R.J. Raison

Chemical properties of ash derived from Eucalyptus litter and its effects on forest soils

The concentration of elements in ash from eucalyptus litter varied several-fold depending upon the type of fuel and combustion conditions. In vegetation fires, and where ash is used as a soil amendment, significant amounts of nutrients and heavy metals will be added to soils. Three patterns of dissolution of ash constituents in water were observed: (1) high (> 70% of element content of ash quickly soluble), but with a residual component which was not solubilized by further dilution (elements K, S, and B); (2) relatively insoluble, but where the amount dissolved is related to dilution (Ca, Mg, Si and Fe), and (3) highly insoluble (such as P). The capacity of ash to neutralize acid was well correlated with the total amount of basic cations (K+Ca+Mg) in the ash. Addition of 4–20 t ash ha−1 to a range of forest soils increased pH (KCl) by one to three units depending on soil C content, decreased exchangeable Al and H content, increased exchange sites (variable negative charges) and increased basic-cation levels. Ash addition increased respiration rates in all soils, especially in those with higher organic matter content. Addition of ash increased N mineralization rate and nitrification in most of the soils, although the magnitude of response was not clearly related to assessed soil properties. This study suggests that account must be taken of soil and ash properties when assessing the utility of plant ash for soil amelioration.

Effects of soil phosphorus availability, temperature and moisture on soil respiration in Eucalyptus pauciflora forest

Rates of soil respiration (CO2 efflux) were measured for a year in a mature Eucalyptus pauciflora forest in unfertilized and phosphorus-fertilized plots. Soil CO2 efflux showed a distinct seasonal trend, and average daily rates ranged from 124 to 574 mg CO2 m−2 hr−1. Temperature and moisture are the main variables that cause variation in soil CO2 efflux; hence their effects were investigated over a year so as to then differentiate the treatment effect of phosphorus (P) nutrition.Soil temperature had the greatest effect on CO2 efflux and exhibited a highly significant logarithmic relationship (r2 = 0.81). Periods of low soil and litter moisture occurred during summer when temperatures were greater than 10 °C, and this resulted in depression of soil CO2 efflux. During winter, when temperatures were less than 10 °C, soil and litter moisture were consistently high and thus their variation had little effect on soil CO2 efflux. A multiple regression model including soil temperature, and soil and litter moisture accounted for 97% of the variance in rates of CO2 efflux, and thus can be used to predict soil CO2 efflux at this site with high accuracy. Total annual efflux of carbon from soil was estimated to be 7.11 t C ha−1 yr−1. The model was used to predict changes in this annual flux if temperature and moisture conditions were altered. The extent to which coefficients of the model differ among sites and forest types requires testing.Increased soil P availability resulted in a large increase in stem growth of trees but a reduction in the rate of soil CO2 efflux by approximately 8%. This reduction is suggested to be due to lower root activity resulting from reduced allocation of assimilate belowground. Root activity changed when P was added to microsites within plots, and via the whole tree root system at the plot level. These relationships of belowground carbon fluxes with temperature, moisture and nutrient availability provide essential information for understanding and predicting potential changes in forest ecosystems in response to land use management or climate change.

Effects of slash burning on soil phosphorus fractions and sorption and desorption of phosphorus

Abstract Soil P fractions and P sorption-desorption characteristics were studied 7 months after clearfelling and slash burning at a mixed Eucalyptus forest site in eastern Australia. Depending on the fire intensity, three different microsites were generated: unburnt, burnt and intensely burnt (ashbed). Phosphorus fractions were extracted from the soil with NH 4 F (0.03 N) + HCl (0.025 N) (Bray I), NaHCO 3 (0.5 N), NaOH (0.5 N), and H 2 SO 4 (1 N). Adsorption isotherms were obtained by equilibrating soils with solutions having concentrations of P, and desorption of the adsorbed P was studied by extracting the soils with Bray I extract. The effects of fire on soil P were greatest in the surface soil horizons and depended upon fire intensity. Ashbed soils differed from unburnt soils for P fractions and P sorption and desorption characteristics. Labile inorganic P (Bray I) increased from less than 1 mg kg −1 in the unburnt soil to 5–13 mg kg −1 in the ashbed. Inorganic P (NaOH and H 2 SO 4 extractions) increased markedly after fire, especially in the surface layers. The increase in labile organic P (NaHCO 3 -extractable) contrasted with a decrease in total organic P (H 2 SO 4 ) and less labile organic P (NaOH) in ashbed soils, suggesting marked transformation of organic P pools after intense fire. After incubation for 2 months, labile organic P in the unburnt soil increased, whereas large decreases were observed in ashbeds and surface (0–5 cm) burnt soils. The ashbed soil showed an increase in sorption capacity in the 0–5 cm soil layer, but the sorbed P was generally less tightly bound to the solid phase. Seedling growth and foliage P concentrations were greatest in ashbed soils. Harvesting and burning increased the spatial distribution of soil P in the field. The ashbed and burnt microsites represented 19% and 18% respectively, of the surface area of the slash burnt coupe, and about 8 kg P ha −1 was deposited in ash.

