Richard Harper

Soil water repellency (SWR) is a major problem across the globe and often occurs in sandy soils. SWR inhibits seed germination in crops and pastures, enhances surface runoff and erosion, and accelerates the movement of nutrients and pesticides into groundwater. SWR is caused by the accumulation of hydrophobic organic compounds released from plants and soil microbiota, and in some studies the severity of SWR has been related to soil organic carbon (OC) contents and the presence of particular organic compounds. In addition, factors such as root exudates, contributions from multiple plants, compounds produced during the decomposition of organic matter, and fungal bio-products influence SWR. SWR is also related to soil physical properties such as surface area and chemical properties such as pH. In general, SWR increases with increasing soil OC and decreases with increasing soil surface area; therefore sandy soils are more prone to SWR; however if a soil contains sufficient OC, it will be repellent. This review explores the linkage between SWR and physicochemical properties of soils and suggests research directions to uncover the interplay of the various contributing factors to SWR.

Soil water repellency (SWR) is a widespread phenomenon that impacts plant growth, groundwater contamination, surface erosion and runoff in many regions of the world. It is a major constraint to agricultural production in southern Australia, affecting over ten million hectares of arable land and is a feature of natural ecosystems, and is particularly exacerbated by wildfire. Water repellency is caused by hydrophobic organic substances coating soil particle surfaces or within interstitial matter, derived from decomposing organic matter, root exudates and microbial by-products. The incidence of SWR predominantly depends on the interaction of soil organic matter (SOM) and the soil surface area.
Given the strong interest in increasing SOM contents to mitigate climate change, SWR will likely also increase. It is thus important to map soils in terms of both the current degree of SWR and its potential to develop in the future, based on intrinsic soil properties such as soil surface area. This will identify where amelioration techniques such as wetting agents and clay-rich amendments can be better targeted. Conventional SWR measurements are costly, time-consuming and labour-intensive and contain a high uncertainty in their representativeness and applicability in the field. This review will consider how different proximal sensing techniques can be used to assess the current incidence and severity of SWR and the risk of it developing, to provide integrated site-specific management.

Deep soils are located in most continents of the world. Soil carbon measurements are invariably made from the surface horizons, whereas much larger carbon stores occur to depths of many meters, with plant roots providing the main source of carbon. This root biomass persists long after land is deforested for agricultural and other pursuits or forests are killed by pests and fires and may represent a considerable carbon store at the global scale. The impacts on these carbon stores of reforestation or climate change are mostly unknown as the estimation of root biomass and carbon dynamics is challenging in deep soils. This chapter explores deep soil carbon from the perspectives of its definition, source, and persistence; methodologies available to study deep soil carbon; and the effect of land-use change on this carbon store.

Salinization of land is a form of desertification; salinization of rivers is a global threat to biodiversity and compromises the ecosystem goods and services of rivers, wetlands, and lakes. Salinization is caused by flooding or inundation with saline waters, breaching of dykes, storm surges, tsunamis, or the drying of large inland water bodies. Salinization can result where irrigation waters are compromised by salinity. Salinity intersects with major global concerns, including food security, desertification, and biodiversity protection. Soil salinity results from an excess of salts in the soil that reduces plant growth and crop productivity and affects soil biological activity. Salinized soils impose an osmotic stress on plants, reducing water uptake and concentrating toxic level of sodium and chloride. Different plant species exhibit different degrees of salinity tolerance. Salinization removes arable land from production, causing abandonment globally of 0.3–1.5 million hectare year−1. With adequate drainage, salts can be leached and the soil recovered but where the water table remains near the surface, the salinity problem will remain. It may be possible to reverse the effects of salinization. A crucial consideration is whether the desired end point is stabilizing the soils against further change, or reversing the process and restoring soils to another state. Approaches include prevention, stabilization, active management, or land retirement or abandonment. Successful restoration of salinity at the landscape-scale relies on broadscale land-use change. This is problematic where the most profitable land-use is agriculture, thus there has therefore been considerable investigation of land-use systems that at least replicate the profitability of the current agricultural system. Recent approaches have explored how to make the higher water using farming systems acceptable by making the replacement plants profitable in their own right.

As outlined in Chapter 2, our analysis of forest-water relations addresses four important subsystems of a linked planetary social-ecological system: climate, forests, water and people. In this chapter, we consider how each of these subsystems is changing (trend) and what is causing the change (’determinant’). We discuss the critical determinants of change in forests as they relate to water quality and quantity. Chapter 4 then presents the impacts of these changes on water quality and quality.

Water security is key to achieving the United Nations’ Sustainable Development Goals (SDGs). In what has been called a new era of the Anthropocene, the changes we are inflicting on our planet are influencing the current and future availability of ecosystem services of importance to our survival. And provision of clean water is the most basic ecosystem service necessary for life on earth. Yet, increasingly we are facing water shortages, and an estimated four billion people do not have sufficient access to safe and reliable water.

Whereas the link between forests and climate is regularly considered in decision-making, that between water and forests remains under-represented. Even more importantly, water, forests and climate are intrinsically interlinked, but this relationship is still poorly understood when it comes to decision-making.

Forests influence water resources in multiple ways, and at multiple levels. Generalisations fail to capture these intricate interlinkages. Soils, rooting depth, leaf area and stems are key features of trees and forests that determine the way an individual tree uses water and consequently, its impact on water resources. Species diversity and age structure of a forest in turn influence the above four key factors. At landscape scales, the diverse mosaic of land use that typifies human-dominated landscapes, also determines the way forests and trees are able to influence water availability and quality. Recent recognition of the role of forests in downwind generation of precipitation further expands the geographical scope of the intricate relationship between water and forests.

