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.