Gas exchanges and root morphology of lettuce plants grown with insect-treated bio-compost

Composted organic material is usually applied onto agricultural fields for soil fertility restoration, but in a broader prospective it represents an environmentally friendly strategy for the ecosystem management and the integrated in situ/ex situ conservation practices. Indeed, compost enhances the natural revegetation of degraded soils, increases the carbon storage and the plant diversity in grassland ecosystems, and is used in peat-free growing media for the ex situ conservation of rare, endangered and medicinal plant species. As an objective of a regional project, a bio-compost was obtained by an insect-assisted composting of different agro-industrial biowastes using black soldier fly (Hermetia illucens L.) (BSF) larvae (Fig. 1). According to the Italian Legislation (D. Lgs 75/2010), this bio-compost meets the limit values of several chemical, physical and microbiological parameters so that it can be used in agriculture as an organic amendment. However, agronomic tests are also needed before its use as a plant-conditioner. In this respect, the morpho-physiological responses of Lactuca sativa L. plants to bio-compost derived from an insect-based bioconversion of dry olive cake (DOC) mixed with poultry manure (CM) and olive leaves (OL) (CM:DOC:OL, 45%:45%:10%, w/w) were evaluated. The lettuce plants were cultivated in pots filled with soil:sand ratio (50/50, v/v) and fertilized with different bio-compost rates (1, 2 and 3 kg m-2) (Fig. 2). The control plants were not treated with bio-compost fertilized. Biometric parameters were evaluated as plant performance. The gas exchanges (photosynthetic and transpiration rates, stomatal conductance and water use efficiency intrinsic) were measured by LICOR 6400XT. The root morphological parameters (length, surface area, root length of different diameter classes, root length ratio, root mass ratio, root fineness and root tissue density) were analyzed by WINRHIZO system. The results pointed out 1) an increase varying between 13.3-16.5% of plant fresh weight in all compost treatments, 2) an enhancement of the root length, surface area and fine root length at 2 kg m-2 of compost rate and 3) the improvement of the root length was determined by an increase of the biomass allocation, 4) these morphological performances were sustained by higher levels of photosynthetic rates although an increase of water losses by higher transpiration rates was observed.