Sewage sludge addition modifies soil microbial communities and plant performance depending on the sludge stabilization process

Despite the widespread use of sewage sludge as an organic amendment to improve soil stability and plant productivity, relatively little is known about how the different sludge stabilization processes affect the microbial composition and diversity of the sludge and the soil microbial populations as well as plant performance. In this study, the effects caused by addition of thermophilic aerobic (ATAD) and mesophilic anaerobic (MAD) sludge and inorganic fertilization on soil microbial community structure and diversity was assessed by pyrosequencing of 16S and 18S rRNA genes. Melon (Cucumis melo L., cv. Giotto) was used as model crop and its performance (growth and physiological state) was monitored together with changes in soil chemical parameters. Our results showed that the stabilization process of sewage sludge determined the feasibility of the final by-product as an organic amendment by altering in different manner the soil environment and modifying the soil microbial community structure and functioning. Changes in soil microbial community were related more to changes in the soil chemical environment rather than to the introduction of sludge-borne microorganisms. We also have shown that changes in a single physicochemical parameter (electrical conductivity) due to sludge application are associated with a pronounced shift in microbial community structure and activity as well as in plant performance. Along these lines, we showed that the application of ATAD sludge into soil resulted in less pronounced changes in its chemistry and microbial community structure, while enhancing soil microbial activity and plant performance. This study shows, therefore, that ATAD sludge could be applied as an excellent alternative to MAD sludge or inorganic fertilization.