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Information on decomposition of harvest residues may assist in the maintenance of soil fertility in second rotation (2R) hoop pine plantations (Araucuria cunninghumii Aiton ex A. Cunn.) of subtropical Australia. The experiment was undertaken to determine the dynamics of residue decomposition and fate of residue-derived N. We used [sup.15]N-labeled hoop pine foliage, branch, and stem material in microplots, over a 30-mo period following harvesting. We examined the decomposition of each component both singly and combined, and used [sup.13]C cross-pularization and magic-angle spinning nuclear magnetic resonance ([sup.13]C CPMAS NMR) to chart C transformations in decomposing foliage. Residue-derived [sup.15]N was immobilized in the 0- to 5-cm soil layer, with approximately 40% [sup.15]N recovery in the soil from the combined residues by the end of the 30-mo period. Total recovery of [sup.15]N in residues and soil varied between 60 and 80% for the combined-residue microplots, with 20 to 40% of the residue [sup.15]N apparently lost. When residues were combined within microplots the rate of foliage decomposition decreased by 30% while the rate of branch and stem decomposition increased by 50 and 40% compared with rates for these components when decomposed separately. Residue decomposition studies should include a combined-residue treatment. Based on [sup.13]C CPMAS NMR spectra for decomposing foliage, we obtained good correlations for methoxyl C, aryl C, carbohydrate C and phenolic C with residue mass, [sup.15]N enrichment, and total N. The ratio of carbohydrate C to methoxyl C may be useful as an indicator of harvest residue decomposition in hoop pine plantations.

This study aimed to evaluate the improvement in soil fertility and plant nutrient use in a macadamia orchard following biochar application. The main objectives of this study were to assess the effects of poultry litter and green waste biochar applications on nitrogen (N) cycling using N isotope composition (δN) and nutrient availability in a soil-plant system at a macadamia orchard, 5 years following application. Biochar was applied at 10 t ha dry weight but concentrated within a 3-m diameter zone when trees were planted in 2007. Soil and leaf samples were collected in 2012, and both soil and foliar N isotope composition (δN) and nutrient concentrations were assessed. Both soil and foliar δN increased significantly in the poultry litter biochar plots compared to the green waste biochar and control plots. A significant relationship was observed between soil and plant δN. There was no influence of either biochars on foliar total N concentrations or soil NH-N and NO-N, which suggested that biochar application did not pose any restriction for plant N uptake. Plant bioavailable phosphorus (P) was significantly higher in the poultry litter biochar treatment compared to the green waste biochar treatment and control. We hypothesised that the bioavailability of N and P content of poultry litter biochar may play an important role in increasing soil and plant δN and P concentrations. Biochar application affected soil-plant N cycling and there is potential to use soil and plant δN to investigate N cycling in a soil-biochar-tree crop system. The poultry litter biochar significantly increased soil fertility compared to the green waste biochar at 5 years following biochar application which makes the poultry litter a better feedstock to produce biochar compared to green waste for the tree crops. [ABSTRACT FROM AUTHOR]

Purpose: Residue retention is important for nutrient and water economy in subtropical plantation forests. We examined decomposing hoop pine ( Araucaria cunninghamii Ait. Ex D. Don) residues-foliage, branches, and stem wood-to determine the changes in structural chemistry that occur during decomposition. Materials and methods: Residues were incubated in situ using 0.05 m microplots. We used solid-state C nuclear magnetic resonance (NMR) spectroscopy to determine the structural composition of harvest residues in the first 24 months of decomposition. Results and discussion: The spectral data for branch and stem residues were generally similar to one another and showed few changes during decomposition. The lignin content of branch and foliage residues decreased during decomposition. When residues were mixed together during decomposition, the O-alkyl fraction of foliage decreased initially then increased up to 24 months, while the alkyl carbon (C) fraction exhibited the opposite pattern. The decomposition of woody hoop pine residues (branch and stem wood) is surprisingly uniform across the major C forms elucidated with C NMR, with little evidence of preferential decomposition. When mixed with branch and stem materials, foliage residues showed significant short- and long-term compositional changes. This synergistic effect may be due to the C/N ratio of the treatments and the structure of the microbial decomposer community. Conclusions: Twenty-four months of decomposition of hoop pine residues did not result in substantial accumulation of recalcitrant C forms, suggesting that they may not contribute to long-term C sequestration. [ABSTRACT FROM AUTHOR]

