Organic Fertilizer Sources Distinctively Modulate Productivity, Quality, Mineral Composition, and Soil Enzyme Activity of Greenhouse Lettuce Grown in Degraded Soil

Intensive greenhouse vegetable production is often associated with a decline of crop productivity due to the increase of soil salinity and/or a reduction of biological fertility. The aim of the current work was to assess the effects of three organic fertilizers on morpho-physiological and agronomic traits of greenhouse lettuce as well as soil enzyme activity under poor soil quality conditions. The tested organic fertilizers (poultry manure, vinasse-based fertilizer, and insect’s frass fertilizer) were applied pre-planting at the same equivalent nitrogen (N) rate (90 kg N ha−1). Laboratory incubation assay results showed that vinasse-based fertilizer was the most suitable fertilizer in supplying the mineral N in the short term. All fertilizers increased shoot fresh and dry weight compared to unfertilized control with a more pronounced effect (+75%) with vinasse-based fertilizer and insect’s frass. Insect frass reduced by 27% the leaf nitrate concentration in comparison with the other treatments. The toxic heavy metal Pb was 46% lower in all organically fertilized lettuce leaves. Soil enzymatic activities of acid phosphatase, alkaline phosphatase, arylsulfatase (ArS), N-acetyl-β-D-glucosaminidase (NAGase), dehydrogenase, and total hydrolase (THA) were enhanced by poultry manure and insect’s frass in comparison with unfertilized control while vinasse-based fertilizer increased ArS, NAGase, and THA. Taken together, our data demonstrate that the application of organic fertilizers especially vinasse-based fertilizer and insect’s frass during intensive crop production is a suitable approach for mitigating the negative impact of soil salinity, enhancing soil biological fertility, and improving agronomic performance of greenhouse lettuce.