Journal list menu

Volume 23, Issue 3 e20269
ORIGINAL ARTICLE
Open Access

The role of hydropedology when aiming for the United Nations Sustainable Development Goals

Johan Bouma

Corresponding Author

Johan Bouma

Department of Soils, Wageningen University, Wageningen, The Netherlands

Correspondence

Johan Bouma, Wageningen University, Wageningen, The Netherlands.

Email: [email protected]

Contribution: Conceptualization, Data curation, Formal analysis, Methodology, Visualization, Writing - original draft, Writing - review & editing

Search for more papers by this author
First published: 14 July 2023
Citations: 3

Assigned to Associate Editor Thorsten Knappenberger.

This paper is based on the virtual keynote presented on August 29 at the Kirkham Conference, Nombolo Conference Center, Kruger Parrk, South Africa, August 28–September 1, 2022.

Abstract

Soil physics defines soil water, air, and temperature regimes that are crucial for agricultural production, the quality of water resources, and waste management. Positioning such activities in the context of the United Nations Sustainable Development Goals (SDGs) will provide a valuable link to the international policy arena and should also include consideration of other SDGs dealing with climate change, biodiversity preservation, and soil health, thereby covering aspects of SDG 2 (zero hunger), SDG3 (good health and wellbeing), SDG6 (clean water and sanitation), SDG13 (climate action), and SDG15 (life on land). Combined expertise of soil physicists and pedologists in hydropedology offers a basic soil package, to be completed by soil chemists and biologists, to interdisciplinary teams studying ecosystem services in line with SDGs. Soils not being isotropic and homogeneous, pedology offers descriptions of soil heterogeneity, requiring innovative measuring methods. Digital mapping significantly improves the quality of traditional soil maps by being more quantitative at lower cost. Socioeconomic conditions are important elements of sustainable development as is recognized by the soil security concept, distinguishing soil connectivity with users and soil codification, linking soil with the policy arena. The latter has been successfully achieved in South Africa. Innovative, “applied” interdisciplinary research is needed to develop operational methods, assessing ecosystem services that are relatively of low cost, simple, and transparent, allowing acceptance by critical stakeholders. Modern proximal sensing techniques offer a promising perspective. An exploratory case study of a Dutch: “Living Lab” is referred to as an example of such an “applied” interdisciplinary approach.

Webinar

Hydropedology: Soil, Water, and the Human Connection

1 INTRODUCTION

There is general agreement on the need for sustainable development. Long a rather abstract concept, the introduction of the 17 United Nations Sustainable Development Goals (SDGs), approved by193 countries in 2015, has introduced goals, targets, and indicators, allowing a focused approach to the sustainability challenge ahead (http://sdgs.un.org). SDGs consider economic, social, and environmental aspects, thereby defining the general concept of sustainable development. This needs emphasis because some of the “hard” sciences, including soil physics and soil science in general, may tend to focus on environmental aspects alone, neglecting the societal and political context of the work. That should indeed not receive primary attention, as this is beyond their professional expertise, but it should form the context within which the work is executed as it will determine whether or not research results will be implemented in practice. The recently proposed concept of soil security defines not only the more traditional aspects of soil condition, capability, and capital but also soil connectivity, describing links with stakeholders, society at large and the policy arena, and soil codification describing links with environmental rules and regulations (Bouma, 2015; Field et al., 2017; Koch et al., 2013). The soil science profession would be well advised to embrace the broad soil security concept in their endeavors.

This special section of the VZJ focuses on the role of soil physics in agricultural production, water resources, and waste management. This discussion can and should, in the opinion of the author, be positioned in the still broader societal context of the SDGs as these goals are well recognized by the policy arena. Attention should, therefore, also be paid to the role of soil physics in studying climate change, biodiversity, and soil health. In this broader context, environmental aspects of the SDGs can be characterized by considering soil functions (including input by soil physics) that contribute to ecosystem services “services provided by the ecosystem to mankind” (MEA, 2005) that, in turn, contribute to certain SDGs (Figure 1; Keesstra et al., 2016). Combining soil physical and pedological expertise in terms of hydropedology, to be discussed below, with related soil chemical and biological data, is proposed to improve soil science contributions to the overall effort to characterize the SDGs.

