Anthony M. O'Connell

Nitrogen release from eucalypt leaves and legume residues as influenced by their biochemical quality and degree of contact with soil

Large areas of short-rotation eucalypt plantations are being established in south-western Australia on land previously used for agriculture. Options for maintaining soil N supply include retention of harvest residues and legume inter-cropping. We evaluated the effects of adding the residues of five legume species and Eucalyptus globulusleaves on inorganic N dynamics in two soils (a Rhodic Ferralsol or red earth and a Haplic Podzol or grey sand) using two modes of residue application in a laboratory incubation experiment (519 days). The time course of net N immobilisation and mineralisation in both soils was strongly influenced by the type and mode of application. Eucalypt leaves caused strong N immobilisation (−7 mg N g−1 residue-C) over the entire 519-day incubation, whereas for the legume species, N that was eventually immobilised at the start of the incubation, remineralised later to different degrees. Amongst the legumes, largest amounts of N were released from lupin residues (18 mg N g−1 residue-C) and lowest amounts from field pea (2 mg N g−1 residue-C). However, initial residue quality parameters were not significantly (P > 0.05) correlated with N release from the residues. Grinding and incorporating of the residues caused a much greater immobilisation of N than when residues were cut and surface applied. When ground residues were incorporated, immobilisation of N was more severe and endured for longer in the finer textured red earth than in the coarse textured grey sand. Where residues were surface applied, N dynamics were similar for both soil types. The results of this study suggest that legumes used as a mulch in eucalypt plantations are a readily available source of N for trees, and that the benefits from retention of harvest residues are more likely in maintaining soil N fertility on the long-term.

How does residue management impact soil organic matter composition and quality under Eucalyptus globulus plantations in southwestern Australia

This study investigated the influence of harvest residue management practices on soil organic matter (SOM) composition and quality from two second-rotation Eucalyptus globulus plantations in southwestern Australia, using solid-state 13C nuclear magnetic resonance (NMR) spectroscopy with cross-polarisation and magic-angle-spinning (CPMAS). Soil samples (0–5 cm) were collected every 12 months for 5 years from two sites that had contrasting soil types and fertility. Harvest residue management treatments established at both sites were (a) no harvest residues; and (b) double harvest residues. The use of 13C CPMAS and DD NMR spectroscopy enabled the successful non-destructive detection of SOM quality changes in the two E. globulus plantations. Relative intensities of 13C CPMAS NMR spectral regions were similar at both sites, and for both harvest residue treatments, indicating that SOM composition was also similar. Dipolar dephasing (DD) NMR spectra revealed resonances in SOM assigned to lignin and tannin structures, with larger resonances in the carbonyl and alkyl C regions that were indicative of cuticular material, enabling detection of changes in SOM quality. Retention of double harvest residues on the soil surface increased the soil quality compared with removal of all harvest residues at both sites as indicated by the NMR aromaticities, but this was most noticeable at Manjimup, which had greater initial soil fertility.

Changes in soil N status and N supply rates in agricultural land afforested with eucalypts in south-western Australia

In south-western Australia, plantations of Eucalyptus globulus are being established on land that has previously been used for conventional agriculture. Sustaining the productivity of these tree plantations in second and subsequent rotations will depend partly on maintenance of soil fertility, especially soil nitrogen (N) supply rates. We compared soil N status and supply rates between adjacent pasture and 6–11 year old first-rotation eucalypt plantations at 31 paired sites in south-western Australia. Total soil N varied widely among sites (0.07–0.68% in the fraction <2 mm of the 0–10 cm soil layer), but concentrations averaged over all sites did not differ between land-use types. However, measurements of the indices of mineralization (mineral N produced during incubation of intact cores), potentially available N (from short-term anaerobic incubation) and model-predicted mineralization rates during 28-day aerobic incubations were generally lower in afforested soils than in pasture soils. This finding was supported by in situ field estimates of N mineralization over a full year at two contrasting paired pasture-plantation sites. At each site there was a marked reduction (2–3-fold) in net annual mineral N flux rates in soils under eucalypt plantations. Reduced N mineralization associated with tree plantations was due to both changes in soil organic matter quality and the generally lower soil moisture content under trees in comparison with pasture. These results suggest that N supply rates of pasture soils are likely to decline when the land is planted to successive crops of eucalypts. Eucalypt plantation managers will need to take account of this and implement management strategies to maintain adequate N nutrition to sustain tree growth in future rotations.

