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 (P_i) and organic (P_o) P pools following Hedley sequential extraction, and the oxygen isotopic composition of HCl-extractable phosphate (~d¹⁸O_HCl-P), which differentiates P from primary and secondary (previously biologically cycled)minerals.We found that in theHedley 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 P_i stocks in NK exceeded those of NPK plots by 70% in Thyrow, but not in Gießen. The NK treatments showed significantly smaller subsoil ~d¹⁸O_HCl-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.