R.J. Harper

Global change pressures on soils from land use and management

Soils are subject to varying degrees of direct or indirect human disturbance, constituting a major global change driver. Factoring out natural from direct and indirect human influence is not always straightforward, but some human activities have clear impacts. These include land-use change, land management and land degradation (erosion, compaction, sealing and salinization). The intensity of land use also exerts a great impact on soils, and soils are also subject to indirect impacts arising from human activity, such as acid deposition (sulphur and nitrogen) and heavy metal pollution. In this critical review, we report the state-of-the-art understanding of these global change pressures on soils, identify knowledge gaps and research challenges and highlight actions and policies to minimize adverse environmental impacts arising from these global change drivers. Soils are central to considerations of what constitutes sustainable intensification. Therefore, ensuring that vulnerable and high environmental value soils are considered when protecting important habitats and ecosystems, will help to reduce the pressure on land from global change drivers. To ensure that soils are protected as part of wider environmental efforts, a global soil resilience programme should be considered, to monitor, recover or sustain soil fertility and function, and to enhance the ecosystem services provided by soils. Soils cannot, and should not, be considered in isolation of the ecosystems that they underpin and vice versa. The role of soils in supporting ecosystems and natural capital needs greater recognition. The lasting legacy of the International Year of Soils in 2015 should be to put soils at the centre of policy supporting environmental protection and sustainable development.

A multivariate framework for interpreting the effects of soil properties, soil management and landuse on water repellency

This paper reviews recent progress in relating the incidence of water repellency to other soil attributes, relating the severity of water repellency to different soil management and landuses and determining the value of soil survey in predicting the risk of water repellency. Data sets of soils from south-western Australia are used for this analysis. The relationship between water repellency and other soil attributes such as clay and organic matter contents has been found to be multivariate in nature, with a general form: Water repellency ≃ a Organic Matterb/Clay(c) where a, b and c are constants. Multiple regressions, which use a variety of attributes related to soil organic matter and soil surface area (e.g. clay, silt, amorphous iron contents), can explain up to 63% of the variation in water repellency. Water repellency increases in severity with increasing organic carbon content or decreasing soil surface area and vice versa. This relationship explains the poor or non-existent bivariate relationships between various soil attributes and water repellency reported in several studies and allows the re-interpretation of studies into the effects of liming, zero-till and different rotation lengths on water repellency as each affect soil organic carbon (OC) content. It is also consistent with observations that water repellency is more prevalent on sandy soils and reported reductions in water repellency following applications of clay and fine inorganic materials. It similarly explains the occurrence of water repellency on soils with >5% clay; accumulation of sufficient amounts of OC can induce water repellency in any soil. Soil management and landuse affect water repellency. Water repellency is less severe in soils under crops compared to pastures and this has been considered as being caused by differences in organic matter composition. This is re-interpreted as being due to differences in organic matter amount as cultivation mineralises and dilutes soil organic matter in the topsoil. Water repellency is also a feature of soils under natural vegetation (Eucalyptus, Banksia spp.) and may be more severe than that for agricultural soils. Increments of organic matter from natural vegetation can induce water repellency to a greater extent than equivalent amounts of organic matter from agricultural species. Thus, water repellency can be considered as a natural feature of soils, rather than a form of land degradation due to farming. Comparisons between the water repellency associated with different soil management or landuses are problematic due to differences in not only OC content and composition but also other soil properties such as surface area, due to the spatial separation of the areas compared. Given the underlying relationship between clay content and water repellency, field texture data from soil surveys can be used to predict the risk of water repellency developing, this providing a basis for optimum management practice. The degree to which this water repellency develops will be dependent on management practices and their effects on soil organic matter content. Quite costly, but profitable, ameliorative management (e.g. clay application) can thus be applied to sites where required, rather than to large areas on a non-specific basis.

