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The application of organic wastes to soils, such as municipal solid waste compost (MSWC) or treated urban sewage sludge (USS) is a current practice for maintaining soil organic matter, reclaiming degraded soils and supplying plant nutrients. Since USS and MSWC may contain organic contaminants, heavy metals or pathogens, this practice represents a potential problem to the environment. In the case of heavy metals both bioavailability and toxicity seems to be critically dependent on the chemical form of the element. In this work, a pot experiment was carried out with different levels of MSWC and USS, as well as an inorganic solution of Cu and Zn. The aim was to obtain information about their bioavailability, mobility and toxicity and correlation among the different soil extracted fractions (F1--Exchangeable metal associated with carbonated phases, F2--Reducible metal or associated with Fe and Mn oxides, F3--Oxidizable metal bound to organic matter) with the Cu and Zn foliar (Cu(F) and Zn(F)) and pseudo totals contents (Cu(Ptotal) and Zn(Ptotal)), using the BCR sequential and aqua regia extraction procedure. Both of these methods were adequate to predict the Cu and Zn available to the plant, as high values on the Cu(F)-Cu(sigma123), Cu(F)-Cu(Ptotal), Zn(F)-Zn(sigma123) and Zn(F)-Zn(Ptotal) correlations were verified. Copper bounded mainly to F3, while Zn bounded to F1, F2 and F3 fractions and the regression analysis revelled that Cu and Zn ryegrass absorption were made mainly on F1 and F3 for Cu and F1 for Zn.

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The agroforestry systems with a high potential for C sequestration are those degraded by poor management strategies. Studies on changes in soil C status in these ecosystems mostly take into account labile C pools. Labile and stable soil organic matter (SOM) fractions are affected by soil management and land-use changes. Stable C pools are essential to understanding effects of land-use on soil C storage in the long term. The SOM stability is partly enhanced by the interaction of SOM with minerals and its inclusion into soil aggregates. Recalcitrant substances (e.g., lignin and chitin) also contribute to the passive SOM fraction. Macroaggregates mostly reflect the influence of plant roots and coarse intra-aggregate particulate SOM (POM), whereas microaggregates reflect the influence of fine interaggregate POM, clay concentration and humified SOM fraction. Often, POM is more sensitive to soil management changes than total SOM. Glomalin is a recalcitrant protein consisting of chitin produced by mycorrhizal fungi to protect hyphae. Glomalin has implications on C sequestration in agroforestry soils, but further research is needed before any prediction can be made. One challenge is reducing the CO2 emission from roots, and increasing the recalcitrant root C.

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