In the context of a circular economy, special attention should be paid to the rational management of biodegradable waste. Currently, a potentially valuable waste material, rich in ingredients available to plants, is digestate, obtained as a by-product in agricultural biogas plants. The presented study aimed to determine the impact of digestate (DIG) from an agricultural biogas plant on soil reaction (pH), electrical conductivity (EC), sorption properties (SBC, HAC, CEC, BS), and chemical composition of soil. The research was based on a pot experiment in which increasing doses of liquid (LD) and solid (SD) forms of DIG were used in corn cultivation, balanced in terms of the amount of N introduced into the soil. The composition of DIG varied and depended on the LD or SD form. The LD was characterized by a lower pH value and higher EC compared to the SD form. The LD contained much less TC, fewer macroelements, and fewer trace elements. The application of LD significantly increased in the soil the content of TC, N_tot, available K, P, Fe, and Mn, and exchangeable cations K⁺. The SD significantly increased the content of available P, Mg, and Mn and exchangeable cations Ca²⁺ and Mg²⁺ in the soil. Both forms of digestate increased the total content of heavy metals (Cu, Cr, Pb, and Ni) in the soil. However, they did not pose a threat to the environment concerning their legally permissible levels.

Nowadays, we have observed the dynamic development of bio-heating plants that use wood biomass for heating or energy purposes. The result of this process is a reduction in carbon dioxide emissions as well as in the production of biomass ash (BA). Despite the waste nature of BA, it should be carefully analyzed and assessed for various applications, including environmental ones. Due to the features attributed to BA, including its alkaline reaction, the high capacity of its sorption complex, relatively low salinity, and significant content of macro- and microelements, a hypothesis was put forward in this work undertaken about the positive role of BA as an immobilizing factor for Cd-, Pb-, and Zn-contaminated soils. This research was based on a pot experiment in which four series were considered: (1) BA; (2) BA + Cd; (3) BA + Pb; and (4) BA + Zn. BA was used at doses of 30, 60, and 90 mg pot⁻¹, and metals at doses of 2 mg Cd, 100 mg Pb, and 300 mg Zn kg⁻¹ of soil. The test plant was corn grown for green mass. The study took into account the influence of BA on the content of the total forms of heavy metals (Me_tot) and their available forms (Me_av). In the soil without the addition of metals, a significant increase in the content of Cd_tot and Cd_av, and a decrease in the content of Zn_tot were observed due to the application of BA. The addition of metals against the background of the BA used resulted in a significant increase in Cd_tot, Pb_tot, and Zn_tot, as well as an increase in the available forms of Pb_av but a decrease in Zn_av. However, there was no significant increase in the Cd_av content. The obtained results may indicate the potentially immobilizing role of BA only in the case of zinc. They may constitute the basis for further, more detailed research aimed at determining the role of BA in the immobilization of various metals in soil.

This research concerned the possibilities of the natural management of ash, which is a waste product obtained in the process of burning wood chips in a bio-heating plant. The basis of the research was a pot experiment, which was carried out in a greenhouse of the University of Warmia and Mazury in Olsztyn, Poland. This experiment dealt with the influence of increasing doses of wood biomass ash added to soil on selected soil properties. The soil used for the pot experiment was taken from the arable layer (0–25 cm) of the soil. It was characterized by acidic reaction, low salinity, and an average content of total carbon (TC). The test plant was corn. Soil analysis after plant harvest showed an increase in pH and a significant improvement of soil sorption properties, without causing an excessive increase in soil salinity. In addition, a significant increase in the content of available K, P, and Mg was observed, which at the highest dose of WBA reached: 121.9; 109.3, and 41.33 mg kg⁻¹ of soil, respectively. The content of trace metals: iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), cadmium (Cd), lead (Pb), cobalt (Co), chrome (Cr), and nickel (Ni) in the soil was quite varied but did not exceed the permissible values for agricultural soils. The content of available forms of these trace metals at the highest dose of WBA was, respectively, 1004, 129.9, 8.70, 2.08, 5.54, 0.195, 1.47, 0.97, and 1.92 kg⁻¹ of soil. The results confirmed the significant fertilizing potential of wood biomass ash.

