Sites In South-western Australia Research Articles

EDNA metabarcoding of log hollow sediments and soils highlights the importance of substrate type, frequency of sampling and animal size, for vertebrate species detection

AbstractFauna monitoring often relies on visual monitoring techniques such as camera trapping, which have biases leading to underestimates of vertebrate species diversity. Environmental DNA (eDNA) metabarcoding has emerged as a new source of biodiversity data that may improve biomonitoring; however, eDNA‐based assessments of species richness remain relatively untested in terrestrial environments. We investigated the suitability of fallen log hollow sediment as a source of vertebrate eDNA, across two sites in southwestern Australia—one with a Mediterranean climate and the other semi‐arid. We compared two different approaches (camera trapping and eDNA metabarcoding) for monitoring of vertebrate species, and investigated the effect of other factors (frequency of species, timing of visits, frequency of sampling, and body size) on vertebrate species detectability. Metabarcoding of hollow sediments resulted in the detection of higher species richness in comparison (29 taxa: six birds, three reptiles, and 20 mammals) to metabarcoding of soil at the entrance of the hollow (13 taxa: three birds, two reptiles, and eight mammals). We detected 31 taxa in total with eDNA metabarcoding and 47 with camera traps, with 14 taxa detected by both (12 mammals and two birds). By comparing camera trap data with eDNA read abundance, we were able to detect vertebrates through eDNA metabarcoding that had visited the area up to two months prior to sample collection. Larger animals were more likely to be detected, and so were vertebrates that were identified multiple times in the camera traps. These findings demonstrate the importance of substrate selection, frequency of sampling, and animal size, on eDNA‐based monitoring. Future eDNA experimental design should consider all these factors as they affect detection of target taxa.

Chronic historical drought legacy exacerbates tree mortality and crown dieback during acute heatwave-compounded drought

Globally, combinations of drought and warming are driving widespread tree mortality and crown dieback. Yet thresholds triggering either tree mortality or crown dieback remain uncertain, particularly with respect to two issues: (i) the degree to which heat waves, as an acute stress, can trigger mortality, and (ii) the degree to which chronic historical drought can have legacy effects on these processes. Using forest study sites in southwestern Australia that experienced dieback associated with a short-term drought with a heatwave (heatwave-compounded drought) in 2011 and span a gradient in long-term precipitation (LTP) change, we examined the potential for chronic historical drought to amplify tree mortality or crown dieback during a heatwave-compounded drought event for the dominant overstory species Eucalyptus marginata and Corymbia calophylla. We show pronounced legacy effects associated with chronically reduced LTP (1951–1980 versus 1981–2010) at the tree level in both study species. When comparing areas experiencing 7.0% and 11.5% decline in LTP, the probability of tree mortality increased from low (<0.10) to high (>0.55) in both species, and probability of crown dieback increased from high (0.74) to nearly complete (0.96) in E. marginata. Results from beta regression analysis at the stand-level confirmed tree-level results, illustrating a significant inverse relationship between LTP reduction and either tree mortality (F = 10.39, P = 0.0073) or dieback (F = 54.72, P < 0.0001). Our findings quantify chronic climate legacy effects during a well-documented tree mortality and crown dieback event that is specifically associated with an heatwave-compounded drought. Our results highlight how insights into both acute heatwave-compounded drought effects and chronic drought legacies need to be integrated into assessments of how drought and warming together trigger broad-scale tree mortality and crown dieback events.

Response of chickpea (Cicer arietinum L.) to terminal drought: leaf stomatal conductance, pod abscisic acid concentration, and seed set.

Flower and pod production and seed set of chickpea (Cicer arietinum L.) are sensitive to drought stress. A 2-fold range in seed yield was found among a large number of chickpea genotypes grown at three dryland field sites in south-western Australia. Leaf water potential, photosynthetic characteristics, and reproductive development of two chickpea genotypes with contrasting yields in the field were compared when subjected to terminal drought in 106kg containers of soil in a glasshouse. The terminal drought imposed from early podding reduced biomass, reproductive growth, harvest index, and seed yield of both genotypes. Terminal drought at least doubled the percentage of flower abortion, pod abscission, and number of empty pods. Pollen viability and germination decreased when the fraction of transpirable soil water (FTSW) decreased below 0.18 (82% of the plant-available soil water had been transpired); however, at least one pollen tube in each flower reached the ovary. The young pods which developed from flowers produced when the FTSW was 0.50 had viable embryos, but contained higher abscisic acid (ABA) concentrations than those of the well-watered plants; all pods ultimately aborted in the drought treatment. Cessation of seed set at the same soil water content at which stomata began to close and ABA increased strongly suggested a role for ABA signalling in the failure to set seed either directly through abscission of developing pods or seeds or indirectly through the reduction of photosynthesis and assimilate supply to the seeds.

