1 INTRODUCTION
Pore structure, i.e., shapes, connectivity, size distributions of soil pores, defines many functions and processes of the soil (Lucas, 2022; Rabot, Wiesmeier, Schlüter, & Vogel, 2018). It regulates availability of O2, water, and nutrients to soil microorganisms and influences processing of soil organics (Bouckaert et al., 2013; Thomsen, Schjønning, Jensen, Kristensen, & Christensen, 1999). Pore connectivity is especially important for providing a suitable habitat for soil-dwelling organisms and enabling microorganisms to access soil organic matter (SOM) (Negassa et al., 2015; Rabbi et al., 2016). Pores of different size ranges have differential effects on the activity and abundance of microorganisms. Specifically, micro-environments associated with higher enzyme activities and greater microbial abundance are found in pores ranging from tens to hundreds μm Ø (Kravchenko et al., 2019; Strong et al., 2004).
Plant roots are a major driver of soil pore formation and a source of SOM (Bodner, Leitner, & Kaul, 2014; Sokol, Kuebbing, Karlsen-Ayala, & Bradford, 2019). After the root senesces, its residues remain in the soil as detritus, and a several millimeters thick region that surrounds these decaying residues is called the detritusphere (Gaillard, Chenu, Recous, & Richard, 1999; Védère, Vieublé Gonod, Pouteau, Girardin, & Chenu, 2020). Soil pore structure within the detritusphere is distinct from that of the bulk soil due to past activity of live roots as well as due to biological and physical changes after roots’ senescence. For example, rearrangement of soil particles or micro-aggregates during root growth (Mitchell & Soga, 2005) may lead to an increase in porosity adjacent to the root (Helliwell et al., 2017), while soil compaction can occur near growing roots (Lucas, Schlüter, Vogel, & Vetterlein, 2019a). However, upon root senescence pore spaces can be partially or completely refilled by soil particles during the decomposition of root residues (Phalempin et al., 2022). Since the detritusphere is a main arena of microbial activity and carbon (C) processing (Kuzyakov & Blagodatskaya, 2015), characteristics of pore structure within detritusphere likely play its special role for the whole soil volume.
Properties of the pore structure in detritusphere depend on a number of factors, including but not limited to: (i) inherent characteristics of soil particles that influence pore formation, such as soil texture and mineralogy; (ii) inherent pore characteristics, i.e., the pore structure within that specific location prior to the root growth within it; (iii) composition of the soil microbial community; (iv) morphological, chemical, and physical characteristics of the roots that generate the detritus.
The structural stability of the detritusphere pores are affected by sand content and abundance of quartz, both known to decrease stability of soil aggregation (Almajmaie, Hardie, Doyle, Birch, & Acuna, 2017; Rivera & Bonilla, 2020), likely due to the large size and low surface area of sand grains as well as the absence of negative charges (Bazzoffi, Mbagwu, & Chukwu, 1995; Six, Elliott, & Paustian, 2000). Moreover, soils dominated by quartz tend to be more easily dispersed than kaolinitic clays due to their lower binding capacity (Buhmann, Rapp, & Laker, 1996; Neaman, Singer, & Stahr, 1999), thus such soils are prone to be easily disaggregated under disruptive forces such as rainfall (Wakindiki & Ben-Hur, 2002).
Inherent soil characteristics affect root growth patterns and thus formation of root-derived pores. Root systems have been shown to grow more extensively in loose than in compact soil (Bengough et al., 2006; Croser, Bengough, & Pritchard, 1999), as well as in an undisturbed soil than in that homogenized by sieving and packing (Phalempin, Lippold, Vetterlein, & Schlüter, 2021b). The roots preferably utilize existing pore spaces, and indeed, the rhizosphere can be more porous than the bulk soil when roots are able to grow into a highly connected pore system (Lucas et al., 2019a). The established soil biopores that have been frequently and continuously used by roots are more likely to be stable due to root exudate and mucilage inputs (Traoré, Groleau-Renaud, Plantureux, Tubeileh, & Boeuf-Tremblay, 2000). Such pores can maintain their structure in detritusphere upon the root senescence and root residue decomposition.
Vegetation type directly affects pore structure via differences in root types and characteristics. For example, presence of coarse root systems increased the volume of > 70 μm Ø pores by 30%, whereas plant species with dense fine root systems generated larger volume of < 30 μm Ø pores (Bodner et al., 2014). Total volumes of soil biopores, i.e., the pores formed by the activity of living organisms such as roots, in Ø <0.2 mm and 0.2–0.5 mm size classes significantly differed among the plant species with different root system characteristics (Lucas, Nguyen, Guber, & Kravchenko, 2022). The differences in pore structure generated by plants with contrasting root systems are expected to be more pronounced in direct vicinity of the roots (Helliwell, Sturrock, Miller, Whalley, & Mooney, 2019), thus, carried later into the properties of the detritusphere. After plant dies, the root residues located in the biopores are decomposed, and the difference in the magnitude of decomposition is likely to be affected by the detritusphere’s pore structure. Variations in residue decomposition can result in variations in the size of the gap between the residues and soil particles, potentially leading to further alterations of the pore structure.
While the structure of pores within the rhizosphere under different soil texture and contrasting vegetation has been actively explored (Helliwell et al., 2017; Helliwell et al., 2019; Phalempin et al., 2022, 2021b), very little information is available on pore structure of detritusphere. For example, Helliwell et al. (2017) observed micro-scale structural changes in pores surrounding growing root systems in uniformly packed soils and found increases in porosity at the interface between roots and soil as roots grow into loamy sand and clay loam soils. However, it is still unclear what happens to the pores surrounding roots once the roots die and decomposition begins. As the detritusphere is one of the most important microbial hotspots (Kuzyakov & Blagodatskaya, 2015), the lack of such information in the pore structure limits progress in understanding mechanisms of soil C cycling and sequestration.
The objective of this research was to characterize the pore structure in root detritusphere of the soils of two contrasting vegetation systems: monoculture switchgrass, where root detritus originated from switchgrass roots, and polyculture restored prairie, where root residues originated from a variety of herbaceous plant species. The two systems have been in place for over 6 years, generating differences in soil C contents (Sanford, 2014; Sprunger & Robertson, 2018), pore structures (Juyal, Guber, Oerther, Quigley, & Kravchenko, 2021), and microbial characteristics (Jesus et al., 2016; Li et al., 2022). We compared pore connectivity and size distribution within the detritusphere of the two systems at five experimental sites representing three soil types with contrasting texture and mineralogy.