Philip J. Smethurst

Soil solution and other soil analyses as indicators of nutrient supply: a review

This review examines the potential for using soil solution as a tool for managing soil fertility. A review of the current use of other types of soil analyses indicates that, while their use in some cases is justified, there are substantial limitations to the development of reliable and widely applicable calibrations. Factors that govern concentrations of nutrients in soil solution and the methods for measuring them are reviewed in relation to their use in nutrient management of forest plantations and agricultural crops. Topics include a discussion of (i) nutrient supply and uptake mechanisms; (ii) solution culture studies which define critical concentrations in solution; (iii) methods of sampling solution from soils and (iv) estimation of concentrations that can be maintained at root surfaces in soil. By inference, nutrient supply would not limit plant growth if concentrations at most root surfaces (e.g. young roots in surface soil) were maintained at or above concentrations needed to maintain high rates of growth in solution culture, i.e. critical concentrations. Several aspects of this method have been validated for N and P in Eucalyptus nitens plantations. For example, when concentrations of ammonium (the preferred N source for E. nitens) in the field fell below the critical level of 50 μM, plantations of E. nitens responded to applications of N-fertilizer. This method was also useful for predicting P deficiency in corn (Zea mays), Eucalyptus globulus and E. nitens grown in soils of widely different P-supply characteristics. The convergence of concepts based on the principles of soil nutrient supply and uptake, which link soil and solution culture studies, is likely to provide a unifying approach for diagnosing nutrient-supply limitations to plant growth and a practical tool for nutrient management in forest plantations.

Ammonium and nitrate uptake by Eucalyptus nitens: effects of pH and temperature

Ammonium and nitrate uptake by roots of Eucalyptus nitens was characterised with respect to pH and temperature. Uptake of ammonium and nitrate was measured as depletion from solutions by roots of intact 11 week old solution-cultured seedlings. Uptake rates of ammonium were consistently higher than those of nitrate in all experiments. Uptake rates for ammonium were 200% higher at pH 4 than at pH 6, but for nitrate were unchanged. Uptake rates of ammonium and nitrate were both reduced to a similar extent (70%) with a decrease in temperature from 20 °C to 10 °C. For ammonium uptake, there was rapid (<24 hr) adaptation to a reduction in root temperature. The apparent preference shown here for ammonium over nitrate could be indicative of E. nitens growing in cold, acidic forest soils where ammonium is commonly more available than nitrate. These results suggest that N uptake rates of E. nitens may be maximised under a wide variety of conditions if N is supplied predominantly in the ammonium form.

Simultaneous measurement of ammonium, nitrate and proton fluxes along the length of eucalypt roots

Knowledge of the preferred source of N for Eucalyptus nitens will lead to improved fertiliser management practices in plantations. Ion selective microelectrodes were used non-invasively to measure simultaneously net fluxes of NH4+, NO3− and H+ along the tap root of solution-cultured E. nitens. Measurements were conducted in solutions containing 100 μm NH4NO3. The pattern of fluxes was such that there was a large influx of NH4+, a smaller influx of NO3− and large H+ efflux. The ratio of these fluxes was constant, according to the ratio 3:1:−6 (NH4+:NO3−:H+). Within the region 20–60 mm from the root apex of E. nitens seedlings there was spatial and temporal variation in fluxes but flux patterns remained constant. Root hair density did not affect fluxes nor did proximity to lateral roots. Variation was less than that found in previous studies of localised root fluxes using similar high-resolution measurement techniques. It was concluded that small-scale spatial variation in fluxes may have confounded previous studies. There were associations between fluxes of all three ions, the strongest associations being between NH4+ and H+, and NH4+ and NO3−. Overall, these results are consistent with NH4+ being the preferred source N for E. nitens.