Age and growth in the Australian freshwater mussel, Westralunio carteri, with an evaluation of the fluorochrome calcein for validating the assumption of annulus formation

Growth and longevity of freshwater mussels (Unionida) are important for defining life-history strategies and assessing vulnerability to human impacts. We used mark–recapture and analysis of shell rings to investigate age and growth of the hyriid, Westralunio carteri, at 5 sites in southwestern Australia. We tested the utility of the in situ marker calcein for validating the assumption of annulus formation in adults. Calcein was incorporated into the shells of all recovered individuals, but it provided an interpretable reference mark in only 4 of 16 individuals. These 4 individuals produced 1 shell ring subsequent to the mark, supporting the assumption of annulus production during the austral winter. Maximum age ranged among populations from 36 to 52 y and maximum size ranged from 72.9 to 82.8 mm. Mean age and length did not differ between sexes, and growth trajectories differed between sexes at only 1 site. Estimates of growth measured by the von Bertalanffy growth constant, K, ranged from 0.021 to 0.336 among sites. Estimates from mark–recapture experiments were 20 to 52% lower than values from shell annuli at all sites except 1 where K from shell annuli was ∼½ that estimated from mark–recapture. Both methods showed a positive relationship between K and mean water temperature among sites, suggesting a role of riparian shading in regulating stream temperature, and hence, indirectly influencing mussel growth. Mussel growth and mean N or P concentrations were not related among sites, but total N at the site with highest mussel growth was >2× higher than at any other site. Westralunio carteri is a long-lived, slow-growing bivalve. Maximum age, K, and probable age at maturity (4–6 y) are similar to other slow-growing freshwater bivalve groups. This suite of life-history traits is considered an adaptation for stable aquatic habitats. Therefore, W. carteri can be expected to adapt poorly to human impacts, such as riparian clearing and water extraction, which increase the temporal variability of environmental conditions in streams.

MORPHOLOGICAL VARIATION IN MULGA. I. VARIATION AND COVARIATION WITHIN AND AMONG ACACIA ANEURA POPULATIONS

ABSTRACT Morphological variability is extremely high in the Australian mulga, Acacia aneura, even within local populations. This variation appears to be genetically based, and its maintenance can be related to two sorts of hypotheses: (a) ecotypic selection for morphological divergence related to the segregation of individuals into distinct, autogenic microniches, and (b) apostatic selection to foil foliar herbivores or seed predators. Morphological variation in the growth form, phyllodes, and pods of mulga is analyzed within and among populations at five topographically uniform sites in southwestern Australia along a gradient of annual precipitation. Phenotypic variability among sites is negatively correlated with annual precipitation, while within sites growth form and phyllode variability covary, and phyllode morphology covaries with pod morphology. Although the two sorts of hypotheses, ecotypic versus apostatic selection, are not clearly separable by morphological data alone, the former appears to be ...

Survival and growth of perennial halophytes on saltland in a Mediterranean environment is affected by depth to watertable in summer as well as subsoil salinity

