Influence of long-term phosphorus fertilisation history on the availability and chemical nature of soil phosphorus

Data on the dynamics and fate of phosphorus (P) under low soil-test P (STP) conditions is essential for the development of cropping strategies with a high P use efficiency. This study examined the effects of long-term (20 years) mineral and organic fertiliser P applications on a P-depleted sandy loam soil in Denmark. The cycling of P was examined by use of P budgets (inputs-offtake), chemical P extractions, ³³P isotopic exchange kinetics (IEK), and solution ³¹P nuclear magnetic resonance (NMR) spectroscopy on NaOH-EDTA extracts. Recovery of applied P in the topsoil was smaller for animal slurry P compared with mineral fertiliser P. The budgets suggest deeper soil layers play an important role for the cycling of P. Resin-extractable P (2 to 17 mg kg⁻¹), Olsen-P (7 to 16 mg kg⁻¹) and E_1min (1 to 6 mg kg⁻¹) were correlated with the P budgets. Between 63 and 77% of total inorganic P was not exchangeable in a period of three months (E_>3months), with the lowest value observed in no-P treatment N₁K₁. The data show that a redistribution of exchangeable P had taken place under the influence of a strongly negative P budget. Microbial P (6 to 18 mg kg⁻¹) increased under animal slurry inputs compared with mineral fertiliser applications (p < 0.05). All soils were dominated by phosphomonoesters and orthophosphate (98 to 99% of the NMR signal). Concentrations of orthophosphate (86 to 135 mg kg⁻¹) varied significantly between treatments (p < 0.01), whereas forms of organic P remained largely unchanged. The results demonstrate that P applications increased the amount of P that is potentially available for plants, irrespective of input form. Nevertheless, most P applied in excess of crop uptake resulted in an increase of the amount of P that is slowly exchangeable. Under low soil test P conditions such as in the current trial, fertiliser P applied in excess of plant demand that accumulates in soil would thus only be partially available for crops in subsequent years. On the other hand the data suggests that soil P reserves may be utilised for crop growth, but at the low soil P intensity plant access to P will have to be managed carefully.