Biowastes alone and combined with sulfur affect the phytoavailability of Cu and Zn to barnyard grass and sorghum in a fluvial alkaline soil under dry and wet conditions.

Management of degraded soils (i.e., metal contaminated soils, salt affected soils, and soils with low organic matter content) by applying biowastes (e.g., biosolids and compost) and inorganic soil amendments such as sulfur is of great agro-environmental concern. Because Cu and Zn chemical behaviour may be altered with these additions, we aimed at studying the impact of mono- and co-application of different rates (1.25% and 2.5%) of biosolids, compost, and sulfur on the mobilization of Cu and Zn and their uptake in a fluvial soil contains low and high metal concentrations and under two distinct moisture regimes (wet, where we grew barnyard grass; dry, with sorghum). We measured metal fractions and potential availability, along with soil pH, as well as plant yield and metal content in both plants, in an attempt to identify differences in metal behaviour. We found that organic matter (OM) (increased with biosolids and compost application) and soil pH (dramatically reduced with added sulfur) highly affected Cu and Zn mobility. Plant yield increased with increasing soil OM content and decreased with decreasing soil pH, particularly in the 2.5% sulfur treatment. However, Cu absorption was different in the two studied moisture regimes, as it was higher in the wet soil (Cu-DOC complexes, encouraged under wet conditions, may explain this), while it was lower in the dry soil. The biosolid-added Cu was significantly more bioavailable to sorghum plants than the spiked Cu. Co-application of sulfur and biosolids showed significantly higher sorghum uptake of Cu than application of sulfur to the spiked soil with Cu. Zinc uptake decreased in the high compost application rate (2.5%). This behaviour can be explained with the altered geochemical metal fractionation: added metals were distributed mainly in the oxides and organic fraction, but in the wet soil the percentage was higher compared to the dry, possibly due to metal-DOC associations. Also the residual fraction was lower in the wet, denoting higher metal mobility. We conclude that the observed differences between wet and dry soil concerning the metal geochemical behaviour, as were induced by added OM (with biosolids and compost) and reduced pH (with sulfur), are mainly responsible for the markedly different metal uptake patterns. These results may be an aid for effective phyto-management of alkaline fluvial soils with low and high content of Cu and Zn under paddy- and upland cultivation systems.