Farmers need methods for assessing the capability of saltland for productive use based on characteristics that are readily measurable at the paddock scale. We conducted experiments on saltland transects with gradients of salinity and depth to watertable at three sites in south-western Australia. Each was planted with five perennial species with at least some salt tolerance: samphire (Tecticornia mellaria K.A.Sheph.), river saltbush (Atriplex amnicola Paul G.Wilson), small leaf bluebush (Maireana brevifolia (R.Br.) Paul G.Wilson), saltwater couch (Paspalum vaginatum Sw.), and Rhodes grass (Chloris gayana Kunth). Survival and growth of species was related to depth to watertable in summer and average subsoil (0.25–0.50 m depth) electrical conductivity of the saturation extract (ECe). It has been hypothesised that plant zonation on land affected by dryland salinity is affected by the level of salinity and waterlogging on sites. While plant survival ≥60% could be associated with particular ranges of depth to watertable and soil salinity, our data suggest that the most important factor affecting survival and growth was the presence of shallow groundwater in summer. The range of depths to watertable in summer associated with ‘good survival’ (≥60%) was 0.7–1.0 m with samphire, 0.7–1.6 m with saltwater couch, 0.8–1.5 m with Rhodes grass, 0.7–2.4 m with river saltbush, and 0.9–2.4 m with small leaf bluebush. The subsoil ECe (95% confidence interval) associated with ‘good survival’ was 5–14 dS/m for Rhodes grass, 6–11 dS/m for small leaf bluebush, 7–11 dS/m for river saltbush, 6–16 dS/m for saltwater couch, and 27–65 dS/m for samphire. Growth of the perennial grasses was strongly affected by the presence of a shallow watertable in summer; the size of saltwater couch and Rhodes grass increased many-fold as watertable depths decreased from ~1.3 to 0.9 m from the soil surface.

A virological investigation into declining woylie populations

The woylie (Bettongia penicillata ogilbyi) is a critically endangered small Australian marsupial that is in a state of accelerated population decline for reasons that are currently unknown. The aim of the present study was to elucidate the involvement of several viral pathogens through strategic serological testing of several wild woylie populations. Testing for antibodies against the Wallal and Warrego serogroup of orbiviruses, Macropod herpesvirus 1 and Encephalomyocarditis virus in woylie sera was undertaken through virus neutralisation tests. Moreover, testing for antibodies against the the alphaviruses Ross River virus and Barmah Forest virus and the flaviviruses Kunjin virus and Murray Valley encephalitis virus was undertaken through virus neutralisation tests and ELISA mainly because of the interest in the epidemiology of these important zoonoses as it was considered unlikely to be the cause of the decline. Between 15 and 86 samples were tested for each of the four sites in south-western Australia (Balban, Keninup, Warrup and Karakamia). Results indicated no exposure to any of the viral pathogens investigated, indicating that all populations are currently naïve and may be at risk if these pathogens were to be introduced.

Interactive effects of altered rainfall and simulated nitrogen deposition on seedling establishment in a global biodiversity hotspot

Understanding the interactive effects of global change drivers on vegetation is critical for ecosystem management and restoration, particularly in the Mediterranean‐climate biodiversity hotspots of the world. Climate change, habitat loss and nitrogen deposition have been identified as the key threats to biodiversity loss in these regions, yet their combined effects are poorly understood. We measured the interactive effects of rainfall manipulation (reduction, control, addition) and nitrogen deposition (N addition, N 1 P addition, and unfertilised) on the establishment of 19 Banksia‐woodland species planted at three sites in southwestern Australia. Seedling survival and aboveground biomass was increased with water addition but was not affected by rainfall reduction. N addition alone did not impact seedling survival and growth, but interacted with rainfall manipulation and site in unpredictable ways. Treatment effects were context dependent, which we attributed to historic nutrient enrichment and competitive exotic species that prevented seedling establishment. Plant species (n 5 6) varied greatly in their water‐use efficiency and nitrogen‐use efficiency responses to the imposed treatments, which underscores the difficulty of generalising results to larger numbers of species. Despite our finding that rainfall manipulation and nutrient addition have complex, and in some cases antagonistic effects on seedling survival and growth in Banksia woodlands, our results suggest that local context (i.e. invasive species, land‐use history) will have as much influence on seedling establishment as global changes in climate and nitrogen deposition. We call for more field experiments and predictive models to explore further the importance of ecological context in determining the interactive effects of multiple global change drivers on ecosystems. Finally, to realize effective biodiversity conservation, local management interventions that address invasive species and associated habitat degradation will be as critical in the future as they are now.

