Subsoil phosphorus is affected by fertilization regime in long-term agricultural experimental trials

Arable subsoils store large amounts of phosphorus (P); however, it is unclear to what extent, and under which conditions, subsoil resources might supplement crop P acquisition. Here, we hypothesized that (i) insufficient supply of P in topsoil promotes P acquisition from subsoil and (ii) subsoil P cycling is regulated by nitrogen (N) supply. We sampled two German long‐term fertilizer trials in Thyrow (sandy soil) and Gießen (loamy‐clayey soil) to 100‐cm depth. Treatments received either NPK, NK or PK fertilizer for > 60 years. We assessed soil inorganic (Pi) and organic (Po) P pools following Hedley sequential extraction, and the oxygen isotopic composition of HCl‐extractable phosphate (δ18OHCl‐P), which differentiates P from primary and secondary (previously biologically cycled) minerals. We found that in the Hedley sequential extraction subsoil resin‐P stocks (30–100 cm) in NK plots were 60% (Thyrow) and 8% (Gießen) less than those in NPK plots. Subsoil HCl Pi stocks in NK exceeded those of NPK plots by 70% in Thyrow, but not in Gießen. The NK treatments showed significantly smaller subsoil δ18OHCl‐P values than NPK treatments, indicating a predominance of primary (not biologically cycled) minerals and refuting our hypothesis that P deficiency promotes P acquisition from primary minerals. Under N‐limiting conditions, subsoil resin‐P stocks exceeded those under NPK fertilizer by 117% (Thyrow) and 22% (Gießen), supporting our second hypothesis. We conclude that an efficient use of subsoil P resources is achieved only when nutrient supply in arable topsoils is sufficient