Phosphorus fractions in bulk subsoil and its biopore systems

Abstract

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.

Root growth dynamics inside and outside of soil biopores as affected by crop sequence determined with the profile wall method

“…When wheat was grown on compact soil of deep loamy sand, overall root length was 66 % shorter than those in loosen soil, and shoot N and K contents were reduced by 12-14 % (Atwell 1990). On the other hand, a commercial wheat cultivar, Wyalkatchem, increased root length and root mass by 36 and 24 %, respectively, in response to deep-ripping of the compact layers of sandy soil resulting in 19 % increase in grain yield (Chen et al 2014).…”

Optimising Cropping Techniques for Nutrient and Environmental Management in Organic Agriculture

“…When compared with the bulk soil most crop growth improving soil chemical and soil microbial parameters of the drilosphere are higher (Kautz et al, 2013a). After taprooted lucerne and chicory crops the P CAL concentration in linings of biopores larger than 2 mm was up to 5 fold higher than the P CAL concentration in the bulk soil and higher than after fescue (Barej et al, 2014). Figure 6 shows higher soil N and C concentrations in biopores with earthworms when compared with the bulk soil concentrations.…”

Phosphorus—The Predicament of Organic Farming

“…From another German silty site total P-reserves in the topsoil (0-30 cm) were reported with 2600 kg ha −1 and in subsoil (30-105 cm) with 5600 kg ha −1 (Barej et al 2014). Long lasting fertilization trials in Great Britain (157 years, loam), Germany (98 years, loam) and Poland (77 years, sand) ended up with 480, 370 and 206 mg kg −1 total P in the topsoils without P fertilization and up to 1200, 942 and 580 mg kg −1 in fertilized plots.…”

“…Also bulk soil's available inorganic and organic P reserves are much greater (187 kg ha −1 in 45-75 cm depth) than in the soil biopores (4 kg ha P) on the silty soil. Generally the rhizosphere soil directly around the roots is specially enriched with organic P and C (Barej et al 2014). In any case enhancing root access is essential to require subsoil P (Kautz et al 2013).…”