Purpose: It is anticipated that global climate change will increase the frequency of wildfires in native forests of eastern Australia. Understorey legumes such as Acacia species play an important role in maintaining ecosystem nitrogen (N) balance through biological N fixation (BNF). This is particularly important in Australian native forests with soils of low nutrient status and frequent disturbance of the nutrient cycles by fires. This study aimed to examine N enrichment and N natural abundance techniques in terms of their utilisation for evaluation of N fixation of understorey acacias and determine the relationship between species ecophysiological traits and N fixation. Materials and methods: A trial was established at sites 1 and 2 located at Toohey Forest, Queensland, Australia, a eucalypt-dominated native forest, to examine the determination of BNF using N enrichment and N natural abundance methods. Toohey Forest is an urban forest and subjected to frequent fuel reduction burns to protect the adjacent properties. Plant physiological status was measured to determine the relationship between physiological and N fixation activities. Results and discussion: Both N enrichment and N natural abundance techniques may be used to estimate N fixation of acacia tree species. The estimation of BNF using N enrichment was higher than those of the N natural abundance method. A grass reference plant, Themeda triandra, as well as tree reference plants provided an appropriate δN signal. Potential B values for Acacia spp. between −0.3‰ and 1.0‰ provided an acceptable BNF estimation. This suburban forest is located nearby a busy highway leading to N deposition over time with consequent negative δN signal. This N deposition may explain the separation between the δN signal of the acacias and that of the reference plants which led to the successful use of the N natural abundance technique. Acacia leiocalyx demonstrated greater N fixation as well as photosynthesis and instantaneous water use efficiency than Acacia disparimma. However, no strong relationship between plant photosynthesis and N fixation was observed in this study. A high within-treatment variation may have masked the relationships between plant BNF activities and photosynthesis Conclusions: The N natural abundance technique is preferred to be used for future studies as it is simple and inexpensive compared with N enrichment method. The dependence of both species on BNF at site 2, where fuel reduction burning had not taken place for 8 years, suggests that the frequent burning impoverished the soil, and this has wider implications as higher fire frequencies are to be expected in other Australian ecosystems as a result of global climate change. [ABSTRACT FROM AUTHOR]

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 (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 1x1-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)x3 litter rates (nil litter, SL, and DL)x3 15N injection depths (0, 2.5, and 5.0 cm). Experiment 2 included 2 forest types (NF and HP)x2 litter rates (nil litter and SL)x3 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 δ15N were significantly higher, and C/N ratios and δ13C 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 δ15N in the NF soil, but decreased 15NO3 --N, mineral 15N, and δ15N and increased immobilized 15N in the HP soil. The depth of added 15NH4 + significantly altered total 15N, δ15N, 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.… [ABSTRACT FROM AUTHOR]

Background, aim, and scope Mulching is frequently used to overcome the drought problem in hardwood plantations that are increasingly being established in lower rainfall areas of Queensland, Australia because of increasing land values. In addition to soil water content, soil nitrogen (N) availability is another critical determinant of plantation productivity in these areas. The purpose of this study was to understand how soil mineral N dynamics, in situ N mineralization, and the fate of fertilized N would be affected by mulching during the early establishment of hardwood plantations. Materials and methods Two treatments (mulching, 1.57-kg m[sup-2] site preparation residues, and nonmulching) were applied to six 24-m-wide by 67.2-m-long plots at Pechey (27° 18' S, 152° 3' E) in southeast Queensland, Australia. At each sampling, two in situ N mineralization tubes were driven into the ground to a depth of 10 cm. One tube was immediately removed to provide baseline data for that sampling cycle. Another tube was left in situ for 1 month after which it was removed and a further two tubes were inserted to repeat the cycle. Sampling took place over 12 consecutive cycles, spanning one calendar year, commencing in January 2005. Between January and March 2005, another pair of soil tubes was driven into the ground to examine the fate of [sup15]N-labeled fertilizer. Briefly, 20-mL [sup15]N-labeled (NH[sup4])[sup2]SO[sub4] (10.3 at % [sup15]N) solution, containing 1.58 mg N, was uniformly applied on the soil surface of the tubes, giving 2.75 kg/ha N in each soil tube. Then, one tube was removed immediately and another left for I month as described above. Mineral N in all soil tubes was extracted with 2 M KCI. KCI extracts were analyzed colorirnetrically for NH[sub4][sup+]-N and NO[sub3][sup-]-N using a Lachat Autoanalyzer. The isotope ratio of mineral N and total N in [sup15]N-labeled tubes was analyzed using an isotope ratio mass spectrometer with a Eurovector elemental analyzer (Isoprime-EuroEA 3000). Results and discussion Mulching had no significant effect on net soil N mineralization during the 12-month period but had a significantly negative effect on net nitrification when net soil N mineralization was partitioned into net nitrification and net ammonification. Net nitrification constituted more than 85% of net N mineralization for both treatments. Nitrification was significantly correlated with air maximum temperature but poorly correlated with soil moisture. The results of unlabeled and [sup15]N-Iabeled in situ N mineralization experiments showed that reduced nitrification and increased N immobilization resulting from mulching were the main reasons for the lower mineral N of mulched soil in the early establishment period of hardwood plantation in subtropical Australia. Conclusions, recommendations, and perspectives The field incubation results indicate that if soil is mulched with site preparation residues, net nitrification and hence mineral N are lower than nonmulched soil during the first 12-month establishment period of hardwood plantation in subtropical Australia. Increased soil nitrification in the nonmulching treatment means greater potential for N losses through leaching and denitrification. N immobilization as a result of mulching implied that more N was retained in the soil, which may benefit the trees in the longer term. However, long-term research may be necessary to substantiate the conclusion. [ABSTRACT FROM AUTHOR]