Details are in the caption following the image
Schematic representation of the relations between soil functions, ecosystem services, and the United Nations Sustainable Development Goals.

Considering the above, this review paper will discuss the following: (i) the contributions of soil physics to assessing environmental SDGs; (ii) Why consider hydropedology rather than just soil physics?; (iii) hydropedological approaches aimed at defining ecosystem services; (iv) hydropedology in an interdisciplinary context; and (v) considering the socioeconomic and political context.

2 CONTRIBUTIONS OF SOIL PHYSICS TO ASSESSING ENVIRONMENTAL SDGS

Lal et al. (2021) have reviewed the role of soils, expressed in terms of soil health, in achieving several SDGs, including SDG1 (end poverty), 2 (zero hunger), 3 (good health and wellbeing), 5 (gender equality), 6 (clean water and sanitation), 7 (affordable and clean energy), 9 (industry innovation and infrastructure), 11 (sustainable cities and communities), 12 (responsible consumption and production), 13 (climate action), and 15 (life on land). The review not only considers environmental aspects but economic and social aspects as well, be it implicitly. This comprehensive approach may, however, result in a blurred vision as to what, in this case, separate scientific disciplines, like soil physics, can specifically contribute as they are not the only game in town. Other scientific disciplines play major roles and political decisions will, after all, determine whatever will be realized in the real world.

Core Ideas

  • Linking soil research with the SDG debate favors its recognition in the policy and public arena.
  • Hydropedology is more effective in the above context than separate activities of soil physics or pedology.
  • Innovative modeling and measurement methods, digital mapping, and proximal sensing are needed to vitalize hydropedology.
  • Hydropedological contributions to ecosystem services requires a lean, interdisciplinary, operational, and cost-effective approach.
We propose, therefore, to restrict the debate to two aspects:
  1. A focus on environmental aspects where soil physics has a direct and significant impact on delivering ecosystem services, limiting attention to the most relevant SDGs, aiming at growing healthy crops (an important, but not unique, aspect of SDGs 2 and 3); protecting water quality (SDG6); climate mitigation (SDG13); and biodiversity preservation and soil health (SDG15).

  2. Broadening the exclusive focus on soil physics by including pedology as hydropedology and by considering soil moisture regimes in the unsaturated Vadose Zone as a basic joint input of both subdisciplines when discussing soil input into the SDG debate. Soil input is complete when soil chemical and biological aspects are added in this hydropedological context, realizing that nutrient uptake by plants and leaching of nutrients is highly affected by the soil moisture regime that also strongly affects biodiversity. Considering soil and water together, where hydrology will focus on ground- and surface water and soil physics on the Vadose Zone, turns out to be quite attractive for the policy arena. The Dutch Government has, for example, recently proposed to consider “water and soil” as guiding principles in future land use planning. Joint hydropedological work of soil physicists and pedologists has already been realized in the science arena. (Lin, Bouma, & Pachepsky et al., 2006; Bouma, 2006; Lin, Bouma, Pachepsky, Western et al., 2006; Lin et al., 2005, 2008).

3 WHY CONSIDER HYDROPEDOLOGY RATHER THAN JUST SOIL PHYSICS?

As mentioned, soils are not isotropic and homogeneous as is strictly speaking required by Richard's flow theory. Morphological descriptions of soils can assist in redefining boundary conditions of the flow system. This is particularly relevant for bypass flow, defined as: “vertical movement of free water through an unsaturated soil matrix, following macropores, like worm- and root channels or cracks (planar voids).” Bypass flow can result in rapid movement of water and solutes to great depth or to a depth where the macropores are discontinuous. Then water accumulates in the form of: “internal catchment.” Van Stiphout et al. (1987) have provided an example for a silt loam soil. Measuring bypass flow as a function of rain intensity and quantity as well as of the soil surface microrelief is possible (Booltink & Bouma, 2002), and when results are applied in a flow model, a realistic picture of actual soil water contents can be obtained, while assuming soil homogeneity provides unrealistic results (Bouma, 1989, 2016). Occurrence of particular soil horizons can also present problems when assuming homogeneity. For example, a hard spodic horizon could only be physically characterized by measurements in a large carved-out soil column as taking regular soil-core samples was impossible. (Dekker et al., 1984). Measurements in an argillic horizon containing tongues extending downwards from the overlying albic horizon needed two separate measurements: one in the large structural prismatic elements and one in the cracks. This resulted in two distinct populations of data rather than in a meaningless “average” value, based on random sampling, resulting in a very high standard deviation (Bouma et al., 1989).