Contrasting physiological responses of two co-occurring eucalypts to seasonal drought at restored bauxite mine sites

This study describes the physiological response of two co-occurring tree species (Eucalyptus marginata and Corymbia calophylla) to seasonal drought at low- and high-quality restored bauxite mine sites in south-western Australia. Seasonal changes in photosynthesis (A), stomatal conductance (g(s)), leaf water potential (ψ), leaf osmotic potential (ψ), leaf relative water content (RWC) and pressure-volume analysis were captured over an 18-month field study to (i) determine the nature and severity of physiological stress in relation to site quality and (ii) identify any physiological differences between the two species. Root system restriction at the low-quality site reduced maximum rates of gas exchange (g(s) and A) and increased water stress (midday ψ and daily RWC) in both species during drought. Both species showed high stomatal sensitivity during drought; however, E. marginata demonstrated a higher dehydration tolerance where ψ and RWC fell to -3.2 MPa and 73% compared with -2.4 MPa and 80% for C. calophylla. Corymbia calophylla showed lower g(s) and higher ψ and RWC during drought, indicating higher drought tolerance. Pressure-volume curves showed that cell-wall elasticity of E. marginata leaves increased in response to drought, while C. calophylla leaves showed lower osmotic potential at zero turgor in summer than in winter, indicating osmotic adjustment. Both species are clearly able to tolerate seasonal drought at hostile sites; however, by C. calophylla closing stomata earlier in the drought cycle, maintaining a higher water status during drought and having the additional mechanism of osmotic adjustment, it may have a greater capacity to survive extended periods of drought.

Soil water depletion and replenishment during first- and early second-rotation Eucalyptus globulus plantations with deep soil profiles

Eucalyptus globulus plantations are thought to use stored soil water when planted on ex-agricultural sites, and we hypothesized that this is likely to affect productivity of 2nd and later rotation plantations because the next rotations have access to less stored soil water. We used a combination of experiments and modeling to understand the impact of E. globulus plantations on soil water stores over the first rotation and early second rotation. The experiments were conducted at 3 contrasting sites in south-western Australia, and modeling was used to extrapolate the results to other climatic zones. Soil water dynamics were assessed to 8m depth under a range of management options, including spacing and nitrogen addition in the first rotation, and coppice or seedling re-establishment in the 2nd rotation. We found that soil water stores declined over the course of the first rotation at all sites, with some (incomplete) annual replenishment evident at the higher rainfall sites, but less replenishment at depth, especially in the lower rainfall sites. Only the wettest of the 3 sites fully replenished with soil water after harvest of the 1st rotation. Plots with higher stocking rates had higher soil water depletion early in the rotation, although by the end of the first rotation, most treatments had similar soil water deficits of around 800mm at all of the sites. Of the sites that were responsive to N fertilizer, there was a strong differential in the degree of soil water deficit between N treatments, with N fertilized trees using more of the soil water store each year, but also producing more wood. A process-based plantation growth model, CABALA, was found to be adequate for predicting soil water dynamics under the range of management options that we explored, and we applied it to understanding the potential replenishment of soil under 2nd rotation plantations in a range of climatic zones within the E. globulus estate in south-western Australia. This modeling showed that most sites with soil depths of more than 4m (i.e., most of the estate) are unlikely to be fully replenished in the 2nd rotation, and that this is likely to have a significant impact on the capacity of sites to achieve similar productivity levels in the second rotation as the first unless the sites are given an opportunity for soil water replenishment between rotations. The results from this study suggest that plantation managers will need to understand soil water dynamics at any given site to be able to predict productivity in 2nd and later rotations, and may need to explore novel management options like fallowing between rotations to allow for soil water replenishment.

Managing productivity and drought risk in Eucalyptus globulus plantations in south-western Australia

