Soil health and ecosystem services

Global soils are increasingly affected by acidification (Guo et al., 2010), salinization (Machado & Serralheiro, 2017), pollution (FAO, 2021), loss of biodiversity (Hou, 2022) and other soil health issues (Lehmann et al., 2020; Rinot et al., 2019). While traditionally soil scientists are primarily concerned about the nutrient status and crop productivity of agricultural soils (Delgado-Baquerizo et al., 2013; Havlin, 2020), the scientific community is now extending its focus to the broader ecosystem services that soil environment provides (Banwart, 2011; O'Riordan et al., 2021; Pereira et al., 2018). These ecosystem services include, but are not limited to, global climate regulation (Davidson & Janssens, 2006; Jansson & Hofmockel, 2020), water purification (Abdullah et al., 2020; Skaalsveen et al., 2019), human health protection (Oliver & Gregory, 2015; Tilman & Clark, 2014) and biodiversity conservation (Mader et al., 2002; Wall et al., 2015). This new trend has put an umbrella concept, ‘soil health’, at the centre of stage, which also helped the discussion on soil to go beyond the farming communities (Kibblewhite et al., 2008; Lehmann et al., 2020). Scientific publication pertaining to soil health has undergone rapid growth in recent years. The number of journal papers published in each year, as indexed by Web of Science Core Collections, grew from 101 in 2012 to 1043 in 2022, representing a 10-fold growth over a decade (see Figure 1). In contrast, the number of soil-related papers only grew twofold within the same period. The increased usage of soil health as a concept can greatly enhance communication between soil scientists and non-scientists such as policymakers and the general public (Powlson, 2021). This will undoubtedly provide momentum to better protect soil as a non-renewable resource. To contribute to the rapidly growing knowledge base on soil health and enhance its accessibility, our editorial team at the journal of Soil Use and Management is commissioning a virtual special issue (VSI) with the topic of ‘Soil health and ecosystem service’. So far, we have collected 40 papers, covering topics including (1) soil health concept, indicators and path forward, (2) sustainable plant growth, soil animal and microbial diversity, (3) ecosystem service: climate regulation, (4) ecosystem service: water quality, (5) ecosystem service: human health and (6) soil amendment and soil health improvement. We intend to keep updating this VSI and continuously collect high quality research papers that fall under the umbrella of soil health. In their recent critical review on soil health, Lehmann et al states ‘Soil health is the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals and humans, and connects agricultural and soil science to policy, stakeholder needs and sustainable supply- chain management’ (Lehmann et al., 2020). Similarly, the US Department of Agriculture defines soil health as ‘the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals, and humans’. (https://www.nrcs.usda.gov/conservation-basics/natural-resource-concerns/soils/soil-health). Scientists proposed the link between soil and health over half century ago (Howard, 1945; Voisin, 1959), but the concept of soil health did not gain popularity until the most recent decade. In recent years, environmental pressures have forced scientists and farmers to develop and adopt more sustainable agricultural practices (Hobbs et al., 2008). The recognition of soil's importance has accelerated in recent years due to the ongoing international effort on Agenda 2030, that is towards achieving the 17 sustainable development goals (SDGs) and that soil plays critical roles in many of these SDGs (Hou, 2023). Among the various countries, the US has produced the largest number of scientific papers relating to soil health, followed by India and China (see Figure 2). Nevertheless, soil scientists have faced great difficulties in defining and quantifying ‘soil health’ despite of its increased usage over the last decade both inside and outside of the scientific community (Baveye, 2021; Lehmann et al., 2020). Soils serving different functions require different properties. For instance, soil used as fill material in infrastructure requires no biological abundance, but this characteristic is profoundly important for many other ecosystem services (Baveye, 2021). Soil with low pH and a lack of readily available nutrients may be viewed as healthy in a forest, but not healthy for crop growth (Powlson, 2021). The lack of consensus on the definition and quantitative assessment methodology for soil health has led to worries that scientists, policy makers, activists and farmers may lead to confusion and chaos when a common vision to take practical actions is needed (Baveye, 2021). There are a variety of physical, chemical and biological indicators that have been used to evaluate soil health (Lehmann et al., 2020; Sofo et al., 2022). While traditional assessment tends to rely upon physicochemical indicators that are closely linked to soil organic matter and crop yield (Lehmann & Kleber, 2015; Seufert et al., 2012), recent studies have referred more to biological indicators to reflect the ‘living’ characteristics of soil health and biodiversity (Guerra et al., 2021; Pulleman et al., 2012) (see Figure 3). For instance, microbial biomass carbon and nitrogen have been proposed as indicators of soil health in urban areas (Gąsiorek & Halecki, 2022). The