3 RESULTS
3.1 Soil texture and mineralogy in the studied sites
Sand content varied greatly among the studied sites, with Oregon site
having the lowest content at 10% and Lake City having the highest at
87% (Table 1). The silt content was the highest in Oregon at 73% and
the lowest in Lake City at 9%, and the site with the highest clay
content was also Oregon at 17%. In all five sites, quartz was the
dominant mineral (78-92%) with 6-16% of K-feldspars and P-feldspars,
and < 2% contents of other minerals. Hancock and Lake City
had higher contents of quartz and lower contents of K-feldspars and
P-feldspars compared to Oregon, Lux Arbor, and Escanaba sites. Mica
dominated the clay fraction of all studied soils (55-67%), while
smectite, kaolinite, and chlorite were present at < 20%, and
did not differ among the sites.
Quartz was positively correlated with sand content, while silt and clay
contents were positively correlated with feldspars, dolomites, and
siderites across all studied sites, which (Table S1). Interestingly,
contents of smectite and kaolinite were positively correlated with sand
contents, while mica was negatively correlated with sand but positively
with silt and clay (Table S1).
3.2 Bioporosity and POM located within biopores
The soils of Oregon site had the lowest total and image-based porosity,
measuring at around 21% and 10% in both plant systems, respectively
(Fig. 2A). In soils of Lux Arbor and Escanaba, both types of porosities
were higher than those of Oregon. The soils of Hancock and Lake City had
the greatest total and image-based porosity, while lower bioporosity
than the other three sites (Fig. 2A). There were no significant
differences observed in both types of porosities and bioporosities
between the two plant systems, with the exception of the bioporosity of
Lux Arbor site (Fig. 2A). Proportions of the image-based porosity
occupied by biopores were also the greatest in soils of Oregon and Lux
Arbor compared to those of Hancock and Lake City soils under both plant
systems (Fig. 2C). The significant difference in the proportions
occupied by biopores between the two plant systems was found only in Lux
Arbor site, where biopores constituted 19% and 24% of the image-based
porosities of prairie and switchgrass systems, respectively (Fig. 2C).
The proportion of pore space occupied by biopores was negatively
correlated with sand content and quartz, while positively correlated to
silt, clay and P-feldspar (Table S2).
Neither the two plant systems nor the five studied sites showed
significant differences in POM fractions (Fig. 2B). However, proportions
of biopores occupied by POM varied among the studied soils (Fig. 2D). In
the soils of Hancock and Lake City POM occupied >30% of
the bioporosity, while in Oregon and Lux Arbor it was <20%
(Fig. 2D). The bioporosity occupied by POM was positively correlated
with sand and quartz contents and negatively correlated with silt, clay,
K- and P-feldspar contents (Table S2).
3.3 Pore structure changes with the distance from POM
Image-based porosity decreased with the distance from the POM surface in
all soil cores (Fig. 3). In both systems, at < 0.25 mm
distance from POM the image-based porosity tended to be higher in
finer-textured soils of Oregon, Lux Arbor, and Escanaba than in the
coarser-textured soils of Hancock and Lake City (Fig. 3). These pores
are shown as examples in Fig. 4. However, the differences in the
porosity among the sites at < 0.25 mm distance faded at
0.25-0.5 mm interval, and upon distance reaching the 0.5-1.0 mm the
image-based porosity of Hancock and Lake City exceeded that of the
Oregon, Lux Arbor, and Escanaba. The image-based porosities at
< 0.25 mm distance were negatively correlated with sand
content and quartz, while total image-based porosities were positively
correlated with them (Table S4). The total porosities were negatively
correlated with silt, clay, and P-feldspar.
The vegetation also affected the imaged-based porosity. From 0.1-1.5 mm
distance interval in Hancock and 2.5-3.0 mm in Lake City, the
image-based porosity was greater in prairie than that in switchgrass
system, while the porosity in switchgrass was numerically greater at
<0.25 distance (Fig. 3 and Table S3).
The contribution of the three pore-size classes into the visible
porosity changed with the distance from POM. In the region at
< 0.25 mm away from the POM, pore group within 36-150 μm Ø
size range accounted for 4% of the total porosity in finer-textured
soils and 11% in coarser-textured soils (Fig. 5). Meanwhile, relatively
larger pores (> 300 μm Ø) contributed to 75% and 58% of
the porosity in finer- and coarser-textured soil of the same distance
region, respectively. The contributions of 36-150 μm pores in both
textured groups of soils increased with the distance from POM, while
that of the larger pores decreased. Notably, the increases in the
fractions of smaller pores were more drastic in coarser-textured soils
compared to that in finer-textured soils (Fig. 5). Pores in the coarser
soils were mostly represented by finer size pores (36-300 μm Ø) beyond
1.0 mm distance from POM, whereas the finer soils still had 35% and 11
% of the larger pores at 1.0 mm and at 4.0 mm distances from POM,
respectively. In coarser-textured soils, prairie system had greater
fraction of smaller size pores within interval regions from 0 to 3.0 mm
than that of switchgrass system (Fig. 5C and 5D), while the fractions of
such pores did not differ between two systems in finer-textured soils
(Fig. 5A and 5B).
In both plant systems, in the immediate vicinity of POM (<0.25
mm away from POM), pore connectivity was greater in finer-textured than
in coarser-textured soils. However, when examined across the entire soil
volumes, the connectivity was lower in finer-textured than in
coarser-textured soils (Fig. 6). The connectivity in the vicinity of POM
was negatively correlated with sand, quartz, and smectite contents,
while it was positively correlated with silt, clay, and P-feldspar
contents (Table S5). However, the total connectivity showed the opposite
trend, being positively correlated with sand, quartz, and smectite
contents.