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We studied the long-term effects (12 years) of municipal refuse compost addition on the total organic carbon (TOC), the amount and activity of the microbial biomass (soil microbial biomass C, B(C) and metabolic quotient qCO2) and heavy metal bioavaiability in soils as compared to manuring with mineral fertilizers (NPK) and farmyard manure (FYM). In addition, we studied the relationships between among the available fraction [Diethylenetriaminopentacetic acid (DTPA) extractable] of heavy metals and their total content, TOC and B(C). After 12 years of repeated treatments, the TOC and B(C) of control and mineral fertilized plots did not differ. Soils treated with FYM and composts showed a significant increase in TOC and B(C) in response to the increasing amounts of organic C added. Values of the B(C)/TOC ratio ranged from 1.4 to 2, without any significative differences among soil treatments. The qCO2 increased in the organic-amended soil and may have indicated microbial stress. The total amounts of metals in treated soils were lower than the levels permitted by the European Union in agricultural soils. DTPA-extractable metals increased in amended soils in response to organic C. A multiple regression analysis with stepwise selection of variables was carried out in order to discriminate between the influence exerted on DTPA-extractable metals by their total content. TOC and B(C). Results showed that each metal behaved quite differently, suggesting that different mechanisms might be involved in metal bioavailability

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The capacity of Fe-deficient cucumber plants to utilise water-extractable and pyrophosphate-extractable humic substances as a source of Fe was investigated. Plants were grown for 13 days in nutrient solution in the presence or absence of Fe and during the last 7 days water-extractable and pyrophosphate-extractable humic substances were added to the solution at a final concentration of 5 mu g organic C ml(-1). The water-extractable humic fraction did not significantly modify leaf area and dry matter accumulation, leaf total Fe or chlorophyll content of cucumber plants adequately supplied with Fe. In contrast, pyrophosphate-extractable humic substances caused a slight but significant decrease of all the leaf parameters considered, with the exception of the chlorophyll content. Root Fe content of Fe-sufficient plants was decreased by more than 50% in the presence of each humified fraction. Addition of each humic fraction to Fe-deficient plants led to a partial disappearance of leaf chlorosis symptoms with a significant increase in chlorophyll and leaf Fe content. Fe content of roots was also significantly increased in Fe-deficient plants by the addition of humic substances to the nutrient solution. These results show that Fe-deficient cucumber plants can utilise Fe contained in the two fractions of humified organic matter. However, by calculating the amount of total Fe accumulated per plant in the presence of water-extractable or pyrophosphate-extractable humic substances, it could be seen that Fe contained in the water-extractable humic fraction was almost totally used by Fe-deficient cucumber plants, while that present in the pyrophosphate-extractable fraction could only be partially absorbed. The results strongly support a role of humified organic matter in Fe nutrition of plants and are discussed in terms of a possible interaction between soil humic substances and the biochemical mechanisms involved in the plant response to Fe deficiency