More than 2.5 million ha of Eucalyptus globulus are now planted across the globe including approximately 500 000 ha in southern Australia. In this region average annual rainfall has declined since 1960 and this trend is predicted to continue in the coming decades. E. globulus is a premium species for paper manufacture and grows well under moderate seasonal water stress. The traits that underpin this rapid early growth also make the species vulnerable to prolonged water stress. We established nitrogen rate and nitrogen-by-stocking experiments in five 2-year-old E. globulus plantations along a climatic gradient in south-western Australia. We measured volume growth, predawn leaf water potential and leaf area index over 7 years or until the plantations were 9 years old. These data were used to explore the relationship between growth and water stress, to understand the mechanistic basis for the relationship and to identify best-bet management strategies for E. globulus plantations in southern Australia. Nearly all of the variation in volume growth rate between sites could be explained by a combination of climate wetness index and soil depth. There was a significant growth response to nitrogen at two low rainfall and one high rainfall site. There was no growth response to nitrogen on sites where total soil nitrogen in the top 0.1 m of soil was more than 1.9 mg g −1 and a very rapid increase in relative growth response below this threshold. The observed growth response to nitrogen was associated with an increase in water stress and on at least one site increased mortality. Matching the supply of nutrients to demand will maximise the growth at any site but this may increase the risk of drought death at water-limited sites. This will be exacerbated if forecasted changes in the climate of southern Australia are realised. Thinning to 600 stems ha −1 significantly reduced the level of water stress experienced by E. globulus in Western Australia without significantly affecting end of rotation stand volume compared to unthinned stands. These results indicate that for a range of sites in south-western Australia a final stocking density of 600 stems ha −1 coupled with application of fertiliser to maximise growth will minimise risk without sacrificing any of the site potential.

Evolution over 16 years in a bulk-hybrid population of subterranean clover (Trifolium subterraneum L.) at two contrasting sites in south-western Australia

Evolution was measured over 16 years in a self-regenerating, bulk-hybrid subterranean clover population, consisting of F 2 seed from 253 crosses, sown at Nabawa and Mt Barker, short and long growing season sites, respectively, in south-western Australia. Seed bank samples harvested annually were grown in a common garden. Experiment 1 measured flowering time in plants from each year, while Experiment 2 measured 26 variables in the populations 3 and 16 years after sowing, in comparison with the ancestral population. Changes in population means were observed in 20 characters and variability declined in 11 characters at one or both sites, with much of this occurring within the first three years. Natural selection at Nabawa favoured early flowering of long duration, thick peduncles, high harvest index and high hardseededness. At Mt Barker it favoured late flowering of short duration, large leaves and long, thick petioles at flowering, thick stems with long internodes, long, thin peduncles with a high burial angle, large plants at maturity, low hardseededness and high biochanin A and total isoflavone contents. High seed production capacity, with high seed weight and seeds per burr, was important at both sites. The use of bulk-hybrid populations is demonstrated as a low-input means of breeding and selecting well-adapted subterranean clovers.

Existing fire behaviour models under-predict the rate of spread of summer fires in open jarrah (Eucalyptus marginata) forest

SummaryFire behaviour data from Project Vesta experimental fires were used to evaluate the fire behaviour relationship on the Forest Fire Danger Meter (FFDM), the Forest Fire Behaviour Tables (FFBT) for Western Australia and the fire spread model of Burrows (1999) for predicting the rate of spread of fires in jarrah (Eucalyptus marginata) forest under dry summer conditions. Experimental fires were conducted at two sites in south-western Australia under conditions of moderate to high forest fire danger with fine fuel moisture contents of 6–9% oven dry weight and wind speeds up to 20 km h−1 (at 10 m in the open). Fuels ranged in age from 2 to 22 y since the last fire and had understorey shrub layers from 0.5 to 2.0 m tall. Mean rates of spread for experimental fires were 392 m h−1 and 405 m h−1 at the low and tall shrub sites respectively, with the fastest fires spreading at close to 1200 m h−1. Fires spread two to three times faster than predicted by the FFDM and FFBT fire spread models, and up to five times faster than predicted by the spread model of Burrows. Both the FFDM and FFBT models predicted fire spread better at the low shrub fuel site and when wind speeds were <12.5 km h−1. All models under-predicted fire spread at the tall shrub site and grossly under-predicted fire spread when wind speeds were >12.5 km h−1. We argue that existing fire behaviour models under-predict because they were developed using data from fires that had not achieved a quasi-steady rate of spread, whereas the line ignition technique used during Project Vesta experiments resulted in fires travelling at a speed closer to their potential rate of spread for the prevailing conditions. Existing guides remain valid for predicting spread during the early stages of fire growth when fires are developing from a point ignition and the headfire remains narrow.