Co‐benefits, trade‐offs, barriers and policies for greenhouse gas mitigation in the agriculture, forestry and other land use (AFOLU) sector

The agriculture, forestry and other land use (AFOLU) sector is responsible for approximately 25% of anthropogenic GHG emissions mainly from deforestation and agricultural emissions from livestock, soil and nutrient management. Mitigation from the sector is thus extremely important in meeting emission reduction targets. The sector offers a variety of cost-competitive mitigation options with most analyses indicating a decline in emissions largely due to decreasing deforestation rates. Sustainability criteria are needed to guide development and implementation of AFOLU mitigation measures with particular focus on multifunctional systems that allow the delivery of multiple services from land. It is striking that almost all of the positive and negative impacts, opportunities and barriers are context specific, precluding generic statements about which AFOLU mitigation measures have the greatest promise at a global scale. This finding underlines the importance of considering each mitigation strategy on a case-by-case basis, systemic effects when implementing mitigation options on the national scale, and suggests that policies need to be flexible enough to allow such assessments. National and international agricultural and forest (climate) policies have the potential to alter the opportunity costs of specific land uses in ways that increase opportunities or barriers for attaining climate change mitigation goals. Policies governing practices in agriculture and in forest conservation and management need to account for both effective mitigation and adaptation and can help to orient practices in agriculture and in forestry towards global sharing of innovative technologies for the efficient use of land resources. Different policy instruments, especially economic incentives and regulatory approaches, are currently being applied however, for its successful implementation it is critical to understand how land-use decisions are made and how new social, political and economic forces in the future will influence this process.

Relationships of water repellency to soil properties for different spatial scales of study

In order to predict the occurrence of water repellency, which is a labile property, from field survey data obtained throughout the year, it is necessary to identify predictive relationships between water repellency and commonly measured soil properties. This paper evaluates these relationships for diverse soil assemblages. These soil assemblages include a set of reference soils from the south-west of Western Australia (an area of 250 000 km2), more intensively sampled suites of soils in several smaller soil{landscape associations within the south-west of Western Australia (≅1000 km2), soils from single farms (1-10 km2) and transects (≅0·001 km2), and single soil profiles (≅m2). The severity of water repellency was assessed by measuring water drop penetration time in seconds (WDPT) and was related to intrinsic properties of soils using log-transformed data. For the set of soils from the West Midland Sandplain the type of land use was also considered as a variable. There is a general tendency for WDPT to increase as organic matter content increases and decrease as the content of fine mineral material increases (clay, silt, very fine sand). However, there is no single soil property that is able to predict WDPT adequately. Furthermore, reliability of prediction decreases as the area of sampling increases. There appear to be no systematic differences in the capacity of organic matter from pasture or crop to induce water repellency, but increments of organic matter under bush increase water repellency at a greater rate than does organic matter from crop or pasture.

Current status and future prospects for carbon forestry in Australia

Summary Carbon forestry is part of a suite of land-based activities that can be used to mitigate carbon emissions, and also provide a range of other environmental co-benefits. Components are included in the Carbon Credits (Carbon Farming Initiative) Act 2011. There is large divergence in Australian estimates of the areas of land that may be used for carbon forests and there has been a vigorous public debate about carbon forestry, partly based on concerns about displacement of food-producing land. We identify four distinct afforestation or reforestation (AR) activities that involve carbon mitigation and suggest a terminology based on these. These are (1) ‘plantations’ that also produce timber and wood products, (2) ‘carbon-focused’ sinks, (3) ‘environmental’ or natural resource management plantings and (4) ‘bioenergy’ plantings for use either as a feedstock for stationary energy production or transport fuels. After accounting for AR projects established for other purposes (e.g. timber and pulpwood), we estimate that the current area of carbon forests in Australia is 65000 ha. Despite the national Renewable Energy (Electricity) Act 2000 and its 2010 amendments there are few extant biomass projects. However this may change with the development of new technologies and the imposition of a carbon price on electricity production. The reasons for the gulf between actual and potential carbon AR activity are proposed to include (1) the absence of a formal carbon compliance scheme, (2) challenges in managing carbon through an entire product cycle, (3) the degree of understanding of carbon forestry by financiers, (4) landholder preference, (5) technical barriers and (6) regulatory uncertainty. We suggest an extension of the National Plantation Inventory from traditional plantations to carbon forestry, so that future policy can be developed on the basis of good-quality underpinning information that can be disaggregated to analyse trends in AR for different purposes. To encourage innovation in the sector, we also suggest either the extension or establishment of research and development funding arrangements, similar to those already existing for other rural industries.