Effect of legume understorey on decomposition and nutrient content of eucalypt forest litter

SummaryIn Jarrah (Eucalyptus marginata Donn ex Sm.) forest of south-western Australia dense germination and regeneration of the native legumeAcacia Pulchella R. Br. can occur following moderate to high intensity fire. The effect of this legume understorey on rate of decomposition and change in nutrient content ofE. marginata litter was investigated using the mesh bag techniques and by examining four components of forest floor litter representing increasing stages of decomposition. E. marginata leaf litter confined in mesh bags lost 37% of its initial dry weight in the first 8 months on the forest floor and 44% of its initial dry weight after 20 months. During this period weight loss was similar for leaf litter located in forest without legume understorey and for leaf litter placed under dense stands ofA. pulchella. MixingA. pulchella litter withE. marginata litter had no significant effect on rate ofE. marginata litter breakdown.The presence of understorey vegetation had a marked effect on chemical composition of decomposingE. marginata leaves. After 8 and 20 months exposure on the forest floor, leaf litter in mesh bags placed underA. pulchella understorey had significantly (P<0.001) higher concentration and contained significantly (P<0.001) greater amounts of N, P, K, S, Ca and Mg than leaf litter placed in areas without legume understorey. This effect was particularly marked for N and P. In forest without legume understorey the amounts of these two nutrients inE. marginata leaf litter changed little during the first 20 months of decomposition, but forE. marginata leaf litter in mesh bags underA. pulchella there were absolute gains of up to 68% in the amount of N and 109% in the amount of P during this period. This represents accumulation of N and P from sources outside the litter bags.The concentration of N, P, S, Ca and Mg were higher at each of the four stages of decomposition in eucalypt leaf litter collected from the forest floor beneathA. pulchella compared to eucalypt leaf litter collected in forest without understorey. Concentrations of N, P and S increased with stage of decomposition. Levels of these three nutrients in eucalypt litter from under the legume were 1.5 to 2.9 fold higher than in the same component of litter from forest without understorey.The effect of legume understorey on nutrient concentrations in the forest floor and on Cielement ratios in decomposing litter is discussed in relation to long term rates of litter breakdown and net mineralisation of litter nutrients.

Impact of land-use on soil organic matter quality in south-western Australia—characterization with 13C CP/MAS NMR spectroscopy

The influence of change in land-use from native vegetation to pasture (20-71 yr after conversion), and subsequent change from pasture to eucalypt plantation (7-10 yr after conversion) on soil organic matter quality was investigated using C-13 CP/MAS NMR spectroscopy. We studied surface soil (0-10 cm) from six sites representing a range of soil, and climate types from south-western Australia. Total C in the samples ranged from 1.6 to 5.5%, but the relative proportions of the four primary spectral regions (alkyl, O-alkyl, aromatic and carboxylic) were similar across the sites, and changes due to land-use at each site were relatively minor. Main impacts of changed land-use were higher O-alkyl (carbohydrate) material under pasture than under native vegetation and plantation (P = 0.048), and lower aromatic C under pasture than under native vegetation (P = 0.027). The decrease in aromatic C in pasture soils was related to time since clearing

Nutrient accessions to the forest floor in Karri (Eucalyptus diversicolor F. Muell.) forests of varying age

Abstract Nutrient accessions to the forest floor in rainfall, throughfall and stemflow from the understorey and tree strata were measured in karri (Eucalyptus diversicolor F. Muell.) forest of south-west Western Australia. These data, for stands aged 2, 6, 9, and 40 years and for mature forest, were compared with annual accessions of nutrients in litterfall. With increasing age of the stands, the concentration and amounts of Ca, K, Mg, Na and Cl in throughfall increased. The amounts of Mg, Na and Cl in stemflow from the overstorey were also greatest in the older stands. Although concentrations of nutrients in stemflow from the tree stratum were higher than from the understorey, the quantities of nutrients transported to the forest floor in stemflow from the understorey were greater because of its greater volume. Rainfall is the major source of Na (64–91%) and Cl (51–79%) and it also contributes significantly to Mg (19–50%) accessions. Of the nutrient pathways between canopy and forest floor, litterfall accounts for the largest proportion of N (89–93%), Ca (80–87%) and P (67–79%) recycled in the karri forest. About equal amounts of K are transferred in litterfall, throughfall and stemflow with approximately 80% of stemflow K originating from the understorey strata.

