Seasonal toxicity variation in light-textured soil amended with urban sewage sludge: interaction effect on cadmium, nickel, and phytotoxicity.

Sewage sludge is increasingly used as an organic amendment to agricultural soils, especially to soils containing little organic matter. However, little is known on the impact of this biowaste on seasonal changes of nickel and cadmium toxicity in a sandy loam soil. Accordingly, the aim of this field-scale study was to evaluate the seasonal phytotoxicity according to Cd, Ni, and dehydrogenase variation in an agricultural soil during two successive annual amendments with increasing amounts of urban sludge (0, 40, 80, and 120 t ha-1 year-1). Sampling was carried out at the end of dry season (EDS) and at the end of wet season (EWS) during 2 years 2012/2013. Sludge application significantly increased the amount of organic matter and dehydrogenase activity in the soil. In order to explain the seasonal variation of Cd and Ni, pH and electrical conductivity were also monitored in this study. The increased rate of sewage sludge addition slightly reduced the pH but soil remained above neutrality. The electrical conductivity which reflects soil salinity was strongly correlated with Cd and Ni content that increased with sludge dose. Salinity and heavy metals were highest at EDS 2013. In addition, soil phytotoxicity testing was performed by the evaluation of lettuce seed germination for 120 h. Although heavy metal content did not generally exceed Tunisian thresholds (3 and 75 mg kg-1 for Cd and Ni, respectively), the seed germination index decreased with sewage sludge dose at all seasons. In general, we observed a significant effect of seasonal variation for all studied parameters. Sewage sludge reuse could be a feasible way to improve soil organic matter but toxicity risks consistently increased with time.

Comparative effect of salinity on growth, grain yield, water use efficiency, δ(13)C and δ(15)N of landraces and improved durum wheat varieties.

Supplemental irrigation with low-quality water will be paramount in Mediterranean agriculture in the future, where durum wheat is a major crop. Breeding for salinity tolerance may contribute towards improving resilience to irrigation with brackish water. However, identification of appropriate phenotyping traits remains a bottleneck in breeding. A set of 25 genotypes, including 19 landraces and 6 improved varieties most cultivated in Tunisia, were grown in the field and irrigated with brackish water (6, 13 and 18dSm(-1)). Improved genotypes exhibited higher grain yield (GY) and water use efficiency at the crop level (WUEyield or 'water productivity'), shorter days to flowering (DTF), lower N concentration (N) and carbon isotope composition (δ(13)C) in mature kernels and lower nitrogen isotope composition (δ(15)N) in the flag leaf compared with landraces. GY was negatively correlated with DTF and the δ(13)C and N of mature kernels and was positively correlated with the δ(15)N of the flag leaf. Moreover, δ(13)C of mature kernels was negatively correlated with WUEyield. The results highlight the importance of shorter phenology together with photosynthetic resilience to salt-induced water stress (lower δ(13)C) and nitrogen metabolism (higher N and δ(15)N) for assessing genotypic performance to salinity.