Dynamics of Pinus radiata foliage in relation to water and nitrogen stress: I. Needle production and properties

Abstract Measurements of the following parameters were made over a 4 year period in 10- to 14-year-old stands of Pinus radiata subjected to markedly different degrees of water and nitrogen (N) stress: needle length, weight and specific leaf area (every 2 weeks), foliage biomass production (annually), pre-dawn needle water potential (every 2 weeks) and needle litter N concentration (monthly). Increments in needle length were a useful estimate of increments in needle weight for any given forest treatment and year because there was no consistent variation in weight per unit length of needles as they developed during the growing season. However, for well-illuminated needles, the ratio of weight per unit of needle length showed a large (approximately two-fold) variation attributable to treatment and year of foliage elongation. The ratio was loosely positively correlated with needle length (or the favourability of growing conditions) on non-irrigated plots, and appeared to result largely from increases in needle thickness rather than density. Final needle length, ranging between 40 and 160 mm, depended mainly on the amount of water and N stress experienced by trees during the growing season. The majority (greater than 90%) of needle extension occurred during a 4 month period (October–January) in spring and summer and the pattern of needle growth was affected only by water availability. Needle extension rates were negatively linearly correlated with the water stress integral (Sψ, a temporal integration of the effects of both water and N availability on needle water potentials) for monthly periods during the growing season. Needle extension, was most sensitive to the Sψ in mid-spring (October/November) when needles were elongating rapidly and still less than one-half grown. About 80% of the variation in annual foliage production (3.2–8.5 t ha−1) could be explained in terms of both (a) Sψ during the previous summer (when primordia were initiated), and (b) the water and N status of trees concurrent with needle extension. Final needle length and total foliage biomass production in the same year were poorly correlated. The specific leaf area (SLA, all sides) ranged from 10 to 17 m2 kg−1 and was greater for needles formed under low light. Irrigation or fertilisation had only an indirect effect on SLA by hastening canopy closure.

The Biology of Forest Growth experiment: linking water and nitrogen availability to the growth of Pinus radiata

Abstract The Biology of Forest Growth (BFG) study in a 10- to 14-year-old Pinus radiata stand comprised detailed investigations of stand growth, water balance, soil and tree N cycling, canopy dynamics and stand growth modelling The BFG study: (a) developed a method for measuring soil mineral N dynamics in situ: (b) developed the water stress integral, an index of temporally integrated water stress: (c) identified annual average needle litter N concentration as a useful measure of N uptake by and N status of P. radiata stands; (d) demonstrated a marked positive interactive effect of water and N availability on the rate of canopy development, stand foliage carrying capacity, light-use efficiency and above-ground net primary productivity; (e) identified a water stress threshold at a soil water content of 40% of ‘plant-available water’ below which the level of tree water stress is controlled by oil water content and above which the degree of water stress is determined by N status and soil temperature; (f) demonstrated that the leaf area index (LAI) of P. radiata stands varies markedly both within and between years, and that variations in LAI can be monitored indirectly using light transmision techniques provided that account is taken of the surface area of other stand components (dead foliage, branches, boles); (g) quantified the water use of irrigated plantations and its dependence on LAI; (h) quantified the effects of water and N availability on N retranslocation in foliage, and demonstrated the importance of retained foliage for the provision of N for new growth; (i) elucidated mechanisms resulting in prolonged (greater than 8 year) growth responses to N fertilisation, including long-term increases in soil N mineralisation rates; (j) constructed and evaluated a biological model (BIOMASS) of forest stand growth. The BIOMASS model estimates total CO2 uptake by a forest stand from intercepted radiation and photosynthetic properties of the foliage. Net Primary Production is obtained by subtracting respiration rates of the component parts of trees. BIOMASS simulated well the observed growth patterns of trees, especially stem and foliage components. Close similarities between the processes involved in transpiration and CO2 uptake, and good correspondence between calculated and measured soil water balances over a 4 year period, increased confidence in the total CO2 uptake predictions of the model. Research needed to improve tree growth models is briefly discussed. It is argued that long-term multidisciplinary research, such as the BFG study is necessary for advancing understanding of the links between site conditions and forest productivity.

