Application of (15NH4)2SO4 to study N dynamics in hoop pine plantation and adjacent native forest of subtropical Australia: the effects of injection depth and litter addition
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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 15N 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 15N injection depth and litter additions on the dynamics and fate of mineral 15N and also to compare the difference in litter quality, 15N dynamics, and fate between the hoop pine plantation (HP) and the adjacent native forest (NF). Materials and methods The experiments were done in the Yarraman State Forest (26൲' S, 151൱' 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 litter rates (nil litter, SL, and DL)?3 15N 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). Results Total N and d 15N were significantly higher, and C/N ratios and d 13C were significantly lower in the NF litters than in the HP litters. The NF litters had significantly lower total 15N and total 15N recovery than the HP litters after 15N addition. Litter addition had no significant effect on mineral 15N transformations and d 15N in the NF soil, but decreased 15NO3--N, mineral 15N, and d 15N and increased immobilized 15N in the HP soil. The depth of added 15NH4+ significantly altered total 15N, d 15N, and total 15N recovery in the litters, whereas it did not influence 15NH4+-N, 15NO3--N, mineral 15N, or immobilized 15N in soils in the two forest ecosystems. Discussion The NF litters had significantly higher d 15N 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 15N than the NF litters after 15N 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 NO3- through leaching or denitrification. The depth of 15N injection did not significantly alter concentrations of 15NH4+-N, 15NO3--N, mineral 15N, and immobilized 15N in the NF and HP soils, suggesting that the depth of 15N injection had no significant influence on the evaluation of soil N transformations. Conclusions The NF litters had significantly higher total N and d 15N and lower C/N ratios and d 13C 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 NO3- through leaching or denitrification. The depth of 15N added did not significantly alter concentrations of 15NH4+-N, 15NO3--N, mineral 15N, and immobilized 15N in the NF and HP soils. The application of 15N solution by uniform sprinkling onto the soil surface can be used to study in situ field N (including mineral 15N) transformations in the 10-cm depth soils of both forest ecosystems.
Journal of Soil and Sediments
Soil Chemistry (excl. Carbon Sequestration Science)