Of particular interest is the pioneering work in South Africa based on modeling the effect of subsurface pedogenic soil horizons on regional flow regimes (Van Tol, 2020; van Tol & van Zijl, 2022; Van Tol et al., 2018; Van Tol et al., 2021). An intriguing example was generated in Oman where soil scientists observed locally vegetated patterns in an otherwise barren sand basin. When looking at the soils they found that vegetated patterns were associated with subsurface loamy soil layers that retained more water than the surrounding sand, thus explaining the vegetation growth. A thorough soil physical characterization inspired innovative soil engineering for home gardens and city greens, increasing the water supply capacity in arid regions (Al-Mayahi et al., 2020). One element stands out here: it all started by soil scientists observing vegetation patterns in the field and following this up by studying the associated soil processes. This is how pedology got started in the 19th century: the first pedologists saw certain landscape features, that others had so far only looked at in passing, and started to investigate the role of soils.

Other examples can be generated as every soil in the field shows heterogeneity, as described in pedology, offering challenges to generate accurate simulations of soil moisture regimes.

4 HYDROPEDOGICAL APPROACHES AIMED AT DEFINING ECOSYSTEM SERVICES

Soil moisture regimes are crucial factors determining agricultural production levels that affect SDGs 1 and 2. Simulation models are widely available to estimate production levels as a function of climate conditions (e.g., Holzworth et al., 2018; Kroes et al., 2017; Van Ittersum et al., 2013; White et al., 2013). They require basic soil physical characteristics such as hydraulic conductivity and moisture retention (Reynolds et al., 2002), that can be estimated by pedotransfer functions derived from soil survey databases (Bouma, 1989; Van Looy et al., 2017). Unfortunately, many of these databases also contain values of so-called: “available water,” arbitrarily defining water contents between “field capacity” (set at 0.3 bar) and “wilting point” (set at 15 bar). These values date from the 1930s and more recent work has shown that “field capacity” and “wilting point” vary strongly as a function of soil types, crops being studied and environmental conditions. Dynamic models, using hydraulic conductivity and moisture retention data as well as crop-specific “sink-terms” for root uptake of water should therefore be used now, avoiding the “available water” concept (e.g., Kroes et al., 2017).

Soil moisture regimes also define water fluxes during the year that are important to:
  1. characterize the purifying potential of soil during percolation of liquid waste, protecting groundwater quality (SDG6) by defining travel times of soil water. Viruses were removed in 60 cm of a sandy soil when liquid waste was applied at a rate of 5 cm/day while viruses moved beyond 60 cm at flow rates of 50 cm/day. Comparable results were found in a structured silty loam soil where pathogenic bacteria moved rapidly beyond 60 cm along planar voids when liquid waste was applied at rates higher than 1 cm/day. At lower rates water moved through the peds and purification was complete (Bouma, 1979). These examples illustrate soil biological phenomena as a function of the soil water regime;

  2. fine-tuning fertilizer applications in soils to the needs of plants during the growing season in precision practices with the objective to increase fertilization efficiency and reduce groundwater pollution (SDG6) (Stoorvogel et al., 2015). Here, different adsorption rates and flow patterns of nutrients in different soils play a key role when proposing optimal fertilization rates in space and time. These examples demonstrate soil chemical processes as a function of the soil water regime.

