Changes in soil phosphorus (P) fractions and P bioavailability after 10 years of continuous P fertilization

Phosphorus fertilization modify soil P fractions differing in availability, but little is known about bioavailability of these pools for the crops when increasing P fertilizer rates in the medium term. Our objectives were to evaluate: i) changes in Bray-1 P content (extracted with 0,03 M NH4F and 0,1 M HCl) and soil P fractions as estimated by physical (Cambardella and Elliot method, POM-P) and chemical fractionation (Hedley method), ii) the relationship between these fractions and P bioavailability quantified in a test crop, and iii) the interaction with sulfur (S) fertilization after 10 years of continuous P and S fertilization. Soil samples were taken after 10 years in an experiment that followed a maize -full season-soybean -double-cropped wheat /soybean sequence that received P and S fertilization when cereals were sown. Treatments consisted of a factorial combination of three P rates (0, 20, and 40 kg P ha-1) and four S rates (0, 12, 24, and 36 kg S ha-1) applied to cereals from 2000 2010. After that, a maize used as a test crop was sown and P uptake was considered as bioavailable P. Soil samples were taken before sowing the test crop, and P in soil was fractionated chemically by the Hedley method and physically by wet sieving. After 10 years, when P fertilizer was not applied, soil Bray-1 P decreased 50 % respect to the initial P values, and increased up to 2.3 times when evaluating the highest P fertilizer rate. P fertilization did not change in POM-P. P soil labile (NaHCO3-Pi) and moderately labile (NaOH-Pi and HCl 1 M-Pi) inorganic fractions increased 83 %, 50 %, and 22 % in response to P addition, representing an increase of 4.2, 4.9, and 4.5 mg kg-1 of these fractions per 100 kg of applied P. Likewise, these fractions were related with P uptake in the maize test crop with 1.6, 1.5, and 1 kg of P uptake ha-1 in mg kg-1 increase in each of these fractions, respectively. Then, these fractions may be responsible for the legacy-P after 10 years of continuous fertilization in a Luvic Phaeozem of the Pampas region.

Automated summary

Phosphorus fertilization can alter the availability of phosphorus in soil, and it is important to understand the bioavailability of different phosphorus pools for crops. This study evaluated changes in various phosphorus fractions in soil and their relationship with bioavailability in a test crop, as well as the interaction with sulfur fertilization. The results showed that certain soil phosphorus fractions increased in response to phosphorus fertilization and were related to phosphorus uptake in the test crop, indicating their importance in sustaining phosphorus availability in the long term.