In situ mineral 15N dynamics and fate of added 15NH4+ in hoop pine plantation and adjacent native forest in subtropical Australia

Background, aim, and scope Hoop pine (Araucaria cunninghamii) is a nitrogen (N) demanding indigenous Australia softwood species with plantations in Southeast Queensland, Australia. Soil fertility has declined with increasing rotations and comparison study of N cycling between hoop pine plantations, and adjacent native forest (NF) is required to develop effective forest management for enhancing sustainable forest production and promoting environmental benefits. Field in situ mineral N-15 transformations in these two forest ecosystems have not been studied. Hence, the present study was to compare the differences in soil nutrients, N transformations, N-15 fluxes, and fate between the hoop pine plantation and the adjacent native forest. Materials and methods The study sites were in Yarraman State Forest (26 degrees 52' S, 151 degrees 51' E), Southeastern Queensland, Australia. The in situ core incubation method was used in the field experiments. Mineral N was determined using a LACHAT Quickchem Automated Ion Analyzer. N-15 were performed using an isotope ratio mass spectrometer with a Eurovector elemental analyzer. All statistical tests were carried out by the SPSS 11.0 for Windows statistical software package. Results Soil total C and N were significantly higher in the NF than in the 53-year-old hoop pine plantation. Concentrations of NO3- were significantly higher in the NF soil than in the plantation soil. The plantation soil had significantly higher N-15 and C-13 natural abundances than the NF soil. The NF soil had significantly lower C/N ratios than the plantation soil. NO3--N was dominated in mineral N pools in both NF and plantation soils, accounting for 91.6% and 70.3% of the total mineral N pools, respectively. Rates of net nitrification and net N mineralization were, respectively, four and three times higher in the NF soil than in the plantation soil. The (NO3-)-N-15-N and mineral N-15 were significantly higher in the NF soil than in the plantation soil. Significant difference in (NH4+)-N-15-N was found in the NF soil before and after the incubation. Discussion The NF soil had significantly higher NO3--N, mineral N, total N and C but lower delta N-15, delta C-13, and C/N ratios than the plantation soil. Moreover, the rates of soil net N mineralization and nitrification were significantly higher, but ammonification rate was lower in the NF than in the plantation. The NF soil had many more dynamic N transformations than the plantation soil due to the combination of multiple species and layers and, thus, stimulation of microbial activity and alteration of C and N pool sizes in favor of the N transformations by soil microbes. The net rate of N and N-15 transformation demonstrated differences in N dynamic related to the variation in tree species between the two ecosystems. Conclusions The change of land use and trees species had significant impacts on soil nutrients and N cycling processes. The plantation had larger losses of N than the NF. The NO3--N and (NO3-)-N-15-N dominated in the mineral N and N-15 pools in both forest ecosystems. Recommendations and perspectives Native forest soil had strong N dynamic compared with the plantation soil. Composition of multiple tree species with different ecological niches in the plantation could promote the soil ecosystem sustainability. The N-15 isotope dilution technique in the field can be quite useful for studying in situ mineral N-15 transformations and fate to further understand actual N dynamics in natural forest soils.