Predicting wood production and net ecosystem carbon exchange of Pinus radiata plantations in south-western Australia: Application of a process-based model

Over much of south-western Australia there was a rapid decrease in rainfall of around 20% in the mid-1970s. Further declines in rainfall are predicted along with increases in temperature and atmospheric CO 2 concentration. This study focused on the implications of such a change for wood production and net ecosystem carbon exchange of pine plantations in that region. The process-based model CenW was evaluated using measurements of basal area and soil moisture in Pinus radiata plantations at seven sites in south-western Australia, encompassing a range of climatic conditions and silvicultural treatments (fertiliser application and thinning). The model predicted basal area very well and soil moisture reasonably well, except for one site where a water table may have provided additional water supply to trees. The model was then applied to assess the effects of the climatic changes that have occurred since 1975 by running the model for whole 30-year rotations, using observed climate data from either the 1945–1975 or 1975–2005 periods. The simulations showed that out of the six sites considered, wood production and net ecosystem carbon exchange were increased in the latter period at three sites, remained almost unchanged at two sites, and decreased at one site. Analysis of the model outputs, aided by additional simulations revealed that the response to the climate shift was determined by soil type (i.e. decreased growth rate was confined to the site with a sandy soil), and by the interaction between rising CO 2 levels (positive effect) and changes in rainfall (positive or negative depending on the direction of the change in rainfall).

Predictions of summer diapause in the redlegged earth mite, Halotydeus destructor (Acari: Penthaleidae), in Australia

A prediction for the onset of a summer diapause in the eggs of the redlegged earth mite, Halotydeus destructor, was developed for Australia. In this species diapause eggs pass the summer in the cadavers of adult female mites. Adult female mites were collected for several weeks from pastures in spring at 18 sites in south-western Australia and dissected to determine the timing of the production of diapause eggs. Some sites were sampled for several years between 1990 and 1997. A model was developed to predict the time for onset of diapause. The week at which 90% of eggs were in diapause was predicted best by daylength (80.1% of the variability), then by duration of the long-term plant growing season (10.4%, of variability), leaving 9.5% due to other factors. A single chemical spray in spring 2 weeks before the production of 90% diapause eggs resulted in 99% fewer mites present in autumn 7–8 months later at three sites. The timing of the spring spray was the factor leading to successful control. This model was tested at 17 sites across the whole geographical distribution of the redlegged earth mite in Australia between 1998 and 2001. The observed week of 90% diapause was within 1 week of the predicted week on 81% of occasions, and 2 weeks earlier on 15% of occasions. A database was created for the predicted date of onset of 90% diapause for the whole distribution of the redlegged earth mite in southern Australia on a 10 km 2 grid. Australian farmers are using this for timing a spring spray to control mites in the following autumn.

Does transported seagrass provide an important trophic link in unvegetated, nearshore areas?

The contribution of detritus from seagrass and other primary producers to faunal production in unvegetated nearshore areas was examined primarily using stable isotopes. Fish, macroinvertebrates, meiofauna and primary producers (seagrasses, macroalgae, seston and benthic microalgae) were sampled from sites in south-western Australia. All samples were analysed for δ 13C and δ 15N values and fish gut contents were determined. δ 13C values for seagrasses in the region were high compared to other macrophytes, ranging from 49.9 to −8.2‰ compared to −19.8 to −12.6‰ for macroalgae. The δ 15N values ranged between 4.0 and 7.7‰ for the red, brown and green algae, and between 3.2 and 5.9‰ for seagrasses. Seston and benthic microalgae samples had a mean δ 13C value of −12.8 and −14.0‰, respectively, and their δ 15N values were comparable to the macroalgae. All invertebrate fauna had mean δ 13C values considerably lower than seagrasses. However, individual samples harpacticoid copepods and polychaetes had a value as high as −11.7‰. δ 15N values for consumers were higher than those of the primary producers, except for copepods and amphipods. The δ 13C values for fish had a relatively small range, between −16.6 and −13.1‰, and the δ 15N values of fish were elevated compared to the invertebrates and primary producers, ranging mostly between 10.0 and 12.6‰. Mixing model analysis based on δ 13C values indicated that seagrass ranked low as a likely carbon source for all invertebrates other than harpacticoid copepods at a single site and some samples of polychaetes. The δ 13C values for fish were similar to those of a combination of harpacticoid and calanoid copepods, amphipods and polychaetes. The consumption of harpacticoid copepods by some fish species indicates that Amphibolis and Posidonia species in south-western Australia can contribute to the food web of unvegetated nearshore areas as detritus, but brown algae is likely to make a greater contribution. At least for the time of year that was sampled, the flow of detrital seagrass material into the foodweb may be mediated by specific detrivores, in this case harpactacoid copepods, rather than by all detritivores.

