and management practices. The sampling size for Soil delta C-13 was much smaller than the other soil chemical and biological properties due to the different patterns of spatial variation in these soil properties.