Kinetics of ammonium and nitrate uptake by eucalypt roots and associated proton fluxes measured using ion selective microelectrodes

Ion-selective microelectrodes were used non-invasively to measure the concentration dependence of NH4+ and NO3- fluxes around the roots of intact solution-cultured Eucalyptus nitens (Deane & Maiden) Maiden. In addition, NH4+ and H+ fluxes were measured simultaneously at a range of NH4+ concentrations, and NO3- and H+ fluxes were measured simultaneously at a range of NO3- concentrations. Nitrogen concentrations ranged from 10-250 μM, i.e. in the range corresponding to the high affinity transport system (HATS). Both NH4+ and NO3- fluxes exhibited saturating Michaelis-Menten-style kinetics. The Km was 16 μM for NH4+ and 18 μM for NO3-. Values of Vmax were 53 nmol m-2 s-1 for NH4+ and 37 nmol m-2 s-1 for NO3-. Proton fluxes were highly correlated with NH4+ and NO3- fluxes, but the relationships were different. Proton efflux increased with increasing NH4+ concentration and mirrored the changing NH4+ fluxes. The ratio between NH4+ and H+ fluxes was 1 : -1.6. Proton influx was evident with initial exposure to NO3-, with the flux stoichiometry for NO3- : H+ being 1 : 1.4. Subsequent increases in NO3- concentration caused a gradual increase in H+ efflux such that the flux stoichiometry for NO3- : H+ became 1 : -0.8. The presence of 100 μM NH4+ greatly reduced NO3- fluxes and caused a large and constant H+ efflux. These results are evidence that E. nitens has a preference for NH4+ as a source of N, and that the fluxes of NH4+ and NO3- are quantitatively linked to H+ flux.

Spatial scale and opportunities for choice influence browsing and associational refuges of focal plants

1. Foraging decisions by herbivores depend on variation in food types, the scale(s) at which this variation occurs and the opportunity and capacity for herbivores to respond to such variation. These decisions affect not only the herbivores themselves, but also the vulnerability of individual plants to being eaten. Associational plant refuges, in which neighbouring plants alter focal plant vulnerability, are an emergent property of foraging decisions. 2. Using the red-bellied pademelon (Thylogale billardierii) as a model generalist mammalian herbivore, we investigated the spatial scale(s) at which animals made foraging decisions and the resultant effect on focal plant vulnerability. In a replicated design, we varied vegetation at the individual plant scale, generating intraspecific differences in Eucalyptus nitens seedlings by altering their nutrient status (high, low). We varied vegetation at the patch scale, in which seedlings were planted, using high- (grass) and low- (herbicided) quality patches. Animals were allowed to choose where they fed and what they ate. Animal behaviour was recorded and intake of seedlings measured. 3. We found that animals made foraging decisions first at the patch scale then at the scale of individual plants; both patch and focal seedling characteristics influenced browsing. Pademelons spent most of their time in high-quality patches, and seedlings were consequently more vulnerable there than in low-quality patches. Pademelons also ate more foliage from high- than from low-nutrient status seedlings. This behaviour concentrated resources, increasing foraging efficiency and making focal plants more vulnerable to browsing. 4. The opportunity and capacity to choose at both plant and patch scales resulted in a pattern of focal plant vulnerability consistent with the repellent-plant hypothesis. This contrasts with our previous study, in which animals were only provided with choice at the plant level and plant vulnerability followed the attractant-decoy hypothesis. These combined results demonstrate that the influence of neighbouring vegetation on consumption of a focal plant depends on the spatial scale of selection and on opportunities (and capacity) for herbivores to choose.

Nitrogen fixing plants in forest plantation management

There is a wide range of nitrogen fixing plants that have been used in forestry with the presumed objective of raising soil nitrogen levels and subsequently improving the growth of the non-nitrogen-fixing forest species. The nitrogen fixing plants that have been tried in managed forest systems range from trees to herbs and shrubs which fix nitrogen through root associations with either Rhizo bium strains in the case of legumes,or an actinomycete of the genus Frankia in the case of non legumes.

