Field evaluation of liquid fraction of digestates from different feedstocks on soil properties, water productivity, maize yield and economic performance.

Abstract

The sustainable management of livestock and plant residues is a key challenge in modern agriculture, and the recycling of organic wastes through liquid fraction of digestates (LDs) represents a promising solution within a circular bioeconomy framework. As the reuse of organic waste in agriculture gains rapid importance within the scope of circular bioeconomy, field studies that simultaneously assess the hygienic safety, organic pollutants, and pesticide residues of biogas sludge alongside its effects on water use, yield, kernel quality, soil chemistry/enzymology, and economic performance remain limited. This study evaluated the effects of LDs obtained from three different feedstocks (digestate A: poultry+poppy plant residues; digestate B: cattle+plant residues; digestate C: poultry+cattle residues) applied at doses of 10, 30, 50 and 70 t ha⁻¹ (fresh matter basis) on yield and yield components, kernel quality, soil health, water productivity and economic performance on sustainable maize production under field conditions during the 2022-2023 growing season. The results showed that LDs significantly increased maize grain yield, thousand kernel weights, cob weight, and kernel yield per cob, whereas cob diameter, cob length, harvest index, crude protein, and plant height were not significantly affected. In the soil, LDs significantly increased electrical conductivity (EC), available P₂O₅, K₂O, Ca, Mg, Fe, Mn, and B, as well as cation exchange capacity, microbial biomass carbon, microbial respiration, urease activity, and phosphatase activity, thereby improving nutrient availability and soil biological functioning. Furthermore, kernel nutrient composition was improved, with significant increases in total N, P, K, Ca, Mn, Zn, and B contents, while total Mg, Fe, Cu, and Cr remained unaffected. Water productivity (WP) and irrigation water productivity (IWP) showed numerical variation among treatments, with the highest values observed under digestate C; however, these differences were not statistically significant. Although Salmonella spp., Listeria monocytogenes, and E. coli O157 were not detected, elevated Clostridium spp. counts indicated a potential microbiological risk, emphasizing the need for appropriate hygienization prior to agricultural use. Additionally, certain pesticide residues (e.g., permethrin) and high EC in digestate A highlight the importance of dose optimization and monitoring. Although no significant increases in exchangeable Na or potentially toxic metals were observed during the two-year field experiment, the long-term environmental fate of pesticide residues and trace metals remains uncertain, indicating that repeated LDs require continuous monitoring to ensure environmental and food safety. Overall, the findings demonstrate that LDs can be an effective organic amendment for sustainable maize production, enhancing crop productivity, soil health, and resource efficiency while supporting the transition to a circular bioeconomy. However, their safe agricultural utilization requires integrated management strategies, including optimized application rates, hygienization (e.g., thermal pasteurization), and continuous chemical and microbiological monitoring to minimize long-term environmental and public health risks.