Today, the fact that the world has mobilised around 17 SDGs, all of which have a connection to water, provides a crucial argument for paying more attention to the forest-water link. Policymakers are facing new challenges in implementing the multiple water-related objectives across the portfolio of SDGs. While the international community agreed the SDG framework based on moral principles, science is essential for developing the policies and practices required in achieving the related targets.

Led by IUFRO, the Collaborative Partnership on Forests’ Global Forest Expert Panels (GFEP) initiative undertook a comprehensive scientific assessment of the state of knowledge on the forest-water relationship. This policy brief summarises the key messages of the report completed by the GFEP on Forests and Water.

Water security is key to achieving the United Nations’ Sustainable Development Goals (SDGs). In what has been called a new era of
the Anthropocene, the changes we are inflicting on our planet are influencing the current and future availability of ecosystem services of importance to our survival. And provision of clean water is the most basic ecosystem service
necessary for life on earth. Yet, increasingly we are facing water shortages, and an estimated four billion people do not have sufficient access to safe and reliable water.

Whereas the link between forests and climate is regularly considered in decision-making, that between water and forests remains under-represented. Even more importantly, water, forests and climate are intrinsically interlinked,
but this relationship is still poorly understood when it comes to decision-making.

Forests influence water resources in multiple ways, and at multiple levels. Generalisations fail to capture these intricate interlinkages. Soils, rooting depth, leaf area and stems are key features of trees and forests that determine the way an individual tree uses water and consequently, its impact on water resources. Species diversity and age structure of a forest in turn
influence the above four key factors. At landscape scales, the diverse mosaic of land use that typifies human-dominated landscapes, also determines the way forests and trees are able to influence water availability and quality. Recent recognition of the role of forests in downwind generation of precipitation further expands the geographical scope of the intricate
relationship between water and forests.

Today, the fact that the world has mobilised around 17 SDGs, all of which have a connection to water, provides a crucial argument for paying more attention to the forest-water link. Policymakers are facing new challenges in implementing the multiple water-related objectives across the portfolio of SDGs. While the international community agreed the SDG framework
based on moral principles, science is essential for developing the policies and practices required in achieving the related targets.

Led by IUFRO, the Collaborative Partnership on Forests’ Global Forest Expert Panels (GFEP) initiative undertook a comprehensive scientific
assessment of the state of knowledge on the forest-water relationship. This policy brief summarises the key messages of the report completed by the GFEP on Forests and Water.

The association of organic carbon (SOC) with clay in soils means that additions of clay to soils can increase the capacity of the soils for storage, and, eventually, sequestration of C. Addition of a fine-textured waste from bauxite processing to sandy soils for up to 29 years has led to increases of about 12 Mg C ha−1, with a strong (r 2 = 0.93, P < 0.001) correlation between clay content and SOC. An increase of 2.2 Mg C ha−1 has also occurred after 8 years in a sandy topsoil amended with subsoil clay-rich material. Bentonite addition increased plant yield in degraded and light-textured soils in tropical Australia. In Thailand, addition of clay-rich materials, particularly bentonite, but also clayey termite mound material, greatly increased the productivity of a degraded light-textured soil. Examination of soil modified by redistribution of subsoil clay into sandy topsoil by mechanical inversion showed the growth of roots in incorporated lumps of clay. Electron micrographs of clay-rich soils showed that fine mineral material (clay) can become closely associated with roots and other organic matter, which can protect them from decomposition. Roots within added or redistributed clay, along with microbes and their products, may become coated, enabling carbon sequestration in the long-term.

Agriculture, Forestry, and Other Land Use (AFOLU) is unique among the sectors considered in this volume, since the mitigation potential is derived from both an enhancement of removals of greenhouse gases (GHG), as well as reduction of emissions through management of land and livestock (robust evidence; high agreement). The land provides food that feeds the Earth’s human population of ca. 7 billion, fibre for a variety of purposes, livelihoods for billions of people worldwide, and is a critical resource for sustainable development in many regions. Agriculture is frequently central to the livelihoods of many social groups, especially in developing countries where it often accounts for a significant share of production. In addition to food and fibre, the land provides a multitude of ecosystem services; climate change mitigation is just one of many that are vital to human well-being (robust evidence; high agreement). Mitigation options in the AFOLU sector, therefore, need to be assessed, as far as possible, for their potential impact on all other services provided by land. [Section 11.1]

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. Two complementary strategies are being used to restore landscape function across this drier region, through increased reforestation. The first is to shift from the paradigm of forestry comprising tall trees grown in relatively long rotations and producing timber to one based on the production of a range of biomass products (bioenergy, chemicals, sequestered carbon), and environmental services such as providing fresh water. As a consequence of breaking this paradigm, silvicultural practices such as stand densities and rotation length can also be redefined. The second strategy is to integrate these new systems into the existing dryland farming systems. Four broad approaches are being assessed viz. (a) belts of trees with farming maintained in inter-row alleys, (b) blocks of trees located on areas of water accumulation or of high recharge, (c) adjusting species selection to soil conditions, such as those that are shallow or saline, and (d) alternating short phases (3–5 years) of trees with farming. These systems offer the prospect of sequestering carbon, and producing wood or biofuels from farmland without displacing food production.