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.