Pollution of surface waters (SDG6), can be caused by runoff of waste products, such as manure or compost, and this can be reduced by providing a permanent soil cover, avoiding soil crusting and allowing adequate water infiltration into the soil surface reducing lateral runoff into surface waters. Growing healthy crops is a contribution to SDG3 and requires limiting uptake of heavy metals and remnants of biocides, if present, by crops from the soil. This is a highly dynamic area of research with increasing concerns about pollution of crops by soil pollutants (e.g., Geissen et al., 2021). Increasing the soil carbon content is seen as an important mechanism contributing to climate mitigation (SDG13). Even though the “4per1000” proposal, suggesting that only a 0.4% worldwide increase of soil carbon content could neutralize all carbon emissions, may be too optimistic, carbon capture by soils can still be a significant process contributing to climate mitigation (Dupla et al., 2021; Smith et al., 2020). Obviously, soil moisture regimes are key to provide optimal physical conditions for carbon capture that will differ in different soils and climates.

Finally, last but not least, biodiversity and soil health (SDG15) are affected by soil and climate conditions where the soil moisture regime again plays a key role in defining optimal management practices resulting in biodiversity preservation and adequate soil health.

5 HYDROPEDOLOGY IN AN INTERDISCIPLINARY CONTEXT

As discussed, soil moisture regimes are essential elements in defining and optimizing ecosystem services that contribute to the five key SDGs being considered above. Cooperation between soil physicists and pedologists is not enough to completely characterize ecosystem services in line with the SDGs. Agronomists, plant breeders, entomologists, and others will play key roles in defining ecosystem services satisfying SDGs 1 and 2. Nutrition and pollution specialists will be involved with SDG3, hydrologists with SDG6, climatologists with SDG13, and ecologists with SDG15. What soil data are most effective in an interdisciplinary context? Simulation of soil water regimes, as discussed above, that can include chemical transformations, is an effective procedure in the interdisciplinary process, as all participants have to deliver basic data for the models. Simulation models also provide the unique possibility to explore effects of climate change, an increasingly important issue (e.g., Bonfante et al., 2019, 2020).

Soil maps are important as they define the occurrence of different types of soil in a landscape context. Two developments in soil mapping are important in a hydropedological context: (i) Digital mapping techniques have greatly improved the spatial expression of soil patterns by being more quantitative than classic soil maps and all that at lower cost (Mc Bratney et al., 2003; Minashy & Mc Bratney, 2015; Thompson et al., 2012; van Zijl, 2019; van Zijl et al., 2020). (ii) Functional characterization of soil types by comparing soil moisture regimes of soils with different soil classifications, has resulted in the Netherlands in a reduction of 368 soil types into 79 identical hydropedological units, contributing significantly to the efficiency of practical applications (Bouma, Bonfante et al., 2022; Hack-ten Broeke et al., 2019).

6 CONSIDERING THE SOCIOECONOMIC AND POLITICAL CONTEXT

Contributing to sustainable development is not just yet another activity in the scientific arena. Climate change threatens the very existence of mankind and the science arena has a major responsibility to rise to the occasion by developing operational and practical approaches that can help to reduce and, if possible, to avoid unfavorable effects of the activities of man.

But research on environmental ecosystem services in line with the SDGs, as discussed, will only have an effect when results are applied in practice. In other words, necessary adoption by stakeholders and citizens will only be realized if research is in line with their socioeconomic conditions and requirements. This implies that research should be performed in close interaction with stakeholders, in line with the soil connectivity concept of soil security concept discussed above. The new soil research program of the European Union: “A Soil Deal for Europe” (EC, 2021) emphasizes, therefore, the establishment of “Living Labs” and “Lighthouses,” where scientists work closely together with stakeholders in developing innovative management systems, in a transdisciplinary mode, that satisfies the requirements of the targets and indicators of ecosystem services in line with the SDGs (e.g., Bouma, de Haan et al., 2022; Bouma, 2022).