Dynamics of Pinus radiata foliage in relation to water and nitrogen stress: II. Needle loss and temporal changes in total foliage mass

Abstract The pattern of production and fall of needles was measured over a 4 year (1983–1987) period in 10- to 14-year-old stands of Pinus radiata near Canberra. Australia which were subjected to markedly varying degrees of water and N stress. Annual needle loss (death of green needles) was estimated as annual needle fall plus the increment (or minus the decrement) in the mass of dead needles held in the crown between successive winter measurements. Monthly estimates of needle production and loss were used to calculate seasonal (3-month) changes in total live foliage mass between annual winter measurements of foliage biomass. Annual needle fall ranged from about 1.5 to 5.0 t ha−1, and although well correlated with needle loss, the two parameters differed by up to 2 t ha−1 as stands approached canopy closure. In stands closing canopy. 2–3 t ha−1 of dead needles were retained in the crown. Loss in mass of needles due to leaching and decomposition during the period between senescence and fall results in needle fall being an underestimate of the mass of needles senescing in any year. Annual needle loss was positively linearly correlated with total foliage biomass (r2 = 0.70) or stand basal area (r2=0.75) measured in the previous winter. The average life of needles declined from about 4 years in open stands not suffering severe water stress, to about 2 years after canopy closure or where water stress induced significant loss of foliage. Monthly needle fall varied from less than 100 to greater than 500 kg ha−1 and was positively linearly correlated with a measure of cumulative tree water stress (the water stress integral, Sψ) during the same month. The correlation was highest (r2=0.68) for fertilised stands which were most water stressed, and declined with declining water stress, Sψ useful as a guide to the timing of needle fall, but not as a predictor of total annual needle fall which was mainly determined by foliage biomass of the stand. Older (mostly more than 2 years old) needles were shed largely in response to water stress, and in most years needle fall peaked in the summer-autumn period. In wet years and in irrigated stands the peak in needle fall was delayed by 3–6 months.

Effects of water availability and fertilization on nitrogen cycling in a stand of Pinus radiata

Nitrogen cycling was studied for four years (1983–1987) in an N-deficient 10-year-old stand of Pinus radiata growing on a yellow podzolic soil which had a low water-holding capacity. Trees were subjected to combinations of irrigation of N-fertilization resulting in a wide range of N uptake and tree growth. Net mineralization, plant uptake and leaching of soil N was monitored using a sequential coring and in-situ incubation technique. Nitrogen concentrations were measuredd monthly in live needles and litterfall. Average rates of weight loss and release of N from decomposing litter were estimated over a 3-year period using a budgeting approach. Trees responded only to N (not to P, and there was no N×P interaction), but there was a large positive interaction between N supply and water availability. Response to fertilizer averaged + 24% over a 4-year period, but was zero during a growing-season which contained a 4-month drought. Irrigation alone increased growth by 60%, but in combination with high N availability growth increased 2–3 fold. Annual uptake of N ranged from <10 (irrigated plots in years 2 and 3 after enhanced mineralization during the initial year) to 166 kg ha−1 (during a wet growing season following heavy N fertilization). Although soil mineral-N concentrations were elevated for only about 1 year after fertilization, fertilization enhanced rates of N mineralization throughout the soil N mineralization may have resulted from re-mineralization of the large quantity (147 kg soil N mineralization may have resulted from re-mineralization of the large quantity (147 kg ha−1) of fertilizer N immobilized by the soil during the initial 8 months after fertilization, or the N released from decomposition of fine roots having higher N content. Nitrification was negligible in unfertilized soils, but increased markedly 50–100 days after fertilization and resulted in the leaching of about 60 kg N ha−1 during autumn and winter of the first year after fertilization. Fertilized soils have continued to nitrify readily. Irrigation increased rates of weight loss and N release from decomposing litter. The rate of N uptake by trees markedly affected the concentrations of N in newly emerging and older needles, and the concentration of N in needlefall. The weighted mean concentration of N in annual needlefall ranged from 0.42% in the irrigated-only plot (most N-stressed) to 0.94% in the heavily fertilized plot during the first year after treatment. These weighted concentrations are a useful index of N uptake from the soil and of growth rate where water supply is not limiting. Except for the initial year after heavy N fertilization, annual uptake of N was equivalent to annual soil N mineralization, and N uptake was positively linearly correlated with annual basal-area increment of trees.