conversion of multiple indicator score into a single soil health score can be achieved; however, the ratings for individual indicators are viewed as being more important, especially when it comes to management practices (Moebius-Clune, 2016). Moreover, expert opinion may be combined with non-linear scoring techniques to offer a more comprehensive assessment of soil quality and soil health (Ghorai et al., 2023). Existing studies have found that sustainable plant growth often co-occurs with and sometimes heavily relies upon soil biodiversity (Guerra et al., 2021). The microbial community in rhizosphere is thousands of times richer than that in the bulk soil (Bamdad et al., 2022). Arbuscular mycorrhizal fungi (AMF) are naturally occurring in agricultural soils, forming symbiotic relationships with 72% of plant species, and contribute to phosphorus uptake by crops (Christensen et al., 2022). The mycelium developed by AMF expands the volume of soil from which soil nutrients can be extracted from, and also results in diverse response to soil disturbance (Conceição et al., 2023). Soil animals also play an important role in maintaining soil health and enhance crop productivity (Van Groenigen et al., 2014; Wu et al., 2011). A study in Cambodia found that soil mounds originated from termite bioturbation host abundant soil animals and plant species (see Figure 4). The mounds were widely used by local farmers to increase the fertility of their rice fields (Muon et al., 2023). A global meta-analysis shows that the presence of earthworms in agroecosystem increased crop yield by 25% (Van Groenigen et al., 2014). Soil use and management affects both crop yield and biodiversity of agricultural soil. A field study showed that long-term lime application, started in 1942 and applied every 5–9 years, resulted in higher level of arbuscular mycorrhizal fungi, improved root growth and higher crop yield (Christensen et al., 2022). In comparison with chemical fertilizer applications, agroecological practice in tea farms significantly increased AMF frequency and intensity, soil macrofauna and mesofauna abundance. The tea yields were slightly lower, but still enabled farmers to earn $8400/ha/year owing to the production of higher quality organic tea (Le et al., 2023). Conservation tillage practices, including no-till, ridge tillage and subsoiling tillage were found to all significantly increase soil organic carbon in North-eastern China, but no-till reduced crop yield in areas where mean annual temperature is below 3°C (He et al., 2022). Soil plays an important role in regulating the atmospheric composition and climate change (Jansson & Hofmockel, 2020; Lal, 2004). Soils represent the largest terrestrial carbon pool, with surficial soil storing 1500–2400 GtC, far exceeding atmospheric carbon at 860 GtC (Friedlingstein et al., 2020). Land degradation from agricultural practices results in significant loss of soil carbon stock. Grassland accounts for nearly 70% of global agricultural land, and its mismanagement has led to the loss of approximately 300 GtC in grassland soil (Abdalla et al., 2022). Peatland represents one of the most carbon rich and fertile soils on earth, storing 30% of global soil carbon; but drainage and intensive cultivation has resulted in a huge loss of peat (Matysek et al., 2022). The soil environment also emits a large amount of CH4 and N2O greenhouse gases, especially under inundated environments such as peat land and rice paddy fields (Page et al., 2022; Zhu et al., 2018). Sustainable agricultural practices, such as rotational grazing in grassland and residue return in cropland, can significantly reduce net CO2 emission and increase soil carbon stock (Abdalla et al., 2022; Islam et al., 2022). The wastage of peat can be regulated by careful water-table management, but it also causes trade-offs among respiratory loss of carbon, CH4 emission and plant productivity (Matysek et al., 2022). Organo-mineral interactions involving Al, Fe and Mn elements affect the decomposing of soil organic carbon (Antony et al., 2022). Climate smart agriculture technologies must incorporate the latest scientific findings, to mitigate climate change potential (Paustian et al., 2016), as well as enabling farmers to better adapt to ongoing climate change stresses (Nyagumbo et al., 2022). Soil carbon management can also play a role for large landholding institutions to reach carbon zero goals. For instance, a recent study found that Newcastle University in the United Kingdom can off-set 50% of its current emission over a 40-year period by using alternative land management strategies (Wang, Werner, & Manning, 2022). In order to feed an ever-growing global population with a limited supply of cropland, there is an increased usage of fertilizers and pesticides in agriculture to improve crop yields (Lu & Tian, 2017). While these various chemicals can be beneficial to supply soil nutrients and suppressing crop diseases, they also pose a threat to aquatic environment due to their eutrophication potential and exposure toxicity (Wallman & Delin, 2022). The irony also lies in that only a small fraction of the nutrients and pesticides can reach the plant root system and target organisms, respectively. Under worst scenarios, only 1% of applied pesticides may reach target organisms, while the majority of the reminder may enter surface water or groundwater, causing potential water pollution (Ali et al., 2019). Water pollution from soil management is affected