Abstract

CACCHIARELLI, Josefina; GALANTINI, Juan A.  and  ROSELL, Ramón A.. Structural stability and phosphorus in soil aggregate fractions in El Divisorio stream catchment area (Coronel Pringles, Buenos Aires province). Cienc. suelo [online]. 2008, vol.26, n.1, pp.71-79. ISSN 1850-2067.

In the last years, water quality in the Paso de las Piedras reservoir has been affected by eutrophication, leading to problems in the provision of potable water to the population of Bahía Blanca city and surrounding areas. Agricultural activity has been pointed out as the possible non-point source of P contaminating the El Divisorio stream, which flows directly into the reservoir. The objective of this work was to assess some selected edaphic properties associated with the contamination potential of soils such as structural stability, aggregate distribution and concentration of organic (Po) and inorganic (Pi) P in the aggregates. Sixteen locations were selected along the stream in farmers' fields. Soil samples from the surface 5 cm-layer were collected in the upper, intermediate and lower positions (L, M and B respectively) on each location. Structural stability was determined by the difference between dry and wet mean weight diameters. Soil Po and Pi were measured in each wet aggregate fraction. The values of structural stability were high; however, a great amount of fine material (<1 mm) was present, probably as a consequence of the destruction of less stable aggregates due to agricultural mechanic forces. A large concentration of P was present in this fraction, representing around 30% of total soil P. Pi concentration was similar among all fractions in L and M (200-240 mg P kg^-1) while it was significantly higher in B (360-400 mg P kg^-1). These differences may be attributed to finer texture and weaker stability in the B positions. Differences among landscape positions were less evident for Po, although greater concentrations were encountered in B in most aggregate fractions. According to these results, P losses could be of significant magnitudes even at low storm intensities, because the small soil particles that are carried in runoff water contribute with great amounts of P. Management practices tending to reduce the impact of agricultural P on water quality should be aimed to reduce the risk related to B positions due to their higher P content and lower structural stability.

Keywords : Eutrophication; P losses; Organic P; Inorganic P.

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