Biopores provide nutrients from root debris and earthworm casts. Inside large biopores, root function is limited due to the lack of root–soil contact. However, the immediate surroundings of biopores may hold a key function as “hotspots” for root growth in the subsoil. To date, sufficient quantitative information on the distribution of roots and nutrients around biopores is missing. In this field study, the biopore sheath was sampled at distances of 0–2, 2–4, 4–8, and 8–12 mm from the surface of the pore wall. The results show a laterally decreasing gradient from the pore towards 8–12 mm distance in root length density (RLD) of spring barley (Hordeum vulgare L.) and faba bean (Vicia faba L.), as well as in total nitrogen (Nt)- and total carbon (Ct)-content. In the biopore sheath (2–12 mm), the share of roots with a diameter of less than 0.4 mm was 92% for barley and 89% for faba bean. The findings support the view that roots can utilize biopores to gain access to deeper soil layers and may use the sheath for nutrient uptake and entrance through to the bulk soil. However, especially for barley, the inner layer of the biopore sheath appeared to be more important for root growth than the sheath of farer distance.

Root growth through biopores is facilitated by low mechanical impedance and nutrient enrichment due to the deposition of organic material at the biopore sheath. Plant roots and earthworms impact biopore sheath properties differently. However, the literature lacks a quantitative study of the root distribution within the sheath of pores, which were originated by taproots or earthworms. According to previous literature on pore connectivity, it can be hypothesized that precrops encourage root growth into the biopore sheath in comparison to an earthworm characterized sheath. A pot experiment was performed to compare the root distribution of spring oilseed rape (Brassica napus L.) and faba bean (Vicia faba L.) within the biopore sheath of two different biopore types. The biopore sheath was characterized by taprooted chicory (Cichorium intybus L.) or anecic earthworm (Lumbricus terrestris L.). Roots were sampled at the biopore lumen and at lateral distances of 0–2, 2–4, 4–8 (sheath) and 20–36 mm (bulk soil) from the biopore wall surface. In both pore types >50% of the root length (cm) and >70% fine roots of oilseed rape were found in a comparatively small soil area (Lumen + 2 mm). On the contrary, faba bean grew primarily through the bulk soil with >75% root length and rarely into the biopore sheath in both pore types. In both species there was a lateral decrease of the total nitrogen (Nt)-content from biopore wall (Mean ± SE: 0.061% ± 0.002%) to bulk soil (0.053% ± 0.002%), but no significant difference between the pore types. The results of the current study illustrate that the root growth of spring oilseed rape and faba bean was not encouraged by the precrop in comparison to the earthworm characterized sheath.

Aims Biopores offer favorable chemical, biological and physical properties for root growth in untilled soil layers. There they are considered as nutrient “hotspots” with preferential root growth. However, the literature lacks a quantification of metabolic activity due to nutrient acquisition of main crops while growing in the biopore sheath. Methods A pot experiment was performed to map the metabolic activity of roots, as indicated by pH change. The roots of spring barley (Hordeum vulgare L.), spring oilseed rape (Brassica napus L.) and faba bean (Vicia faba L.) were growing through the biopore sheath influenced by an earthworm (Lumbricus terrestris L.) or a taproot (Cichorium intybus L.), in comparison to subsoil without a pore (bulk soil). pH sensitive planar optodes were applied in order to image a planar section of the sheath, while preserving an intact biopore sheath during the experiment. Results Roots were first found in the field of view in worm biopore then root biopore and bulk soil. At time of the first measurement the pH value was highest in worm biopore sheath (LS-Mean±SEM: 7.16a±0.11), followed by root biopore sheath (6.99ab±0.12) and bulk soil (6.61b±0.12). In spring oilseed rape a significant alkalization (+0.80 Δ pH) was found over time in bulk soil. Faba bean significantly acidified the root biopore sheath (-0.73 Δ pH). Spring barley showed no significant pH changes. Conclusions The results of the current study reveal a trend of faster root growth through biopores and a higher initial pH value in the biopore sheaths compared to the bulk soil. Biopores serve not only as an elongation path for roots, but their sheaths also provide an environment for root activity in the subsoil.