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.