4 DISCUSSION
The results demonstrated that soil pore structure in the root
detritusphere and in the whole soil volumes were affected by both soil
texture and plants. Coarser-textured soils had much higher image-based
porosity, yet fewer pores of biological origin than finer-textured
soils. The biopores of fine-textured soils were numerous and constituted
a significantly greater portion of the overall pore space, yet not
holding as much remaining POM as those in the coarse-textured soils.
Pore-size distributions in detritusphere as well as their spatial
distribution trends with distance from the decomposing roots also
markedly differed between the finer- and coarser-textured soils. Pores
in the immediate vicinity of POM were better connected in finer-textured
soils than in coarser-textured soils in both plant systems. While
switchgrass soil had more biopores than prairie, its detritusphere pores
consisted of relatively large size pores than those of the prairie,
especially in coarser-textured soils.
4.1 Influences of soil texture on detritusphere pores
The greatest porosities found at < 0.25 mm away from the POM
in soils of both plant systems indicated that the vicinity of the POM
was mostly air-filled (Fig. 3). This “POM gap” between soil particles
and root residues can be explained by incomplete filling of existing
pores by roots during their growth and decrease of roots’ volume due to
shrinking upon drying and/or their decomposition (De Gryze et al.,
2006). Consistent with this explanation, roots of Agave desertiwere found to shrunk by 34% in 24 days of natural drought in a
greenhouse study (North & Nobel, 1997), and transpiration shrank roots
of Lupinus albus (Koebernick et al. 2018). Decreases in POM
volume due to decomposition were both visually observed and quantified
using X-ray μCT images of intact soil samples (Juyal et al., 2021; Kim,
Guber, Rivers, & Kravchenko, 2020).
Our findings of inherent texture and mineralogy characteristics
influencing the contribution of biopores to the overall soil porosities
(Fig. 2C and Table 1) were consistent with expectations and previous
reports. In relatively sandy soils the biopores formed by roots were
partially or completely refilled by sand grains after root
decomposition, while in loamy soils the biopores that the roots left
behind still maintained their structure (Phalempin et al., 2022). Sand
grains have high volume-to-surface area ratios, and quartz on grain
surfaces often lacks negative charge (Bazzoffi, Mbagwu, & Chukwu, 1995;
Schrader & Zhang, 1997), resulting in low stability of particle
arrangements (Almajmaie et al., 2017). Thus, the subsidence and
displacement of the dispersed sand grains near decaying POM residues is
likely among the reasons for the lower contributions of biopores to
overall porosities in coarser-textured soils (Hancock and Lake City
sites) compared to that in finer-textured soils (Oregon, Lux Arbor, and
Escanaba sites) (Fig. 2C) and for the greater proportions of biopore
space occupied by POM (Fig. 2D). The lower pore connectivity near the
POM in coarser-textured than in finer-textured soils (Fig. 6) is another
outcome of low stability. Finer, i.e., lower sand and quartz contents,
soil particles are expected to facilitate maintenance of the structure
by pores around POM, as compared to that of pores in coarser-textured
soils.
The other two contributors to the observed differences in biopore
volumes and in POM presence within the biopores are the inherent
differences between coarser- and finer-textured soils in terms of (i)
root growth and (ii) root residue decomposition rates. The volume of
biopores and their occupation by roots might be overall lower in
coarser-textured soils due to poorer root growth conditions (Dodd &
Lauenroth, 1997; Sainju, Allen, Lenssen, & Ghimire, 2017). POM in soils
with high sand contents might decompose slower than that in the soils
with low sand contents due to lower microbial activity at organo-mineral
surfaces of sand grains (Haddix et al., 2020; Kaiser, Mueller,
Joergensen, Insam, & Heinemeyer, 1992; Kögel-Knabner et al., 2008).
Indeed, a negative correlation between sand contents and microbial
biomass C was found across our experimental sites in a parallel study
(Lee, Lucas, Guber, Li, & Kravchenko, 2023). Thus, in coarser-textured
soils, the size of POM residues might not be decreasing as quickly as in
the finer-textured soils, and the region around the POM may not be
completely empty yet (Fig. 3). However, if the differences in plant
growth and decomposition rates had indeed played a significant role in
generating the observed differences in the biopore occupation by POM
(Fig. 2D), we would expect to also detect the differences in terms of
POM occupation between the two plant systems. Soils of restored prairie
have developed higher SOM (Sanford, 2014; Sprunger & Robertson, 2018),
thus better plant growth conditions, and much more active and abundant
microbial communities (Lange et al., 2015), e.g., significantly higher
microbial biomass C (Lee et al., 2023), than those of the monoculture
switchgrass. Yet, there were no significant differences between the two
systems in terms of POM occupation of the biopores (Fig. 2D) as well as
the porosity in the detritusphere at least 1.0 mm away from POM (Table
S3), ruling out the importance of these contributors.
Thus, we conclude that the loss
of structure and collapsing of biopores in coarser-textured soils is the
main reason of the observed effects and is likely a wide-spread
phenomenon.
Larger proportion of 36-150 μm Ø pores in close proximity (<
0.25 mm distance) to POM in coarser-textured soils (Fig. 5) is
consistent with lower soil porosity at the same distance (Fig. 3). Sand
grains dominating coarser-textured soils can sporadically fill the POM
gaps (Phalempin et al., 2022; Schrader & Zhang, 1997), and the filling
by the grains may fragment the space of the gaps into finer pores.
Indeed, porosities within the <0.25 mm distance to POM were
negatively correlated with sand contents (Table S5). However,
coarser-textured soils had larger contribution of such pores in
intervals of > 0.25 mm compared to finer-textured soils,
showing positive correlations between sand contents and porosities of
the entire volume (Table S5). Typically, in such regions beyond the
root-influenced zone – areas where root-induced pores are negligible –
the porosity tends to increase with higher sand content (Ding, Zhao,
Feng, Peng, & Si, 2016; Fan et al., 2021; Nimmo, 2013). Indeed, gaps
between sand particles are likely to primarily consist of pores that
range between 50-200 μm Ø (Bantralexis, Markou, & Zografos, 2023).
Therefore, the contrasting contributions of finer pores by distances are
an indication of a localized effect (~ 0.25 mm) of roots
on the pore structure, beyond which the porosity was mostly controlled
by the soil texture.
4.2 Influences of vegetation on detritusphere pores
The overall influence of the studied plant systems, 5-6 years after
their establishment, on the pore characteristics of detritusphere was
much lower than that of the inherent soil characteristics, i.e., texture
and mineralogy. An important exception was the image-based porosity in
remote portions of detritusphere (> 1.0 mm): it tended to
be greater in the soils of restored prairie than in those of switchgrass
(Fig. 3 and Table S3). Switchgrass roots often reuse existing biopores
(Lucas, Santiago, Chen, Guber, & Kravchenko, 2023), and their thick
roots were likely responsible for soil compaction and low porosity at
>1 mm distances (Aravena, Berli, Ghezzehei, & Tyler, 2011;
Liu, Meng, Huang, Shi, & Wu, 2022). On the contrary, finer and heavily
branching roots of many plant species of restored prairie likely
promoted formation of finer pore networks throughout the entire
detritusphere stabilizing them via root exudates and rhizodeposits
(Hairiah, Widianto, Suprayogo, & Van Noordwijk, 2020; Smith,
Wynn-Thompson, Williams, & Seiler, 2021). We surmise that these very
fine roots rapidly decomposed after soil sampling and thus could not be
detected as POM in the current study.