Juan Manuel Martín-García

Carbonate and Phosphate Precipitation by Saline Soil Bacteria in a Monitored Culture Medium

Carbonate and phosphate precipitation by bacteria isolated from a saline soil was studied in vitro in a liquid culture medium over 45 days. Physicochemical parameters of this medium were continuously monitored using both selective electrodes (continuous monitoring, CM) and individual measurements by other techniques on days 5, 10, 15, 20, 25, 35 and 45 (discontinuous monitoring, DM). In DM, the precipitated minerals were studied (XRD and SEM-EDX) and the saturation index of the mineral phases was analyzed (PHREEQC program). Using the CM and DM data it was possible to distinguish several temporary stages in which both the medium and the mineralogy changed: 1) 0 to 10 days: pH reaches 8.4; significant loss of Mg2+ (incorporated into the bacterial biomass) and Ca2+ (through mineral precipitation); formation of crystals, although not in sufficient quantity to be studied until day 10. 2) 10 to 25 days: pH decreases but remains above 8; appreciable loss of Mg2+ and Ca2+ due to formation of spherical carbonate bioliths with traces of phosphates occluded within these carbonates. 3) After 25 days: biomineralization slow down; pH returns to initial values and struvite is formed (idiomorphic prismatic crystals). These trends are in agreement with the findings of other workers, although with some peculiarities regarding stages and types of mineral precipitated. In some cases the struvite contained small quantities of K and Ca, possibly because these are intermediate mineral species between typic-struvite, K-struvite and Ca-struvite. The bacteria-mediated precipitation of carbonates of Ca and/or Mg and phosphates (struvite) by the bacteria from a saline soil is demonstrated. However, struvite was not found in the soils of origin of the bacteria, possibly because it is a metastable mineral in most soils.

Impact of spent coffee grounds as organic amendment on soil fertility and lettuce growth in two Mediterranean agricultural soils

ABSTRACT The use of spent coffee grounds (SCG) as organic amendment is a triple solution: a reuse of this bio-residue (6 millions of tons per year), an increase in soil organic carbon (SCG contain 82% of carbohydrates and 13% of proteins) and a decrease in CO2 emissions into the atmosphere. Thus, we investigate the effect of SCG on soil and plants in an in vitro assay. The variables considered were SCG dose (2.5 and 10%), two agricultural soils (Calcisol and Luvisol), and four cultivation times (15, 30, 45 and 60 days). The chemical and physicochemical soil properties, SEM images and growth parameters were analyzed. The highest dose of SCG increased organic carbon, total N and available K and P by 286%, 188%, 45%, and 9%, respectively, while decreasing lettuce growth by 233% compared to control. The SEM study revealed that SCG particles are incorporated into soil aggregates and microorganisms grow over them, which is also corroborated by a 10-times increase of soil respiration rate. The behavior of both soils differs for some SCG effects. In conclusion, SCG have a great potential to increase soil fertility and further investigations are needed in order to improve the use of SCG as amendment.