Application of (~(15)NH_4)_2SO_4 to study N dynamics in hoop pine plantation and adjacent native forest of subtropical Australia: the effects of injection depth and litter addition

摘要

Purpose Hoop pine (Araucaria cunninghamii) is a nitrogen (N)-demanding native Australian softwood plantation species. Litter quality and its effects on soil mineral N and ~(15)N transformations have not been well studied in the hoop pine plantation and adjacent native forest. The present study was conducted to determine the impact of ~(15)N injection depth and litter additions on the dynamics and fate of mineral ~(15)N and also to compare the difference in litter quality, ~(15)N dynamics, and fate between the hoop pine plantation (HP) and the adjacent native forest (NF).rnMaterials and methods The experiments were done in the Yarraman State Forest (26°52' S, 151°51' E), southeastern Queensland. Materials of litter addition were prepared on the basis of ten random samples of litters taken from the NF and HP sites using a 1 × 1-m quadrat. Litter additions were defined as: SL represented the average condition of forest floor in the forest ecosystems and DL represented the double average amount of litters in the forest ecosystem. Experiment 1 covered 2 forest types (NF and HP) × 3 litterrnrates (nil litter, SL, and DL)×3 ~(15)N injection depths (0, 2.5, and 5.0 cm). Experiment 2 included 2 forest types (NF and HP)×2 litter rates (nil litter and SL)×3 injection depths (0, 2.5, and 5.0 cm) of distilled water. The in situ core incubation method was used with an incubation period of 28 days. The isotope ratio of mineral N or/and total N (soil and litter) were analyzed using an isotope ratio mass spectrometer with a Eurovector elemental analyzer (Isoprime-EuroEA 3000).rnResults Total N and δ~(15)N were significantly higher, and C/N ratios and δ~(13)C were significantly lower in the NF litters than in the HP litters. The NF litters had significantly lower total ~(15)N and total ~(15)N recovery than the HP litters after ~(15)N addition. Litter addition had no significant effect on mineral ~(15)N transformations and δ~(15)N in the NF soil, but decreased ~(15)NO_3~- -N, mineral ~(15)N, and δ~(15)N and increased immobilized ~(15)N in the HP soil. The depth of added ~(15)NH_4~+ significantly altered total ~(15)N, δ~(15)N, and total δ~(15)N recovery in the litters, whereas it did not influence ~(15)NH_4~+-N, ~(15)NO_3~- -N, mineral ~(15)N, or immobilized ~(15)N in soils in the two forest ecosystems.rnDiscussion The NF litters had significantly higher δ~(15)N than the HP litters, indicating that the NF soil had a higher rate of nitrification than the HP soil. Higher litter quality in the NF was an important driving force for N cycling to promote strong N dynamics in the NF soil over the HP soil. The HP litters had significantly higher total ~(15)N than the NF litters after ~(15)N addition, implying that soil mineral N was relatively deficient in the HP in comparison with the NF. Litters decreased nitrification and increased immobilization in the HP soil, showing forest litters resulted in more N immobilization to prevent the loss of substantial quantities of NO_3~- through leaching or denitrification. The depth of ~(15)N injection did not significantly alter concentrations of ~(15)NH_4~+ -N, ~(15)NO_3~- -N, mineral ~(15)N, and immobilized ~(15)Nrnin the NF and HP soils, suggesting that the depth of ~(15)N injection had no significant influence on the evaluation of soil N transformations.rnConclusions The NF litters had significantly higher total N and δ~(15)N and lower C/N ratios and δ~(13)C than the HP litters. Mineral N was relatively insufficient in the HP soil relative to the NF soil. The HP litters facilitated more N immobilization in the soil to reduce the loss of substantial quantities of NO_3~- through leaching or denitrification. The depth of ~(15)N added did not significantly alter concentrations of ~(15)NH_4~+-N, ~(15)NO_3~+ -N, mineral ~(15)N, and immobilized ~(15)N in the NF and HP soils. The application of ~(15)N solution by uniform sprinkling onto the soil surface can be used to study in situ field N (including mineral ~(15)N) transformations in the 10-cm depth soils of both forest ecosystems.