Phosphorus fractions in bulk subsoil and its biopore systems

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 ( P CAL ) 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 P CAL . Both P CAL 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 NaHCO 3 ‐extractable P i and P o 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.