Soil properties affecting metal extractability patterns in periurban calcareous agricultural soils in the mediterranean area

The potential risk of metal accumulation in periurban agricultural areas is a matter for concern. The climate characteristics and carbonate content of calcareous agricultural Mediterranean soils typical in these areas favour metal accumulation at the surface level; however there is also a risk of potential metal mobility. Our study focuses on the soil properties affecting metal mobility in these soils. Metal extractability patterns were assessed in soils after they were spiked with a mixture of Cd, Cu, Pb, and Zn and incubated up to 12 months, using one-step extraction methods [NaNO[3], a mixture of low molecular weight organic acids - LMWOA and diethylene triamine pentaacetic acid -DTPA-]. The concentration of extractable Cd was 50% lower in the soil with the highest carbonate content. LMWOA-extractable Cu was highest in soils with the lowest organic matter [OM] content at day 1 of the incubation experiment and in soils with the lowest carbonate and recalcitrant OM contents at 12 months. Fine mineral fractions determined the lowest DTPAextractable Cu. The highest Pb retention was in soils with the highest carbonate and clay contents. However, we were unable to establish any soil component affecting Pb extractability patterns. The Zn extractability pattern was related to particle-size distribution, which was highest in soils with a low proportion of fine mineral fractions. To summarise, carbonate, particle-size distribution and OM are relevant to potential metal mobility in these soils. The high DTPA-extractable metal values are evidence of a potential risk of metal mobility in the soils in the study

Sewage sludge application in Mediterranean agricultural soils: effects of dose on the soil carbon cycle

This work investigates the effect of the application rate and type of sludge throughout the soil carbon cycle in a semiarid Mediterranean agro-ecosystem. We study the two-year evolution of the various pools of soil organic and inorganic carbon and their influence on soil respiration. We applied three rates [40, 80 and 160 Mg/ha] of two types of sludge -aerobically and anaerobically digested sewage sludge- in a calcareous Mediterranean soil. The study area is located in the southeast of Madrid [Spain] and is characterised by a Mediterranean climate with a marked seasonal and daily contrast. We analysed soil organic carbon, CO[2] emissions, organic carbon fractions, soluble carbon, and inorganic carbon forms. Measurements were madeat three times over two years, and bimonthly for organic carbon and CO[2]. The results show that sludge type and rate of application exert a significant influence throughout the soil carbon cycle. Aerobic sludge has a greater effect over the short-term. Anaerobic sludge treatment appears to have less effect on the cycle at the beginning of the amendment, but is prolonged over time, as the differences with untreated soil persist even after two years. The application of organic amendments in calcareous Mediterranean soils also modifies the inorganic carbon pools and greatly increases the soil soluble hydrogen carbonates. All of these results are reflected in the rates of soil CO[2] emissions, with the highest values recorded in soils amended with aerobic sludge. Our data points to the advisability of a review of the European Union's recommendations regarding sludge and agriculture. We propose including a sludge stabilization process and recommended application ratesaccording to the effects on soil biogeochemical cycles