Nutrient concentrations in foliage, litter and soil in relation to wood production of 7- to 15-year-old Pinus radiata in Victoria, Australia

Summary We report the changes in nutrient concentrations in unthinned stands of Pinus radiala aged 7, 11 and 15 years old planted across 11 soil types. Correlations between nutrients in foliage, litter and soil (0–20 cm and 20–40 cm) indicated that the links between the components increased with stand age for N and P. Litter weight on the forest floor was dependent upon standing wood volume, and the mass of nutrients in the litter, particularly N and Ca, increased concomitantly with wood volume. The litter layer provided a large pool of nutrients which was related to nutrient concentrations in the surface soil. Concentrations of N and P declined in foliage with age concomitantly with an increase in the amounts immobilized in litter. However, wood volume at age 15 years was positively correlated with foliar N and P concentrations, and the rate of decline in foliar N or P was not correlated directly with a decrease in the rate of wood production.

Nitrogen fluxes in surface soils of young Eucalyptus nitens plantations in Tasmania

Nitrogen (N) fluxes in surface soils (0–10 cm) were measured by an in situ soil-core technique at 5 sites supporting 1- to 4-year-old Eucalyptus nitens plantations in Tasmania. Rates of net N mineralisation (NNM) ranged from 13 to 188 kg Nsha.year. The highest rate was from an ex-pasture site, which was almost double the highest rate of NNM measured on an ex-forest site. Although there was a tendency towards lower rates of NNM in uncultivated versus cultivated soils, and in the final versus previous year or two of measurement, these differences were not significant. Concentrations of mineral N, however, decreased at all study sites during the first 3–4 years, significantly at 4 of these sites. Within the errors of measurement, most N that was mineralised was also nitrified and leached. Seasonal patterns in NNM and concentrations of nitrate (NO3–) occurred only at one site, which also had the highest annual rate of NNM. The balance of mineral N remaining in surface soil each October was 2–17 kg Nsha. We concluded that there was little evidence of a decrease in rates of NNM between 1 and 4 years after planting, and that high rates of NNM at some sites were accompanied by high rates of leaching that limited the accumulation of mineral N. Sitespecific management of N fertiliser may be warranted to compliment the wide range of N availability in eucalypt plantations. mineralisation, nitrification, leaching, uptake, forests, Australia.

Soil types as classes for managing the nutrient status of planted Pinus radiata in Victoria, Australia

Summary The study reported here investigates the utility of using soil profile types as classes for managing the nutrient status and productivity of stands of planted Pinus radiata in Gippsland, Victoria. The first objective of the study was to determine from stands of P. radiata planted on eleven soil types, which chemical variables in the sampled components foliage, forest floor litter, and soil (0–20 and 20–40 cm) were best correlated with wood volume production. The second objective was to determine whether soil types as classes defined on observable soil profile criteria reflected their innate nutrient status and wood volume production. Nitrogen and Ca in foliage, N, P, and K in forest floor litter, and all measured chemical variables in soil were correlated strongly with wood production. However, analyses of variance of individual chemical variables across soils did not show exclusive and consistent grouping of similar soil profile types. Discriminant analyses based separately on a subset of nine so...

Preferences of two mammalian herbivores for tree seedlings and potential cover crops in plantation forestry

Summary Newly established plantations are often damaged by heavy browsing. This is a major economic problem. Vegetation management involving the use of cover crops is a potential method for reducing browsing damage to plantation seedlings. An important first step in determining whether this is feasible is to determine the relative preferences of browsing herbivores for seedlings and crops. We conducted a no-choice feeding trial with red-bellied pademelons (Thylogale billardierii) and common brushtail possums (Trichosurus vulpecula) to determine their relative preferences for two common plantation species: Eucalyptus nitens and Pinus radiata, and three potential legume cover crops: bitter lupin (Lupinus albus), sweet lupin (L. angustifolius) and broad bean (Vicia faba L. cv Coles Dwarf). Pademelons exhibited a strong preference for P. radiata, but did not eat much of the legumes. Possums showed no significant preferences for any of the plants offered. A paired-choice feeding trial with pademelons demonstrated that bitter lupin was less preferred and oats (Avena sativa variety Esk) were more preferred than E. nitens. These results can now be integrated with an understanding of feeding behaviour at higher scales (e.g. vegetation patch) in order to design effective vegetation management for reducing browsing damage.