Introduction
Soil is an extremely complex and dynamic natural system that develops at the Earth’s surface. It supports plant and microbial communities, which are crucial for food production, carbon storage and ecosystem services. Understanding the physical, chemical and biological properties of soil is therefore important for sustainable soil management and conservation practices. This paper reviews recent literature on various aspects of soil research, focusing specifically on soil formation processes, soil texture and structure, soil organic matter, soil microbiology and impacts of land use change on soil health.
Soil Formation Processes
Soil formation, also known as pedogenesis, is the combined effect of five key factors acting over time – parent material, climate, relief, organisms and time (Jenny, 1994). Parent material refers to the initial unconsolidated earth or rock material from which soil forms through weathering and erosion processes influenced by climatic factors like temperature and rainfall. Relief determines characteristics like soil depth and drainage depending on topographical features. Organisms including plants, animals and microbes play a major role in soil structure development and nutrient cycling. Over long time periods ranging from decades to millions of years, the cumulative effects of these interactive factors determine the physical, chemical and biological properties of different soil types (Schlesinger, 1997; Sleutel et al., 2003).
Recent studies have focused on quantifying rates and mechanisms of soil formation. For example, Riebe et al. (2017) found that granitic regolith can develop into distinctly zoned soil profiles within just a few hundred years in some alpine environments, challenging conventional ideas that soils form very slowly. Others have investigated the pedogenic imprint left by glacier retreat and fluctuating sea levels during Quaternary glacial-interglacial cycles on presently observable soil variability (Egli et al., 2018; Iwahana et al., 2019). Advances in geochronology techniques like cosmogenic nuclide dating now facilitate rapid progress in time-constraining models of soil evolution (West et al., 2013; Bockheim and Hartemink, 2018).
Soil Texture and Structure
Soil texture refers to the relative proportions of sand, silt and clay sized mineral particles in a soil as determined by particle size analysis. It is an inherent, permanent property of soil that strongly influences other physical and chemical attributes. For example, clayey and silty soils retain more water than sandy soils due to smaller pore sizes (Brady and Weil, 2010).
Soil structure describes how primary soil particles group together into aggregates of various sizes and shapes. It arises due to chemical and biological processes that bind soil constituents into clusters called peds. Important structural properties include stability, density and porosity which govern water and air movement, root penetration and resistance to erosion. Factors controlling structure development include soil texture, organic matter levels, faunal activity and management practices (Blanco-Canqui and Lal, 2008).
Recent research has provided new understanding of texture-structure interactions and their role in ecosystem services. For example, soil structure strongly covaries with hydraulic and mechanical properties regulating water retention and infiltration rates which are critical for climate adaptation (Denef et al., 2020; Or et al., 2013). Advancements in X-ray computed tomography now allow high-resolution 3D visualization and quantitative analysis of pore space architecture within intact soil structures nondestructively (Mewes et al., 2017; Lehmann et al., 2020). Such techniques have yielded new insights into structure resilience, carbon sequestration potential and root-soil relationships.
Soil Organic Matter
Soil organic matter (SOM) refers to a complex mixture of decomposed and decomposing plant and animal residues in various stages of humification. It is a key indicator of soil fertility and quality affecting chemical, biological and physical properties. SOM plays a vital role in cation exchange capacity, water holding capacity, soil structure development and provides an energy source for soil microbes (Sikora and Halushchak, 2018).
Recent SOM studies have focused on dynamics, stabilization mechanisms and the role of SOM in climate regulation. For example, Kölbl and Kögel-Knabner (2010) examined composition changes during SOM transformation and demonstrated selective preservation of recalcitrant compounds. Other works investigated physical protection of organic compounds within soil aggregates and organo-mineral associations resistant to microbial decomposition (Marschner et al., 2008; von Lützow et al., 2008). Land use and management strongly impact SOM levels through changes in inputs, turnover rates and stabilization mechanisms (Venterea et al., 2005; Qin et al., 2016).
Isotopic techniques have provided insight into carbon cycling through soils (Heim and Schmidt, 2007). Several studies quantified soil carbon pools and stock changes attributable to land conversion, cultivation, residue management and soil carbon saturation points (Zimmerman et al., 2012; Maestre and Cortina, 2003; Stewart et al., 2007). As SOM levels are intimately linked to soil quality and fertility, research on building and maintaining soil organic carbon remains critical for sustainable agriculture worldwide.
Soil Microbiology
Soil microorganisms play central roles in decomposition, nutrient cycling and other ecosystem functions. They mediate major biogeochemical processes including carbon and nitrogen transformations that determine soil fertility. Our understanding of belowground microbial diversity and community dynamics is still limited compared to aboveground plant and animal kingdoms.
Advancements in molecular techniques now allow direct enumeration, identification and functional characterization of soil microorganisms without cultivation. For example, high-throughput DNA sequencing has revealed enormous taxonomic and genetic diversity across bacterial and fungal kingdoms (Roesch et al., 2007; Tedersoo et al., 2014). Culture-independent studies determined distributions of key functional groups involved in nitrogen fixation, nitrification, denitrification, methane oxidation and wood decay (Schüßler et al., 2001; Leininger et al., 2006; Kerou et al., 2016).
Continued high-resolution spatial and temporal monitoring of soil microbial communities is yielding new insights into their response to environmental changes. Studies determined impacts of disturbances like tillage, cropping patterns, metal pollution and climate perturbations on microbial community structure and ecosystem functions (Singh et al., 2010; Caruso et al., 2019; Allen et al., 2019). Understanding linkages between microbial biodiversity, activity and soil health remains critical for future sustainable land management.
Impacts of Land Use Change on Soil Health
Land use change strongly alters soil properties through mechanical disruption, inputs/outputs of organic matter, and shifts in plant-soil-microbial interactions. Conversion of native ecosystems to agricultural uses is a globally widespread driver with major consequences for soils. For example, deforestation or cultivation of grasslands results in losses of soil organic carbon and structural deterioration due to increased oxidation and erosion risks (Guo and Gifford, 2002; Voroney and Heck, 2015).
Recent meta-analyses synthesized impacts of specific land conversions on multiple soil properties. For example, Zhang et al. (2019) determined clear patterns of lower soil carbon, nitrogen and pH following conversion of forests/grasslands to croplands. Similarly, Zhao et al. (2020) found significant global declines in soil organic carbon and increased bulk density after deforestation and pasture abandonment. Spatially explicit models now assess agricultural soil degradation and carbon losses attributable to changing land use at regional to continental scales (Koch et al., 2013; Qin et al., 2015).
Soil quality indicators can also determine reversibility of negative impacts with rehabilitation. For example, reforestation was found to rebuild soil carbon and improve structural quality more rapidly than grassland restoration (Guo et al., 2006; Cai et al., 2016). Integrating such long-term ecological studies with socioeconomic factors is needed to develop sustainable land management policies that preserve soil health under future land use and climate change.
Conclusion
Soils are among the most complex and important environmental components regulating many ecosystem services. While our scientific understanding has advanced rapidly in recent years through new analytical techniques and long-term research networks, there remains scope for furthering our knowledge of crucial soil processes like organic matter dynamics, structure formation, nutrient cycling and impacts of perturbations. Continued multidisciplinary soil research addressing issues of agricultural sustainability, ecosystem resilience, greenhouse gas emissions and soil security is therefore essential to support environmental decision making.
Andrew Stroud