Subsoils—a sink for excess fertilizer P but a minor contribution to P plant nutrition: evidence from long-term fertilization trials

Abstract

Background The phosphorus (P) stocks of arable subsoils not only influence crop production but also fertilizer P sequestration. However, the extent of this influence is largely unknown. This study aimed to (i) determine the extent of P sequestration with soil depth, (ii) analyze P speciation after long-term P fertilization, and (iii) compare soil P tests in predicting crop yields. We analyzed four long-term fertilizer trials in Germany to a depth of 90 cm. Treatments received either mineral or organic P, or a combination of both, for 16 to 113 years. We determined inorganic and organic P pools using sequential extraction, and P speciation using 31P nuclear magnetic resonance (NMR) and X-ray absorption near edge structure (XANES) spectroscopy. In addition, we applied three P soil tests, double-lactate (DL), calcium acetate lactate (CAL), and diffusive gradients in thin films (DGT). Results The results suggested that plants are capable of mobilizing P from deeper soil layers when there is a negative P budget of the topsoil. However, fertilization mostly only showed insignificant effects on P pools, which were most pronounced in the topsoil, with a 1.6- to 4.4-fold increase in labile inorganic P (Pi; resin-P, NaHCO3–Pi) after mineral fertilization and a 0- to 1.9-fold increase of organic P (Po; NaHCO3–Po, NaOH–Po) after organic P fertilization. The differences in Po and Pi speciation were mainly controlled by site-specific factors, e.g., soil properties or soil management practice rather than by fertilization. When modeling crop yield response using the Mitscherlich equation, we obtained the highest R2 (R2 = 0.61, P < 0.001) among the soil P tests when using topsoil PDGT. However, the fit became less pronounced when incorporating the subsoil. Conclusion We conclude that if the soil has a good P supply, the majority of P taken up by plants originates from the topsoil and that the DGT method is a mechanistic surrogate of P plant uptake. Thus, DGT is a basis for optimization of P fertilizer recommendation to add as much P fertilizer as required to sustain crop yields but as low as necessary to prevent harmful P leaching of excess fertilizer P.

The fate of phosphorus from bone char-based fertilizers in soil pools in a 5-year crop rotation

“…As indicated by the positive P budget of all fertilized treatments, P fertilization in both iSPTCs was above the plant requirements, which however was not reflected in the corresponding increase in the soil's P t stocks. Such underestimation of topsoils P stockswhen considering the individual P budgets (Table 2) -were also reported in other studies based on longterm P fertilization trials (Siebers et al, 2021;Zimmer et al, 2018). For example, Siebers et al (2021) reported that, despite positive P budgets, most of the P treatments of four different long-term P fertilization trials showed negative P balances for the topsoil (30 cm) in the range between − 17 to − 46%.…”

“…Such underestimation of topsoils P stockswhen considering the individual P budgets (Table 2) -were also reported in other studies based on longterm P fertilization trials (Siebers et al, 2021;Zimmer et al, 2018). For example, Siebers et al (2021) reported that, despite positive P budgets, most of the P treatments of four different long-term P fertilization trials showed negative P balances for the topsoil (30 cm) in the range between − 17 to − 46%. Siebers et al interpreted the fact that for some sites and treatments, the gradual inclusion of even deeper soil layers in the calculation improved the P balance as an indication of the site-and soil-specific leaching of excess fertilizer P from the topsoil to the subsoil.…”

Fertilizers and Fertilization Strategies Mitigating Soil Factors Constraining Efficiency of Nitrogen in Plant Production

“…Current studies document that these P resources are used by plants, provided that the P balance in the topsoil is negative. This conclusion is probably the result of using powerful extractants to determine the available P [ 63 ]. A study by Barłóg et al [ 64 ] clearly showed that extraction solution for N min determination can also be used to determine the resources of other nutrients.…”

Fate of P from organic and inorganic fertilizers assessed by complementary approaches

“…The double-lactate method (P-dl) is one of the German standard soil P tests. It uses double-lactate in an acid medium and is mainly applied for soils with a pH < 6 (Siebers et al 2021).…”

“…Practical P management is mostly guided by analyzing topsoil samples. However, although P contents commonly decrease with increasing depth the subsoil should be considered as relevant P reservoir (Siebers et al 2021). The vertical P translocation depends on agronomic management and a full understanding of P cycling processes is only possible if deeper soil depths are considered, too (Glaesner et al 2011;Rubaek et al 2013;van der Bom et al 2017).…”