by soil amendment type, rainfall intensity and interval, as well as topography and hydrogeology conditions (Wang, Guo, et al., 2022; Wang, Wang, et al., 2023; Yang et al., 2022). A study in Ireland found that increasing the time interval between slurry application and first rainfall event to 49 days could reduce phosphorus loading in runoff by 80% (O'Rourke et al., 2022). In New Zealand, farm dairy effluent is recycled in land application to supply nutrients to soil, however, it can also unintentionally contaminate surface water and groundwater by surface runoff or leaching of dissolved reactive phosphorus and E. coli. Treatment with poly-ferric sulphate can reduce phosphorus and E. coli leaching by 93% and 98%, while maintaining plant biomass and phosphorus uptake (Che et al., 2022). The effect of soil management on water quality also depends on soil texture. For instance, the risk of pesticide transmission from soil to waterways was the most significant for soil with less than 20% of clay or more than 45% of sand (McGinley et al., 2022). Soil health is closely linked to human health. Soil is the ultimate source of nearly 95 per cent of food consumed by human beings (FAO, 2015). Food produced on unhealthy soil results in food of poor nutritional quality and jeopardizes human health (Gashu et al., 2021; Oliver & Gregory, 2015). Enhanced biodiversity can provide more fertile soil and more diverse food supply, as well as the cultivation of medicinal plant species (Muon et al., 2023). Moreover, soil pollution is increasingly ubiquitous due to industrial emission, as well as usage of fertilizers and pesticides (Jia & Hou, 2023; Sun et al., 2023). Both accidental ingestion of contaminated soil particles and consumption of contaminated foodstuff can cause detrimental health effects (Hou, 2021; Hou et al., 2020). Heavy metals such as cadmium in agricultural soil represents one of the biggest threats to human health (Arao et al., 2010; Qin et al., 2023), and crops have particularly high rates of cadmium uptake (Dong et al., 2022; Zhang, Tan, et al., 2022). Open burning of contaminated straw or its usage as livestock feed can further increase exposure risks via inhalation or ingestion of meat and milk. Therefore, it is essential to mitigate soil pollution and enhance soil health, to avoid the detrimental health effects associated with soil contaminants while increasing food product nutritional values (Jin et al., 2021). A variety of intervention measures have been proposed to enhance soil's ecosystem service in protecting human health. For example, foliar spray with nano-silicon and nano-selenium can effectively reduce Cd uptake by rice (Deng et al., 2021). Ecological restoration was used to reduce the mobility and health risk associated with cadmium and lead (Zhao et al., 2022). Biochar derived from biological waste can be used to reduce the bioavailability of soil heavy metals (Ayaz et al., 2022). The use of plastic in agriculture and accumulation of microplastics/nanoplastics may affect soil microbial community and plant health (Palansooriya et al., 2022; Wang et al., 2023; Zhou et al., 2023). Biochar was also found to be effective in mitigating soil pollution caused by microplastics (Dissanayake et al., 2022). Soil amendments such as biochar and compost have been widely used to improve soil health (Gao et al., 2022; Thakur et al., 2023; Zhang, Liu, et al., 2022). In recent years, biochar has attract attention because it offers a wide variety of soil health benefits (Abel et al., 2013), such as supplying slow-release nutrients, mitigating climate change by carbon sequestration and reducing soil contaminant leaching and bioavailability (Hou et al., 2023; Wang, Deng, et al., 2023). However, the effect of biochar on soil health depends on its feedstock and production conditions (Li et al., 2023). Therefore, to ensure sound soil health management, appropriate feedstock type, pyrolysis temperature and application rate must be selected to match with specific site soil characteristics. Soil amendment based on microorganisms and low-cost biological waste is of huge potential in soil health management. Microbial soil amendments, also named biofertilizers, offer a variety of environmental and socioeconomic benefits. Its market was valued at $1.57 billion in 2018 and its annual growth rate was expected to reach 12% for the next 5 years (O'Callaghan et al., 2022).Three major types of microorganisms offer nutritional benefits: nitrogen fixing rhizobia, arbuscular mycorrhizal fungi (AMF) and plant growth-promoting rhizobacteria (Bamdad et al., 2022). However, soil microbial inoculants can only be effective if they can be incorporated into existing farming practices and the intended microbial community be well established (O'Callaghan et al., 2022). Differences in soil texture, associated with eroded or depositional landscape positions, also play an important role in how soil amendments affect soil ecosystem service functions (Abagandura et al., 2022). For sandy soil with relatively low clay content and aggregate stability and capacity to store cationic nutrient and water, applying liming materials such as limestone represents a best management practice to maintain optimal soil pH, supplying calcium and magnesium, and improve soil structure for root development (Christensen et al., 2022). With careful assessment of soil characteristics and proper selection of soil amendments, soil health can be greatly improved and maintained.