The nonmycorrhizal root – a strategy for survival in nutrient‐impoverished soils

The nonmycorrhizal root – a strategy for survival in nutrient‐impoverished soils

Soil particulate organic matter effects on nitrogen availability after afforestation with Eucalyptus globulus

We examined the hypothesis that changes in the quality and/or quantity of soil particulate organic matter (POM) after afforestation of pasture land with Eucalyptus globulus Labill. plantations caused increased nitrogen (N) immobilization and a decline in N availability. The quantity of POM was measured on soils from 10 paired pasture/plantation sites in south-western Australia. Net mineralization of C and N were measured over a 14-day incubation of POM, whole soil, and a mix of POM (33%) and whole soil (67%) at 25 °C and optimal moisture content (matric potential of 25 kPa). There was no significant difference in total organic C between pasture and plantation. However, the POM fraction C was higher in plantation soils (75%) than under pasture (62%), reflecting the coarser nature of organic inputs under plantation. Total soil N concentration was 20% lower under plantation compared to pasture, and the proportion in the POM was higher (74% compared to 57% for pasture soil). The C:N ratios in POM under both pasture and plantation, and in the whole soil under plantation were around 19, but C:N ratios of whole soil under pasture was 17. Average C mineralization was 13% lower in plantation relative to that in pasture soil. The isolated POM fraction had 18% higher C mineralization rate than that in whole soil. The change in net N mineralization with afforestation was marked, with 50% lower net N mineralization in plantation than pasture whole soils. Net N mineralization in the isolated POM fraction was also about 50% of that in the whole soil for both pasture and plantation soils. Although, the pasture and plantation POM had similar C:N ratios, the net N mineralization was 2-fold greater in pasture POM than in plantation POM, suggesting that biochemical characteristics other than the C:N ratio had the main influence on net N mineralization rates. The POM fraction did not significantly immobilize N from the whole soil when placed in a mixture of POM and whole soil, suggesting that N immobilization was not the main mechanism for POM to influence N availability in these soils.

Soil particulate organic matter effects on nitrogen availability after afforestation with Eucalyptus globulus

We examined the hypothesis that changes in the quality and/or quantity of soil particulate organic matter (POM) after afforestation of pasture land with Eucalyptus globulus Labill. plantations caused increased nitrogen (N) immobilization and a decline in N availability. The quantity of POM was measured on soils from 10 paired pasture/plantation sites in south-western Australia. Net mineralization of C and N were measured over a 14-day incubation of POM, whole soil, and a mix of POM (33%) and whole soil (67%) at 25 °C and optimal moisture content (matric potential of 25 kPa). There was no significant difference in total organic C between pasture and plantation. However, the POM fraction C was higher in plantation soils (75%) than under pasture (62%), reflecting the coarser nature of organic inputs under plantation. Total soil N concentration was 20% lower under plantation compared to pasture, and the proportion in the POM was higher (74% compared to 57% for pasture soil). The C:N ratios in POM under both pasture and plantation, and in the whole soil under plantation were around 19, but C:N ratios of whole soil under pasture was 17. Average C mineralization was 13% lower in plantation relative to that in pasture soil. The isolated POM fraction had 18% higher C mineralization rate than that in whole soil. The change in net N mineralization with afforestation was marked, with 50% lower net N mineralization in plantation than pasture whole soils. Net N mineralization in the isolated POM fraction was also about 50% of that in the whole soil for both pasture and plantation soils. Although, the pasture and plantation POM had similar C:N ratios, the net N mineralization was 2-fold greater in pasture POM than in plantation POM, suggesting that biochemical characteristics other than the C:N ratio had the main influence on net N mineralization rates. The POM fraction did not significantly immobilize N from the whole soil when placed in a mixture of POM and whole soil, suggesting that N immobilization was not the main mechanism for POM to influence N availability in these soils.

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.