Allocation of carbon in a mature eucalypt forest and some effects of soil phosphorus availability

Pools and annual fluxes of carbon (C) were estimated for a mature Eucalyptus pauciflora (snowgum) forest with and without phosphorus (P) fertilizer addition to determine the effect of soil P availability on allocation of C in the stand. Aboveground biomass was estimated from allometric equations relating stem and branch diameters of individual trees to their biomass. Biomass production was calculated from annual increments in tree diameters and measurements of litterfall. Maintenance and construction respiration were calculated for each component using equations given by Ryan (1991a). Total belowground C flux was estimated from measurements of annual soil CO2 efflux less the C content of annual litterfall (assuming forest floor and soil C were at approximate steady state for the year that soil CO2 efflux was measured). The total C content of the standing biomass of the unfertilized stand was 138 t ha-1, with approximately 80% aboveground and 20% belowground. Forest floor C was 8.5 t ha-1. Soil C content (0–1 m) was 369 t ha-1 representing 70% of the total C pool in the ecosystem. Total gross annual C flux aboveground (biomass increment plus litterfall plus respiration) was 11.9 t ha-1 and gross flux belowground (coarse root increment plus fine root production plus root respiration) was 5.1 t ha-1. Total annual soil efflux was 7.1 t ha-1, of which 2.5 t ha-1 (35%) was contributed by litter decomposition.The short-term effect of changing the availability of P compared with C on allocation to aboveground versus belowground processes was estimated by comparing fertilized and unfertilized stands during the year after treatment. In the P-fertilized stand annual wood biomass increment increased by 30%, there was no evidence of change in canopy biomass, and belowground C allocation decreased by 19% relative to the unfertilized stand. Total annual C flux was 16.97 and 16.75 t ha-1 yr-1 and the ratio of below- to aboveground C allocation was 0.43 and 0.35 in the unfertilized and P-fertilized stands, respectively. Therefore, the major response of the forest stand to increased soil P availability appeared to be a shift in C allocation; with little change in total productivity. These results emphasise that both growth rate and allocation need to be estimated to predict changes in fluxes and storage of C in forests that may occur in response to disturbance or climate change.

An assessment of biomass for bioelectricity and biofuel, and for greenhouse gas emission reduction in Australia

