J.F. McGrath

The effects of Eucalyptus globulus Labill. leaf letter on C and N mineralization in soils from pasture and native forest

The effects of addition of Eucalyptus globulus leaf litter on carbon and nitrogen mineralization in soils from a pasture and a native forest were evaluated using a long-term laboratory aerobic incubation assay (29 weeks at 20°C) in leaching microlysimeters. The amount of added leaf litter significantly influenced microbial respiration, microbial biomass and N turnover in both the native forest and pasture soils. Cumulative CO2-C respired increased with increasing rate of leaf litter addition when leaf litter was mixed through the soil or placed on the soil surface. These increases were associated with increases in microbial biomass C content. Cumulative net N mineralization declined in all treatments when litter was added and was lowest when leaf litter was mixed with soil. When leaf litter was added in increasing amounts to the soil surface, there was a concomitant increase in microbial biomass N content (r2=0.79, n = 8), indicating that the reduction in net N mineralization was primarily due to immobilization of N in microbial tissues. In contrast, when litter was mixed with soil in increasing amounts, there was a decrease in microbial biomass N in forest soil and an increase in pasture soil. Consequently, changes in the rate of net N mineralization were not well related to changes in microbial biomass N content. It is suggested that this may be due to the greater activity and more rapid turnover of microorganisms where litter was incorporated resulting in more of the immobilized N being partitioned into metabolic products or dead microbial cells. Incorporation of litter may also have enhanced loss N through denitrification.

Fertilizer and previous land use effects on C and N mineralization in soils from Eucalyptus globulus plantations

Rates of microbial respiration and net N mineralization were measured in soils from Eucalyptus globulus Labill. plantations established on ex-pasture and ex-native forest sites using a long-term laboratory aerobic incubation (226 d at 20°C) in leaching microlysimeters. The plantations had been fertilized with either N or P, or both N and P 15 mo prior to soil sampling. Carbon and net N mineralization rates in soils from plantation sites were compared with rates from adjacent pasture and forest sites. Potentially mineralizable N (N0) and net mineralization rate constant (k) were determined using a first-order kinetic model. Application of nutrients to eucalypt plantations significantly influenced net N mineralization processes and the effects differed with previous land use. For both ex-native forest and ex-pasture plantation sites, addition of N fertilizer markedly increased (about 2-fold) k, and reduced N0. Addition of P in the absence of N to the ex-native forest plantation soil reduced cumulative net N mineralization. Both the amount and rate of net N mineralization were associated with previous land use. The proportion of total soil N mineralized during incubation was similar in the ex-native forest plantation, ex-pasture plantation and pasture soils and higher than in the native forest soil. The relative amounts of CO2-C respired per unit of net N mineralized differed between the land use units and was greatest on the native forest site, reflecting the high C-to-N ratio of soil organic matter and potential for N immobilization in soil from the native forest relative to the managed ecosystems. Amounts of net N mineralized (kg ha-1 in 0-200 mm soil) during the initial 28 d aerobic incubation declined in the order: pasture (29)>ex-pasture (18)>ex-native forest (14)> native forest (3), reflecting the low net N mineralization in native forest soil compared to managed ecosystems. The reason for the large difference (about 40%) between the amounts of net N mineralized from soils of the pasture and the ex-pasture plantation sites is uncertain and requires further investigation.

Selecting species for recharge management in Mediterranean south western Australia – some ecophysiological considerations

This paper makes a case for coupling quantitative relationships between plant function and environment with an understanding of local hydrology as a basis for selecting species for recharge control and salinity management in the cropping zone (rainfall < 500 mm per year) of south western Australia, a region with a Mediterranean-type climate. This case is made in two parts: (a) A review of some physiological responses of trees to drought and trends in these responses in relation to the rainfall patterns across the natural distribution of species. In general low-rainfall species use less water during winter and early spring and have greater physiological activity during summer than high-rainfall communities. Trends in leaf water relations and stomatal responses to soil drying are consistent with this observation; species from lower rainfall environments have lower leaf water potential at the turgor loss point and weaker stomatal responses to soil drying than those from high rainfall areas. (b) A physiological growth model for Eucalyptus globulus is used to demonstrate how a quantitative description of key plant-environment interactions may be used to analyse the growth and mortality risk for combinations of site, climate and stand management.

Broad-Scale Restoration of Landscape Function with Timber, Carbon and Water Investment

Salinization threatens up to 17 million hectares of Australian farmland, major fresh water resources, biodiversity and built infrastructure. In higher rainfall (>600 mm/year) areas of south-western Australia a market based approach has resulted in the reforestation of over 280,000 ha of farmland with Eucalyptus globulus plantations. This has had significant collateral environmental benefits in terms of reducing salinity in several watersheds. This model has not been replicated in the lower (300–600 mm/year) rainfall areas of this region, which is a global biodiversity hotspot. In this area, conventional forestry species have lower wood yields and longer rotations, compromising profitability, and reinforcing land-holder preference to maintain existing agricultural activities.

The development of reforestation options for dryland farmland in south-western Australia: a review

Current forest industries in south-western Australia are based on regrowth natural eucalypt forests and Pinus and Eucalyptus spp. plantations, and restricted to areas with >600 mm y−1 annual rainfall. Dryland farming systems have been developed across 20 million ha in a zone with 300–600 mm y−1 annual rainfall and a Mediterranean climate. This zone is beset with land degradation problems, such as salinity and wind erosion, and there has been considerable effort in the last three decades to develop reforestation options to stabilise the landscapes. Traditional forestry approaches using pulp wood or sawlog production in this zone have been limited by unfavourable economics driven by modest tree growth rates, large transport costs to processing and export facilities, and high labour costs. Given that salinity results from a disruption of the landscape water balance, reforestation has represented a major component in attempts to tackle the problem. Issues with reforestation include (1) obtaining sufficient scale of activity to impact watershed water balances, (2) obtaining a hydrological response without displacing farm production and rural communities and (3) gaining payment for non-forest benefits. This paper reviews the approaches that have been used to integrate trees into the dryland farming (300–600 mm y−1 annual rainfall) systems of south-western Australia, and have resulted in at least 113 286 ha of reforestation. These included both traditional (pine and eucalypt sawlogs) and new (sandalwood, biodiversity restoration and carbon mitigation through bioenergy and carbon sequestration) projects. Ongoing investment has centred on carbon sequestration as this represents one of the few profitable options for the management of dryland salinity in this region. Approaches developed in this region to encourage farmland reforestation will be applicable in other dryland regions, particularly with the interest in using the land-sector to meet climate mitigation targets.