Aluminium fractionation and speciation in bulk and rhizosphere of a grass soil amended with mussel shells or lime

Acid soils have a high proportion of Al in the exchange complex and are usually amended with lime to reduce the toxic effect of the free Al (Al 3+). To this purpose, in some places like Galicia (NW Spain), there is a high production of wastes such as mussel shells that may substitute the lime. The aim of this work was to study the effect of different amending products derived from mussel shells on the Al chemistry in both bulk and rhizosphere, and on pasture production. The experiments were conducted on plots of an acid soil cultivated with grasses (Dactylis glomerata L., Trifolium repens L. and Lolium perenne L.). Each plot received its own treatment consisting of 1) commercial lime, 2) mussel shells dried and ground till 0-2mm (MDF), 3) mussel shells dried and ground till 2-4mm (MDC), 4) mussel shells heated (550°) and ground till <63μm (MHF), or 5) mussel shells heated (550°C) and ground till 0-2mm (MHC); one plot acted as control and was not treated. Each plot was replicated four times. The soil samples were collected after 16months and submitted to a general characterization. The Al was extracted from the soil by different solutions: ammonium oxalate (Alo=non-crystalline Al), sodium pyrophosphate (Alp=organically bound Al), copper chloride (Alcu=organo-Al complexes of low to medium stability) and ammonium chloride (exchangeable Al); the difference Alp-cu estimated the organo-Al complexes of high stability. From the soil water extracts, the Al was fractionated into acid-soluble (Al-polymers, colloids and organic complexes), non-labile Al (organic monomers) and labile Al (inorganic monomers, namely the species Al 3+, Al-OH, Al-F and Al-SO 4). For each plot, the production of the total dry matter and of that produced by the sown species, were also determined. With respect to the control, all the amending treatments increased the pH in the rhizosphere (5.11 in the control plots, 5.48-5.89 in the amended plots), which showed pH values higher than the bulk (5.13-5.60 in the amended bulk soil; 5.48-5.89 in the amended rhizosphere). Also the contents of C and N were higher in the rhizosphere than in the bulk, without difference among treatments. Concerning with Al chemistry, all the mussel shells derived products had effects similar to those of the commercial lime, with no difference among them. With respect to the control, in the soil solid phase all the treatments induced an increase in the formation of hydroxyl-aluminium complexes in the bulk (Alo: 3500mgkg -1 in the control, 3700-5500mgkg -1 in the amended plots) and organo-Al complexes of high stability in the rhizosphere (Alp-cu: 3563mgkg -1 in the control, 3550-5000mgkg -1 in the amended plots). The exchangeable Al diminished with all treatments, more in the rhizosphere (1.36cmol(+) kg -1 in the control, 0.24-0.47cmol(+) kg -1 in the amended plots) than in the bulk (2.83cmol(+) kg -1 in the control, 0.46-1.23cmol(+) kg -1 in the amended plots). Concerning the water extracts, the treatments induced an increase of organic complexes (polymers and monomers), more pronounced in the rhizosphere (non-labile Al: 0.17μmolL -1 in the control, 10.7-35.7μmolL -1 in the amended plots) than in the bulk (non-labile Al: 0.25μmolL -1 in the control, 0.25-3.57μmolL -1 in the amended plots). Further, in the treated plots the labile Al was dominated by Al-OH complexes, which are considered toxic but a lower level than the Al 3+ and abounded in the control. The production of dry matter due to the sown species was about double in the plots treated with CL and MHF with respect to the control. We conclude that the products derived from mussel shells can substitute the commercial lime in the amending of acid soils, so increasing the value of the shells and limiting the environmental problems derived by their disposal.