Limited impact of organic fertilizers on soil phosphorus accumulation in a long-term field experiment with excess fertilization

Understanding how fertilizer phosphorus (P) interacts with soils is necessary in order to use it efficiently. In the long-term field experiment CRUCIAL (Closing the Rural-Urban Nutrient Cycle − Investigations through Agronomic Long-term experiments), organic fertilizers were intentionally overapplied to test soil ecotoxicological limits. This has resulted in yearly P inputs of up to 463 kg·ha⁻¹ and positive P balances (input after crop offtake) for over 20 years. These inputs have occurred in sandy loam soils. Fertilization treatments included sewage sludge, compost, cattle manure, other organic fertilizers, and mineral NPK. We hypothesized that, given the high fertilizer P inputs, P accumulation in the topsoil (0–0.2 m), along with its distribution into sequentially extractable fractions, would vary depending on P fractions in the fertilizers. We measured P fractions and total P concentrations in both organic fertilizers and topsoils, and calculated two P budgets. The first budget compared topsoil total P concentrations from 2003 with those from 2022, considering the total fertilizer P inputs and crop offtake over that time. The second is a fractional P budget that uses P fractions in topsoils treated with NPK as a baseline to assess fertilization induced changes. Treatments with sludge, compost, or manure had positive P balances between 2152 and 2664 kg·ha⁻¹ over 20 years. Total P stocks in the topsoil increased by at least 869 kg·ha⁻¹ in treatments with sludge or compost compared to only 278 kg·ha⁻¹ with manure. Treatments supplying less resin extractable-P and more NaHCO₃-P or NaOH-P retained a higher proportion of the fertilizer P, even in topsoils already saturated with P. Consequently, specific organic fertilizers have caused increases in soil P fractions corresponding to their own P fraction compositions. Still, between 59–87 % of the excess P applied could not be accounted for in topsoils after considering crop P offtake. Consequently, regardless of the fertilizer type, overfertilization led to excess P movement out of the topsoil layer, which is detrimental for the environment.