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Taprooting crop species are capable of creating soil biopores (>2 mm in diameter) in the subsoil due to their large root size and deep-rooting habit. The aim of this study was to quantify root growth dynamics of wheat in the subsoil during its complete growth season as affected by crop sequence. Temporal observation on root length (km m −2 ) of wheat inside and outside of biopores at four growth stages (tillering, booting, anthesis, and milk) was conducted by using the profile wall method under the two crop sequence treatments involving two precrops, viz., chicory and tall fescue. Frequency of biopore presence measured on vertical profile walls depended on the choice of precrops in which chicory precrop resulted in higher frequency (2.3 %) compared with tall fescue (1.5 %). Root length of wheat measured inside biopores was significantly higher when grown after chicory (0.024 km m −2 ) in comparison to tall fescue (0.006 km m −2 ). On average, root length outside biopores after growing chicory was 45.9 % higher than tall fescue until the stage of anthesis. We conclude that at the site under study biopores as pathways for rapid root growth into deeper soil layers allow roots to re-enter and explore the subsoil. Thus, cereals cultivated in rotation with taprooted crops can draw benefit from enhanced uptake of water and nutrients from deeper soil layers during early growth stages. Model simulations with various abiotic and biotic factors will be helpful to reveal the direct evidence of biopore-root-shoot relationship in the future.

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Summary Improving phosphorus (P) accessibility in subsoils could be a key factor for sustainable crop management. This study aims to explain the quantity of different P fractions in subsoil and its biopore systems, and to test the hypothesis that crops with either fibrous (fescue) or tap‐root systems (lucerne and chicory) leave behind a characteristic P pattern in bulk subsoil, biopore linings and the rhizosphere. The crops were cultivated for up to 3 years in a randomized field experiment on a Haplic Luvisol developed from loess. Aqua regia‐extractable P (referred to as total P) and calcium acetate lactate‐extractable P (PCAL) were assessed at 0–30 (Ap horizon), 30–45 (E/B horizon), 45–75 and 75–105 cm subsoil depths. In addition, sequential P fractionation was performed on different soil compartments between 45 and 75 cm depths. The results showed that total P stocks below the Ap horizon (30–105 cm) amounted to 5.6 t ha−1, which was twice as large as in the Ap, although the Ap contained larger portions of PCAL. Both PCAL and sequential P extractions showed that biopore linings and the rhizosphere at the 45–75 cm depth were enriched, rather than depleted, in P. The content of inorganic P (81–90% of total P) increased in the following order: bulk soil = biopores <2 mm ≤ rhizosphere ≤ biopores >2 mm. Biopores >2 mm and rhizosphere soil were clearly enriched in resin‐ and NaHCO3‐extractable Pi and Po fractions. However, we failed to attribute these P distribution patterns to different crops, suggesting that major properties of biopore P originated from relict biopores, rather than being influenced by recent root systems. The stocks of the sum of these P fractions in the bulk subsoil (182 kg ha−1 at 45–75 cm depth) far exceeded those in the biopores (3.7 kg ha−1 in biopores >2 mm and 0.2 kg ha−1 in biopores <2 mm). Hence, these biopores may form attractive locations for root growth into the subsoil but are unlikely to sustain overall plant nutrition.