Bio-mitigation of carbon following afforestation of abandoned salinized farmland

As the global demand for food continues to increase, the displacement of food production by using agricultural land for carbon mitigation, via either carbon sequestration, bioenergy or biofuel is a concern. An alternative approach is to target abandoned salinized farmland for mitigation purposes. Australia, for example, has 17 million ha of farmland that is already or could become saline. At a representative, salinized, low rainfall (350 mm yr−1) site at Wickepin, Western Australia, we demonstrate that afforestation can mitigate carbon emissions through either providing a feedstock for bioenergy or second generation biofuel production and produce salt‐tolerant fodder for livestock. A range of factors markedly affect this mitigation. These include hydrological conditions such as salinity, site factors such as slope position and soil properties and a range of silvicultural factors such as species, planting density and age of the planting. High density (2000 stems ha−1) plantings of Eucalyptus occidentalis Endl. produced a mean total biomass of 4.6 t ha−1 yr−1 (8.5 t CO2‐e ha−1 yr−1) averaged over 8 years. Atriplex nummularia Lindl. produced a mean total biomass of 3.8 t ha−1 yr−1 (6.9 t CO2‐e ha−1 yr−1) averaged over 4 years and approximately 1.9 t ha−1 yr−1 of edible dry matter annually to 8 years of age. With differences in salt tolerance between E. occidentalis and A. nummularia, we propose an integrated approach to treating salinized sites that takes salinity gradients into account, replicates natural wetland ecosystems and produces both fodder and biomass. Continued mitigation is expected as the stands mature, assuming that growth is not affected by the accumulation of salt in the soil profile. Such carbon mitigation could potentially be applied to salinized farmland globally, and this could thus represent a major contribution to global carbon mitigation without competing with food production.

Using soil and climatic data to estimate the performance of trees, carbon sequestration and recharge potential at the catchment scale

There is considerable interest in integrating deep-rooted perennial plants into the dryland farming systems of southern Australia as soil, water supplies and biodiversity are continually threatened by salinity. In addition to wood products, trees could provide new products, such as bioenergy, environmental services, such as the sequestration of carbon, reductions in recharge to groundwater and biodiversity protection. Before marketing these services, it is necessary to determine the optimal distribution of trees across the landscape, in terms of land suitability, their productivity, and proximity to existing processing and transport infrastructure. Similarly, understanding how recharge varies across landscapes will allow the targeting of trees to areas where they are most needed for salinity control. Catchment scale (1:100000) soil and landform datasets are now available across much of the agricultural area of Australia. While these data are at a scale inappropriate for management at the enterprise (farm) scale, they will allow broad planning for new plant-based industries, such as whether there is sufficient suitable land available before embarking on a new enterprise and the likely productivity of that land. In this paper, we outline an approach that combines existing soil and landform data with estimates of climate to produce estimates of likely wood yield, carbon sequestration and potential for recharge to groundwater. Using the 283686 ha Collie catchment of southwestern Australia as an example, this analysis indicated broad areas where land is suitable for forestry, where forestry is unlikely to succeed, or where it was not required because leakage to groundwater is negligible. It also provides broad estimates of wood production and carbon sequestration. The approach is applicable to the integration of deep-rooted perennial plants into farming systems in other regions confronted with multiple natural resource management issues