We provide a quantitative assessment of the prospects for current and future biomass feedstocks for bioenergy in Australia, and associated estimates of the greenhouse gas (GHG) mitigation resulting from their use for production of biofuels or bioelectricity. National statistics were used to estimate current annual production from agricultural and forest production systems. Crop residues were estimated from grain production and harvest index. Wood production statistics and spatial modelling of forest growth were used to estimate quantities of pulpwood, in‐forest residues, and wood processing residues. Possible new production systems for oil from algae and the oil‐seed tree Pongamia pinnata, and of lignocellulosic biomass production from short‐rotation coppiced eucalypt crops were also examined. The following constraints were applied to biomass production and use: avoiding clearing of native vegetation; minimizing impacts on domestic food security; retaining a portion of agricultural and forest residues to protect soil; and minimizing the impact on local processing industries by diverting only the export fraction of grains or pulpwood to bioenergy. We estimated that it would be physically possible to produce 9.6 GL yr−1 of first generation ethanol from current production systems, replacing 6.5 GL yr−1 of gasoline or 34% of current gasoline usage. Current production systems for waste oil, tallow and canola seed could produce 0.9 GL yr−1 of biodiesel, or 4% of current diesel usage. Cellulosic biomass from current agricultural and forestry production systems (including biomass from hardwood plantations maturing by 2030) could produce 9.5 GL yr−1 of ethanol, replacing 6.4 GL yr−1 of gasoline, or ca. 34% of current consumption. The same lignocellulosic sources could instead provide 35 TWh yr−1, or ca. 15% of current electricity production. New production systems using algae and P. pinnata could produce ca. 3.96 and 0.9 GL biodiesel yr−1, respectively. In combination, they could replace 4.2 GL yr−1 of fossil diesel, or 23% of current usage. Short‐rotation coppiced eucalypt crops could provide 4.3 GL yr−1 of ethanol (2.9 GL yr−1 replacement, or 15% of current gasoline use) or 20.2 TWh yr−1 of electricity (9% of current generation). In total, first and second generation fuels from current and new production systems could mitigate 26 Mt CO2‐e, which is 38% of road transport emissions and 5% of the national emissions. Second generation fuels from current and new production systems could mitigate 13 Mt CO2‐e, which is 19% of road transport emissions and 2.4% of the national emissions lignocellulose from current and new production systems could mitigate 48 Mt CO2‐e, which is 28% of electricity emissions and 9% of the national emissions. There are challenging sustainability issues to consider in the production of large amounts of feedstock for bioenergy in Australia. Bioenergy production can have either positive or negative impacts. Although only the export fraction of grains and sugar was used to estimate first generation biofuels so that domestic food security was not affected, it would have an impact on food supply elsewhere. Environmental impacts on soil, water and biodiversity can be significant because of the large land base involved, and the likely use of intensive harvest regimes. These require careful management. Social impacts could be significant if there were to be large‐scale change in land use or management. In addition, although the economic considerations of feedstock production were not covered in this article, they will be the ultimate drivers of industry development. They are uncertain and are highly dependent on government policies (e.g. the price on carbon, GHG mitigation and renewable energy targets, mandates for renewable fuels), the price of fossil oil, and the scale of the industry.

Nitrogen mineralization in relation to site history and soil properties for a range of Australian forest soils

Rates of N mineralization were measured in 27 forest soils encompassing a wide range of forest types and management treatments in south-east Australia. Undisturbed soil columns were incubated at 20°C for 68 days at near field-capacity water content, and N mineralization was measured in 5-cm depth increments to 30 cm. The soils represented three primary profile forms: gradational, uniform and duplex. They were sampled beneath mature native Eucalyptus sp. forest and from plantations of Pinus radiata of varying age (<1 to 37 years). Several sites had been fertilized, irrigated, or intercropped with lupins. The soils ranged greatly in total soil N concentrations, C:N ratios, total P, and sand, silt, and clay contents. Net N mineralization for individual soil profiles (0–30 cm depth) varied from 2.0 to 66.6 kg ha-1 over 68 days, with soils from individual depths mineralizing from <0 (immobilization) to 19.3 kg ha-1 per 5 cm soil depth. Only 0.1–3.1% of the total N present at 0–30 cm in depth was mineralized during the incubation, and both the amount and the percentage of total N mineralized decreased with increasing soil depth. N fertilization, addition of slash residues, or intercropping with lupins in the years prior to sampling increased N mineralization. Several years of irrigation of a sandy soil reduced levels of total N and C, and lowered rates of N mineralization. Considuring all soil depths, the simple linear correlations between soil parameters (C, N, P, C:N, C:P, N:P, coarse sand, fine sand, silt, clay) and N mineralization rates were generally low (r<0.53), but these improved for total N (r=0.82) and organic C (r=0.79) when the soils were grouped into primary profile forms. Prediction of field N-mineralization rates was complicated by the poor correlations between soil properties and N mineralization, and temporal changes in the pools of labile organic-N substrates in the field.