Utilization of carbon sources by the soil microbial communities of different forest types in subtropical Australia

Different soil water and nutrient conditions affect soil microbial ecology and changes in soil moisture and nutrients reflect changes in the soil environment over time. In this study, the utilization of carbon sources by the soil microbial community under slash pine, hoop pine (Araucaria cunninghamii) and kauri pine (Agathis australis) was investigated with manipulated soil moisture contents using MicroResp^TM methods. Results showed that under slash pine, carbon utilization was highest in soil at 60% water holding capacity (WHC) and lowest at 40% WHC. The high carbon utilization at 60% WHC is likely due to respiration from anaerobic microorganisms. Conversely, the hoop and kauri pine showed similar results with carbon utilization at its highest in soil at 20% WHC and lowest at 40% WHC. L-Malic acid, oxalic acid and L-Lysine were the carbon resources resulting in the highest microbial utilization intensity. The difference in carbon utilization efficiency between the forest types was obvious at 20% and 60% WHC where the carbon utilization under slash pine was lower than hoop and kauri pine at 20% WHC and higher than hoop and kauri pine at 60% WHC. However, excluding L-Arginine and L-Lysine, the order of carbon utilization in soils with 60% WHC was hoop pine>slash pine>kauri pine. The utilization of carbon sources in soils at 40% WHC was lower than 1μg CO₂ g^-1 h^-1 and the differences between forest types were not obvious. Owing to the high carbon utilization capacity of the microbial communities at 60% WHC, the soil microbial diversity was analyzed using the Shannon diversity index, Shannon evenness index and Simpson diversity index. The differences in values were not significant between forest types but diversity values were highest under hoop pine. This indicates that the decomposition of hoop pine needles is higher, resulting in a higher soil organic matter content and thus higher microbial growth and metabolism. The microbial metabolic diversity of the different forest soil types was analyzed by PCA. Results showed that it was not easy to distinguish between the metabolic diversity of the different soil microbial communities because the complex forest carbon sources result in vast differences in metabolic function. Principal component scores for each factor from analysis of microorganisms on 15 different carbon sources showed that the soil microbial community related to amino acids (PC1) and sugars (PC2), which explained 33. 80 and 24. 13% of the variance, respectively, were also sensitive to changes in forest soil microbial communities. RDA analysis was performed on each variable independently to maintain the contribution rate of forest types in different combinations of variables for the statistical characterization of a single variable, and the merits of the decision variables were revealed. On soil microbial metabolic characteristics and soil properties, RDA analysis showed that the utilization of L-Alanine, L-Arginine, D-(+) -Glucose and N-Acetyl-glucosamine was greatly influenced by pH in kauri pine. Utilization of Citric acid, L-Malic acid and γ-Amino butyric acid were influenced by total phosphorus in hoop pine, and D-(+) -Fructose, Citric acid and L-cysteine-HCl were influenced by water, total nitrogen and total carbon in slash pine.