Evaluation of Liquid Digestates Derived from Different Biogas Feedstocks on Physicochemical Properties and Heavy Metal Contents of Soil and Wheat Production under Central Anatolian Conditions

This study was conducted to evaluate the effects of liquid digestates (LDs) derived from different biogas feedstocks on soil physical and chemical properties, heavy metal contents and wheat production under the semi-arid calcareous soil conditions of the Central Anatolian Region of Türkiye. The field experiment was established during the 2021–2022 wheat growing season using a randomized block design with an unfertilized control, optimum mineral fertilization (NPK), and LD applications at rates of 15, 30, 45, and 60 t ha⁻¹. Three independent field experiments with the same experimental design were established for each of the three liquid digestate types: poultry manure + poppy residues (PP), cattle manure + plant residues (CP), and a poultry manure + cattle manure mixture (PC). The chemical composition and agronomic performance of the digestates varied depending on feedstock origin. According to the obtained results, soil physical properties, including field capacity, permanent wilting point, bulk density, and saturated hydraulic conductivity, did not differ significantly among the treatments. LDs increased available P₂O₅ and K₂O contents without causing significant changes in soil pH, electrical conductivity (EC), CaCO₃, soil organic matter, available Ca and Mg, cation exchange capacity, and exchangeable cations. In PP digestate applications, available P₂O₅ increased from 52.9 to 81.7 kg ha⁻¹ and K₂O from 2477.6 to 2918.8 kg ha⁻¹. In CP digestate applications, available P₂O₅ and K₂O values ranged between 46.9–85.6 kg ha⁻¹ and 1923.0–2196.7 kg ha⁻¹, respectively. In PC digestate applications, available P₂O₅ increased from 38.8 to 89.5 kg ha⁻¹, while K₂O reached 2196.7 kg ha⁻¹. Only PP digestate significantly increased Fe and Mn contents. Heavy metal analyses revealed that Ni and Pb concentrations in the soil remained low and statistically non-significant, whereas Cd, Co, and Cr concentrations were below detection limits. Although the highest grain yields were obtained at 30 t ha⁻¹ for the PP and CP digestates and at 45 t ha⁻¹ for the PC digestate, the 15 t ha⁻¹ applications also resulted in substantial yield improvements and produced grain yields that were statistically comparable to those obtained with mineral NPK fertilization. Nevertheless, because the study was limited to short-term and single-season conditions, long-term multi-year studies are required to better evaluate the cumulative effects of repeated LD applications on soil quality, salinity dynamics, nutrient cycling, soil biology, environmental sustainability, and food safety. In addition, future studies should include pathogen, contamination, and pesticide analyses to provide a more comprehensive assessment of digestate use in agricultural systems.