Long-term effect of tillage on phosphorus forms and sorption in a Vertisol of southern Spain

The supply of soil phosphorus (P) to plants relies on the replenishment of the pool of phytoavailable P forms. In this work, we assessed the effect of 18 years of direct drilling (DD) and conventional tillage (CT) on P forms and sorption properties in the surface horizon of a calcareous Vertisol (Chromic Haploxerert) of southern Spain. P forms in the 0–6 cm top layer of the soil were quantified by sequential extraction with NaOH, citrate/bicarbonate (CB), citrate/bicarbonate/dithionite (CBD), and HCl. HCl-extractable P (viz. P in poorly soluble Ca phosphates) was found to account for more than 80% of the total extractable inorganic P. The concentration of inorganic NaOH-P was not significantly (P < 0.05) affected by the tillage treatment; by contrast, those of organic NaOH-P and inorganic CB-P, CBD-P, and HCl-P were significantly greater in the DD soil than in the CT soil. The concentration of bicarbonate-extractable P (Olsen P) was significantly greater in the DD soil than in the CT soil, with a mean value of 24 and 12 mg kg−1, respectively. The differences in the concentrations of Olsen P and other P forms between the two soils could be ascribed to the excess fertilizer P over plant P uptake being distributed over a greater depth under CT (26 cm) than under DD (6 cm). With both treatments, about one tenth of the excess was seemingly converted into Olsen P. The P sorption curve for the CT soil lay above, but exhibited the same shape as, that for the DD soil; this suggests that the intrinsic P sorption properties of the soil were not affected by organic compounds—which accumulated to a greater extent under DD than under CT. The main advantage of DD over CT is seemingly the accumulation of a phytoavailable P-rich thin surface layer capable of meeting plant P requirements at the early growth stages. The extent to which mineralization of the relatively abundant organic P forms in the DD soil can supply P to plants is, however, difficult to estimate.