Sewage sludge amendment for rice: a potential alternative to inorganic fertilizer

In a contest of circular economy, municipal solid waste can be adequately treated producing sewage sludge (SS). SSs are rich in organic and inorganic plant nutrients, and exert beneficial effects on soil properties. A greenhouse experiment was conducted on rice plants (Oryza sativa L. var. Sirio CL) to assess the effects of three different SSs: SS1, treated with CaO, SS2, treated with CaO and CO2, and SS3, treated with CaO and H2SO4. SSs were compared with unamended soil (control) and mineral fertilizer (urea, 4 g/pot). Physiological and biochemical analyses were carried out at tillering initiation, booting and panicle emergence. In the first stage, all SSs increased chlorophylls (Chls) content (+49%), and SS3 also in the following stages. At first sampling, antioxidant capacity was 2-, 3- and 2.5-fold higher than controls in SS1, SS2 and SS3, respectively. This result seems related to the increased ascorbic acid (2-fold higher than controls) in SS1 and SS3, whilst in SS2 other antioxidant molecules could be involved. At the end of the experiment, biomass production significantly increased in all SSs (+38%) than controls, while in SS2 and SS3 even compared to urea (+39%). Likewise, a positive correlation (r=0.95) was found between the total amount of nitrogen and biomass in plants grown on SS2 and SS3, which increased their total nitrogen content of 73% and +60% compared to controls and urea, respectively. Apparently, none of SS treatments induced any oxidative stress, as shown by the measurements of malondialdehyde by-products and photosynthetic efficiency (Fv/Fm). In summary, the observed increase of antioxidant activity could help plants to face out potential environmental stress during the transition from vegetative to reproductive stage. Moreover, the increased Chls content may have determined an enhanced photosynthetic activity, inducing a significant increase of total biomass at the end of the experiment.