Phosphorus behavior in unfertilized soils of the Semiarid Argentinean Pampas is not well understood. The objective of this study was to evaluate the effect of cultivation on qualitative and quantitative phosphate concentrations as well as on P sorption of these soils. For this purpose topsoil samples (10 cm) of 10 unfertilized plain soils with different textured parent materials (Ustipsamments, Haplustolls, and Hapludolls) from five sites with increasing mean annual rainfalls (550 to 650 mm) and temperatures (15 to 16°C) were studied. Undisturbed (under Prosopis caldenia forest) and cultivated paired pedons (under rotation of annual crops and pastures for more than 30 years) were compared at each site. Cultivation decreased the inorganic P fractions (Pi) 16 to 19% in one Haplustoll and the Hapludoll, decreased the organic P fraction (Po) 7 to 54% in both Ustipsamments, and decreased the Bray-Kurtz I extractable P (Pa) 62 to 73% in one Ustipsamment, a Haplustoll, and the Hapludoll. Only one Haplustoll, cultivated during long periods with alfalfa pastures, did not show changes in any of studied P fractions. This soil lay for long periods under cattle-grazed alfalfa pastures, under which conditions P losses by wind erosion were decreased, and P recycling caused by additions of cattle and plant residues to the soil, as well as P translocation from the subsoil to the topsoil by deep alfalfa roots, were increased. Total P losses from the four other cultivated soils were larger than calculated plant removal, indicating that other processes than plant uptake decreased P amounts. The Hapludoll and one Ustipsamment were cultivated with annual crops for longer periods than with alfalfa pastures. This produced wind erosion (thinner A-horizons, and losses of easily erodable textural fractions), which depleted Pi and Pa in the Hapludoll and Pa and Po in the Ustipsamment. A Haplustoll and an Ustipsamment, both cultivated for longer periods with alfalfa pastures than with annual crops, did not show wind erosion features but did show P decreases. The Haplustoll lost Pa and Pi, probably because of plant removal without P recycling to the soil by a highly extractive production system (seed and grass-parcel production). The Ustipsamment lost Po because of high mineralization of organic P by tillage. Larger Po losses that occurred in both Ustipsamments were attributed to the higher mineralization rates of organic P compounds, which are known to be high in these textural soil types. The C/Po below 200 of all studied soils indicated that organic P compounds can supply P to the plants by mineralization. This process will be more effective in cultivated than in undisturbed soils because of their lower C/Po quotients. Solubility diagrams showed that the phosphates predominant in all studied soils were fluorapatite (FA) and hydroxiapatite (HA). Cultivation did not change phosphate forms significantly, although when pH values were increased as a consequence of lime accumulation and organic matter (OM) losses, cultivation decreased P sorption capacity of the soils, which contributed to reducing available P to plants. Cultivation with annual crops using conventional tillage systems will increase losses of inorganic P reserves by wind erosion in Hapludolls and of organic P reserves by mineralization in Ustipsamments. Conservation tillage, which leaves large amounts of plant residues on the soil surface, can reduce these losses. Cattle grazing on alfalfa pastures seems to be the best system for reducing losses of available P in any soil type because it reduces wind erosion and increases P recycling to the soil.
1EEA INTA Anguil, Argentina. Dr. Buschiazzo is corresponding author.
2Facultad de Agronomía, UNLPam, Argentina.
Address: Facultad de Agronomía, Universidad Nacional de La Pampa, Cc 300, 6300 Santa Rosa, Argentina. E-mail: email@example.com
Received Nov. 18, 1999; accepted Feb. 3, 2000.