Combustion of woody biomass in professional bioheating plants to generate heat and reduce the dust emissions from this process results in the formation of a huge mass of woody biomass ash (WBA). Due to WBA’s rich chemical composition and the assumptions of the circular economy, this mineral material should be used for environmental purposes to recover valuable macro- and micronutrients. The basis of the research was a pot experiment designed to assess the effect of six doses of WBA (15, 30, 45, 60, 75, and 90 g pot⁻¹) on the growth, development, yield, and chemical composition of corn. Each pot contained 9 kg of soil. Observations show that the use of increasing doses of WBA had a positive effect on the height of corn plants, increasing its yield by 7 to 10% but reducing the dry matter content by 0.47 to 1.37% and the leaf greenness index (SPAD). Moreover, WBA application (T1–T5 treatments) had a positive effect on the content of macroelements (N, K, Mg, Ca, and Na) in corn biomass. A significant increase in the content of K (54%), Mg (38%), Ca (43%), and Na (19%) was observed. However, at the same time, a significant increase in the content of heavy metals—Ni, Cd, and Pb—was observed. Different results were obtained for P, Zn, Cu, Cr, and Co, whose content in corn decreased after WBA application to soil. The obtained results indicate the possibility of using WBA in an environmentally friendly way. However, due to the great diversity of this material in terms of the content of undesirable heavy metals, it is necessary to optimize its dosage and monitor its chemical composition. Considering the growing number of bioheating plants in our country in recent years and the resulting increase in the amount of WBA produced, it is necessary to develop a rational and environmentally friendly method for managing them in the future. The results of our research may provide partial indications of such solutions.

In recent years, there has been growing interest in using wood biomass for energy production, which has led to an increase in post-processing waste in the form of wood biomass ash (WBA). Due to the rich composition of WBA, its fertilising potential should be considered. In the conducted studies, WBA was used both alone and in combination with digestate (DG). The WBA was obtained from the Municipal Heat Energy Company and the DG from the Agricultural Biogas Plant in the form of unseparated liquid digestate (ULD), separated solid digestate (SSD) and separated liquid digestate (SLD). The studies included four series: (1) WBA, (2) WBA + ULD, (3) WBA + SSD and (4) WBA + SLD. In each series, WBA was introduced in three increasing doses (0.5, 1.0 and 1.5, expressed in hydrolytic acidity units (HACs) and determined based on the general alkalinity of the material). The digestates (DGs) were applied in fixed doses, which were balanced with respect to the nitrogen introduced into the soil. The test plant was the maize (Zea mays L.) variety Garantio, which was grown in a vegetation hall. The obtained results indicate that the combined use of WBA and DGs (especially ULD and SLD) had a positive effect on the plant height, leaf greenness index (SPAD), and thus, maize yield and dry matter content. In the series with DG addition, the maize yield ranged from 615.5 g (WBA + SSD) to 729.6 g pot⁻¹ (WBA + SLD), which was 28–52% higher than in the series with WBA alone. In turn, the application of increasing doses of WBA alone did not significantly affect the biomass yield but significantly increased the content of N (34%), K (60%), Mg (56%), Ca (60%) and Na (4%). In the series with WBA and DGs, the increase in the content of the above-mentioned macronutrients depended on the type of DG and the dose of WBA. The exception among the macronutrients was P, whose content generally decreased (by 4–23%) with an increasing WBA dose, regardless of the test series. The most favourable results in terms of the chemical composition, excluding the P content, were observed following the combined application of WBA and liquid forms of DG (ULD and SLD).

Nowadays, waste that can be used for environmental purposes, such as WBA (woody biomass ash), is particularly important. The presented research assessed the effect of soil application of WBA in conjunction with PFM (post-fermentation mass) on the stabilization of soil properties. WBA was applied in three increasing doses (0.5, 1.0, and 1.5 HAC). PFM was applied as follows: ULF (unseparated liquid fraction), SSF (separated solid fraction), and SLF (separated liquid fraction). PFM doses were balanced with the amount of nitrogen introduced into the soil. The study was based on a pot experiment with maize. The applied doses of WBA had a highly significant and positive effect on the stabilization of basic soil properties. After WBA application, hydrolytic acidity decreased (by 30%), soil pH increased (by 1.83 units), total base cation increased (by 66%), available potassium (by 119%), phosphorus (by 44%), and magnesium content (by 38%) as well as electrolytic conductivity increased (by 11%). Furthermore, an increase in soil carbon content and an improvement in the carbon-to-nitrogen ratio were noted. These observed results were further enhanced by the simultaneous application of WBA and the used PFM fractions, of which the liquid fractions (ULF and SLF) had the strongest effect.