Fate of phosphorus in soil during a long-term fertilization experiment in Finland

Accumulation and depletion of soil phosphorus (P) was studied in a long-term (37 y) field experiment in Southern Finland. The loam soil had a high pH (7.5-7.7) due to an earlier liming. Spring barley, spring wheat, oat, and ryegrass, grown in rotation, were annually fertilized with 0, 32, or67 kg P ha(-1) y(-1) (P-0, P-1, and P2K) and sufficient N. The average dry matter grain yield2,600kg ha(-1) of the P-0 plots increased by about 500 kg ha(-1) at P-1 treatment and another 600 kg ha(-1) by P2K. Soil samples were collected in 1978 (beginning), 1995, 2005, and 2015. According to the Chang and Jackson sequential extraction, the P2K and P-1 treatments increased the inorganic soil P by 732 and 32 kg P ha (-1) in 37 years, respectively, while the P-0 plots were depleted by -459 kg P ha (-1). The P2K treatment increased all four P fractions, extracted with NH4Cl (easily soluble), NH4F (Al-P), NaOH (Fe-P), and H2SO4 (Ca-P). Continuous depletion (P-0) decreased the NH4Cl-P and NH4F-P pools, NaOH-P and H2SO4-P pools remaining stable. None of the P pools changed significantly at P-1. The remarkable gap between the measured change and the balance for the P2K and P-1 treatments cannot be explained solely by lateral soil movement, meaning that a significant proportion of the applied P was lost either in surface runoff or transported below the investigated depth of 40 cm. Despite large P applications, the degree of P saturation reached only 20% in the P2K topsoil, assuming a 50% reactivity of Fe and Al oxides. As derived from sorption isotherms, a high EPC0 (i.e., equilibrium P concentration at zero net P sorption or desorption) of 1.30 mg L-1 had been built up in the P2K treatment, while in the P-1 treatment EPC0 (0.33 mg L-1) had remained unchanged and P depletion (P-0) had caused a decrease to 0.12 mg L-1. These results demonstrate that P sorption and desorption properties respond strongly to both P fertilization and null fertilization treatments and that in a long-term field experiment only a low proportion of the residual fertilizer P can be recovered from soil.