Growth, yield and solute content of barley in soils treated with sewage sludge under semiarid Mediterranean conditions

Agricultural soils from many parts of the Mediterranean region are subjected to progressive degradation. Application of sewage sludge provides not only a means for its disposal but also improves soil fertility and physical properties, causing an increase in crop yield. A field experiment was carried out from 1998 to 2001, to investigate the effects of sewage sludge application to barley (Hordeum vulgare L.) var. Sunrise crops on the relationships between plant physiology and some soil properties. Treatments were: (1) fertilization with a conventional inorganic fertiliser (M); (2) 15 t ha−1 of sludge in 1998 only (RS); (3) cumulative sewage sludge application, i.e., repeated applications of 15 t ha−1 every year (CS); and (4) unamended soil as control (C). Cumulative application of sewage sludge to barley crop increased grain yield significantly, which might be associated with improved early establishment of seedlings. The plants had higher dry matter yields and leaf protein concentrations from the beginning of development to ear emergence. These CS plots had lower pH, and increased total organic C (TOC), cation exchange capacity (CEC) and DTPA-extractable heavy metals. This treatment also improved soil microbiological properties, such as basal respiration, microbial biomass and some soil enzyme activities (urease, BAA-protease, phosphatase and β-glucosidase), which promote the recycling of nutrients for crop. Sewage sludge had a positive but short residual effect after only 1-year application. Results indicate that relatively low application rates of sewage sludge could be used for several years to maintain crop production in Mediterranean-type climates. However, there was a significant increase of grain heavy metal concentrations that must be taken into consideration under long-term applications of sludge.