Nitrogen Fertilization Effects on Grassland Soil Acidification: Consequences on Diffusive Phosphorus Ions

Soil acidification due to N supplied from fertilizer or aerial deposition in grasslands is well recognized and documented; however, little is known about its consequences on soil P availability. This question was addressed on a long‐term field experiment done on grassland in Ercé (Central Pyrenees, France), where plots were treated for 6 yr with a factorial combination of mineral N added as NH 4 NO 3 (0 and 190 kg ha −1 yr −1 ) and P (0 and 50 kg ha −1 yr −1 ) and managed with a frequent defoliation regime (3 cuts yr −1 ). Diffusive PO 4 ion dynamics at the soil–solution interface was determined by coupling sorption studies with 32 P labeling of PO 4 ions (Pi) and dilution analysis. Results showed that the soil pH did not change in zero‐N treatments (5.9 ± 0.1) in comparison with the initial soil (5.9 ± 0.1), whereas it decreased to 5.3 ± 0.1 on N‐treated soils. Meanwhile the cation exchange capacity and exchangeable Ca 2+ and Mg 2+ contents decreased significantly and the saturation percentage of the adsorption complex by Al 3+ increased from 2 to 15; no changes were observed for untreated soils. Furthermore, the gross amount of diffusive Pi (Pr) was accurately described as a function of the concentration of PO 4 ions in the solution (Cp) and time ( t ) by a Freundlich kinetic equation; these dynamics were significantly modified by N supply but not by P.