Fish waste as source of humic acids biostimulants : enhancing water use efficiency and rizosphere acidification in lettuce

Plant biostimulants are substances or microorganisms used in low quantities with the aim ofenhancing nutrition efficiency, abiotic stress tolerance and/or crop quality traits, regardless of its nutrient content. Humic acids (HA) are a natural category of biostimulants which result of the transformation of biomolecules as proteins, lignin, carbohydrates and aliphatic polymers from plant, animal and microbial residues. Diverse activities generate organic wastes which could be used for composting with the purpose of producing renewable HA. Aquaculture is the fastest-growing animal food production worldwide and is becoming the main source of aquatic animal food for human consumption (Ottinger et al., 2016). However, large quantities of fish waste are also obtained from beneficiation units, which may cause pollution if improperly disposed in the environment. Since most of the fish waste is formed of protein and fat (Ghaly et al., 2013) its recycling by composting may constitute an important source of HA. The aim of this work was to investigate whether HA from fish waste composting act as a growth biostimulant to lettuce or not. Fish waste from Arapaima gigas (pirarucu) and Pennisetum purpureum (crushed grass) was composted (3:1 ratio, w:w, dry base) for 120 days, and the following chemical characteristics were determined: total nitrogen content (TNC), applying the Kjeldahl method; pH and electrical conductivity (EC), after shaking the compost in 0.01 mol -1 CaCl2; total organic carbon (TOC), using the Walkley–Black procedure; and total P, by colorimetry at 660 nm. The concentration of K, Ca, Mg, Fe, Cu, Zn, Ni, Mo, Mn, Cu, Na, Cr and Cd was determined by inductively coupled plasma-mass spectrometry. Humic acids were extracted from the compost, using the procedure indicated by the International Humic Substance Society. The efficiency of the HA as biostimulant was tested in lettuce (Lactuca sativa L.), by applying solutions with 0, 10, 20, 40 and 80 mg L -1 of HA every 5 days, for 45 days. The plants were evaluated for wet and dry weight, root length and number of leaves. Following this initial work, another experiment was performed using only a control (without HA) and the observed most efficient dose (20 mg L -1 of HA). Leaf gas exchange parameters, such as the CO2 assimilation (A) and transpiration (E) rates and stomatal conductance (gs), were measured on the 45th day of the experiment, with a portable photosynthesis system. Gas exchange determinations were performed between 8am and 11am. The standard protocol to evaluate in vivo proton extrusion (H +ATPase) was applied using media containing plants treated with HA and not exposed to it, for 14 days (Zandonadi et al., 2016). The seedlings were then placed onto agar plates for additional 24 hours, for image acquisition and protons extrusion measurements. The low value of TOC associated with the high TNC resulted in a low C/N ratio (7.8). A C/N ratio below 20 is suggested as indicative of acceptable maturity of the final product and a ratio of 15 or even less is preferable. The EC activity was lower than that reported by Illera-Vives et al. (2013) reflecting low potential to produce phytotoxicity due high to salt concentration. The elements content in the fish waste compost was detected in the following order of abundance: P>Ca>N>Mg>K>Fe>Zn>Mn>Mo>Cu (Table 1). Higher values for P, Ca, Zn, Fe and Cu were detected.