Hardsetting in the Surface Horizons of Sandy Soils and its Implications for Soil Classification and Management

Marked variations in hardsetting occur in the sandy surface horizons of duplex deep sandy soils in a semi-arid area of Western Australia. Hardsetting by definition only occurs in dry soils and increases with field texture. Soil strength measured on remoulded samples in the laboratory conformed with field assessments of strength (consistence). Most (79%) of the variation in strength between Ap horizon samples was explained by clay content, with small differences in clay content resulting in large differences in strength. Half of the maximum measured strength in the Ap horizons was achieved at a clay content of only 8%. The A2 horizons were markedly stronger than corresponding Ap horizons, despite similar clay contents, and this difference in strength is related to the larger organic matter content of the Ap horizons. Hardsetting of these sandy soils may be explained in terms of the cementing action provided by clay which forms bridges between particles. Organic material weakens these bridges. Hardsetting may affect the wind erodibility of sandy soils, through differences in surface conditions (i.e. loose v. compact) and by increasing the resistance to abrasion by saltating sand. It is not clear what effect it will have on plant performance. Hardsetting is a continuous, rather than discrete soil attribute, and if it is to be described in the field, and used in soil classification schemes, objectives class limits should be defined, perhaps using dry consistence ratings.

The effects of clay and sand additions on the strength of sandy topsoils

The clay contents of sandy soils in south-western Australia are often modified, either intentionally or inadvertently, as a result of management practices and erosion. Although the strength of sandy surfaced soils has previously been shown to be related to clay content, in natural soils the effects of induced changes in clay content on soil strength have not been assessed. Increasing amounts of subsoil clay were added to their respective topsoils in increments ranging from 5 to 20% by weight, and these systematically increased soil strength. A strong log-log relationship between clay content and soil strength explained 69% of the variation, with soil strength further affected by sodicity. This enhancement of soil strength has implications for the practice of claying water-repellent soils, particularly where non-uniform application or poor incorporation results in high concentrations of clay, where very high rates (e.g. 300 t/ha) of application are used, or where clayey subsoils are brought to the surface by deep cultivation or the removal of topsoils by erosion. Drift sand, with a clay content of around 1% and negligible strength, was added in increasing increments to typical topsoils, over the range of 0-100% by weight to replicate the effects of wind-induced deposition and winnowing of clay particles. Increasing additions of drift sand systematically decreased soil strength, with a log-log relationship between clay content and strength of the mixtures explaining 81% of the variation. This suggests that wind erosion, and the winnowing of clay or deposition of drift sand, permanently destabilises soil surfaces by reducing soil strength. It is feasible that strategic applications of sand on the surfaces of soils affected by hardsetting may reduce soil strength and encourage soil structure development and seedling emergence.

Bioenergy production and sustainable development: science base for policy-making remains limited

The possibility of using bioenergy as a climate change mitigation measure has sparked a discussion of whether and how bioenergy production contributes to sustainable development. We undertook a systematic review of the scientific literature to illuminate this relationship and found a limited scientific basis for policymaking. Our results indicate that knowledge on the sustainable development impacts of bioenergy production is concentrated in a few well‐studied countries, focuses on environmental and economic impacts, and mostly relates to dedicated agricultural biomass plantations. The scope and methodological approaches in studies differ widely and only a small share of the studies sufficiently reports on context and/or baseline conditions, which makes it difficult to get a general understanding of the attribution of impacts. Nevertheless, we identified regional patterns of positive or negative impacts for all categories – environmental, economic, institutional, social and technological. In general, economic and technological impacts were more frequently reported as positive, while social and environmental impacts were more frequently reported as negative (with the exception of impacts on direct substitution of GHG emission from fossil fuel). More focused and transparent research is needed to validate these patterns and develop a strong science underpinning for establishing policies and governance agreements that prevent/mitigate negative and promote positive impacts from bioenergy production.