The science community faces a new challenge. Not only should they present their expertise in an interdisciplinary context where soil scientists interact with agronomists, hydrologists, ecologists, and climatologists, as discussed above, but they should also be aware of the demands of the stakeholders. Such demands may be unrealistic but even then they should be seriously considered in an often confusing: “post-truth,” “fact-free” modern world (Bouma, 2018). To be successful, this calls for convincing approaches that are relatively simple, operational, and cheap and this is in striking contrast with conventional research that (as such: rightly!) tends to explore and expand scientific boundaries with innovative techniques, aiming for publications in top journals. A new form of “applied research” is needed, next to basic research to keep the profession alive, where different disciplines act together by combining their existing expertise, realizing that: “the better is the enemy of the good.” Much can already be achieved applying existing knowledge and expertise and new research should preferably be focused on gaps in existing knowledge, rather than being guided by curiosity alone. An exploratory attempt along these lines was made for a Dutch “Living Lab,” demonstrating that, indeed, existing data allowed conclusions about a number of ecosystem services and soil health (Bouma, de Haan et al., 2022). But biological aspects of soil health and biodiversity assessment for SDG15 still lacked operational procedures and thresholds. Also, a plea was made for introducing proximal sensing techniques to improve existing monitoring procedures involving expensive and long-duration lab measurements (Reijneveld et al., 2022, 2023; Viscarra-Rossel et al., 2010; Viscarra-Rossel & Bouma, 2016; Viscarra-Rossel et al., 2017). In terms of soil security, soil condition, capability, and capital could be defined in this particular case study. Connectivity was still somewhat limited as contacts were mainly made with workers at an experimental station. But codification is still beyond reach because of the highly volatile political arena in the Netherlands where, unfortunately, research only seems to play a role at the sideline as it has not been able to realize a comprehensive interdisciplinary approach to address the sustainable development issue.

In contrast, in South Africa hydropedology is expressed now in land-use regulations. The University of Free State and other institutions developed the science, some of which funded by the Water Research Commission which is the research funding institution of the Department of Water and Sanitation (DWS). Departmental officials were nominated to serve on reference groups to help guide the research to ensure that the science and tools developed would be appropriate for assessment requirements through the legislative framework for water use authorization. This was a lengthy process but now tools are available and DWS has adopted them, compelling applicants to use these standardized tools and guidelines in the assessment process. This was effectively done not just for hydropedology but also for other assessment tools like wetland delineation, the wetland offset tool, the wetland buffer determination tool and so forth. In terms of soil security, codification is therefore assured!

With due respect, lawyers, governance- and communication experts, that tend to increasingly dominate in governmental agencies in many countries, as “hard” scientists retire, cannot be asked to judge technical hydropedological requirements when judging and regulating land use plans in the type of reference groups mentioned above. In South Africa, a hydropedologist was a member of the reference group, acting as a crucial “anchor” linking the science and policy arena. Such “anchors” are essential to advance the soil mission, where hydropedology turns out to be an effective tool.

7 CONCLUSIONS

  • Measuring soil moisture regimes is crucial to define ecosystem services in line with several UN-SDGs. Emphasizing this link with the SDGs will strengthen the links of this type of research with the policy arena.
  • Hydropedology, combining expertise of soil physicists with pedologists, is more effective than separate activities of either one when contributing to the SDG debate.
  • Pedology is rejuvenated by digital mapping techniques, providing quantitative results and by proximal sensing that can rapidly provide basic soil data, both at relatively low cost.
  • Contributions of soil science to the SDG debate should be based on hydropedological characterization of soil moisture regimes together with related soil chemical and biological data.
  • Widely available simulation models for the soil moisture regime are essential tools to realize hydropedological characterizations and interdisciplinary contributions to define ecosystem services.
  • Acceptance of research results by stakeholders and the policy arena, which is essential to realize the societal value of research, requires development of operational, relatively simple and low-cost methods to engage critical stakeholders, taking into account their socioeconomic conditions.

AUTHOR CONTRIBUTIONS

Johan Bouma: Conceptualization; data curation; formal analysis; methodology; visualization; writing—original draft; writing—review and editing.

CONFLICT OF INTEREST STATEMENT

The author declares no conflicts of interest.