Nutrient accumulation in and release from the litter layer of karri (Eucalyptus diversicolor) forests of southwestern Australia

Abstract Dry weight and nutrient content of the litter layer of pole-stand karri forest growing in southwestern Australia were related to period of accumulation by the function Xt=XSS (1 − e−kt)+ X0e−kt where Xt is the amount of dry weight or nutrients present in the forest floor at time t, XSS is the amount present under equilibrium conditions, X0 is the amount present at t=0, and k is a constant. This relation explained 90% of the variation in litter dry weight and from 65% to 91% of the variation in amounts of nutrients accumulating in the forest floor. Rate of dry weight and nutrient accumulation was initially rapid, reaching 50% of the equilibrium values, XSS, in 2.4 to 5.5 years. Almost half of the litter dry weight and two-thirds of the N, P and S were stored in the fine ( Rates of release of dry weight and nutrients from the forest floor were estimated from annual inputs in litterfall and annual increments in the amounts stored in the forest floor. The mobile elements Na and K were released rapidly from the litter layer, Ca and Mg were released at approximately the same rate as litter dry-weight and N, P and S initially accumulated were then slowly released from the forest floor. The effects of periodic prescribed burning on cycling of nutrients in forest litter are discussed. Nitrogen and P are the elements most affected by current burning practices. More research is needed to quantify the effects of recurrent fires in karri forest on P nutrition. Budgets based on data currently available on rates of N accession in rainfall and through symbiotic and non-symbiotic N2-fixation suggest that, with current burning frequencies, volatilization of N from the litter layer and understorey vegetation during fire may exceed cumulative accretion of N to the ecosystem during inter-fire periods. More precise estimates of N inputs and outputs are needed to confirm this.

Decomposition of leaf litter in karri (Eucalyptus diversicolor) forest of varying age

Rates of weight loss and release of N, P, K, S, Ca, Mg, Na and Cl from decomposing leaf litter were measured in regrowth karri (Eucalyptus diversicolor F. Muell.) forest aged 6, 9 and 40 years and in mature karri forest growing in south-western Australia. Decomposing leaf litter from the four sites lost 47–55% of original dry weight after exposure for 82 weeks on the forest floor. Rates of weight loss from leaf litter at the four sites varied in the order: mature<40-year-old=9-year-old=6-year-old-forest. A composite exponential function, with separate decay functions for labile and resistant litter components, explained 98–99% of the variation in mean dry weight loss in relation to time of exposure. Loss of labile litter components was rapid (half-lives 5–14 weeks). Loss of the resistant components was faster for litter from regrowth stands (half-lives 116–119 weeks) than for litter from mature forest (half-life 189 weeks). Slower rates of leaf litter decomposition together with increasing amounts of leaf and non-leaf (twigs, bark and fruit) litterfall as the stands mature indicate that the rate of accumulation of forest litter will be greater in mature forest than in regrowth stands. The order of element mobility in decomposing karri leaf litter (Na>Cl>K>S>Ca>N>P) was similar to that reported for other eucalyp litters. Nutrient content of fresh leaf litter, and rates of nutrient release during leaf litter decomposition, tended to be greater in the younger regrowth stands than in mature forest. Therefore, although the amounts of leaf litterfall were smaller in the younger stands, the amounts of nutrients released during the initial stages of decomposition were similar to, or greater than, the amounts released from decomposing leaf litter at the mature forest. However, since the proportion of non-leaf components in annual litterfall increases with age of the forest, total release of K, S, Ca, Mg and Na from all litter fractions is likely to be greater on older stands. Nitrogen and P occur at low concentrations in the major non-leaf litters and are imported into these tissues during decomposition. Therefore, in the initial stages of decay, immobilization of N and P will be greater in the litter layer of mature forest than in young regrowth stands.

Increasing and Sustaining Productivity of Tropical Eucalypt Plantations Over Multiple Rotations

Summary Eucalypt plantations in India are an important source of fiber for paper making and fuel for local villagers. Large areas of land have supported eucalypt plantations for several rotations, and productivity has generally been declining through successive rotations. In 1997, we initiated a project to examine site management options as a way to improve the productivity of these sites. We established a large experimental infrastructure at each of four sites in Kerala, consisting of up to five separate fully randomized block experiments at each site, examining interrotation management options (organic matter manipulation, N and P fertilizer input, legume cover cropping, weed control). Two of the sites were typical lowland plantations with Eucalytus tereticornis, and the other two sites were typical upland plantations with Eucalytus grandis. Following treatments, we monitored plantation productivity and impacts on soil and nutrient cycling for one full rotation (6.5 years). We found it is possible to increase the volume growth of E. grandis by up to 48%, and E. tereticornis by up to 268% through a combination of optimum site practices (mostly weed control and nutrient addition), but productivity responses are dependent on site-specific factors. Key outcomes of this research are presented, with special reference to application in the broader context of tropical eucalypt plantations.