The most important component of agricultural system are soils as the basis for the growth of plants, accumulation of water, plant nutrients and organic matter. The main task of our research was to ascertain changes in soil organic carbon (SOC) and mobile humified carbon fractions in digestate-treated soils. We have performed three field experiments using the same design on two soil types in 2019–2020. We studied the fertilization effects of different phases of digestate on Retisol and Fluvisol. Fertilization treatments: control; separated liquid digestate 85 kg ha−1 N; and 170 kg ha−1 170 N; separated solid digestate 85 kg ha−1 N; and 170 kg ha−1 N. We have found a greater positive effect on the increase in SOC because of the use of the maximum recommended fertilization rate of the solid digestate. The content of mobile humic substances (MHS) tended to increase in grassland and crop rotation field in digestate-treated soil. In our experiment, maximum concentration of SOC was found in 0–10 cm soil layer, while in the deeper layers the amount of SOC, MHS and mobile humic acids proportionally decreased. We concluded, that long-term factors as soil type and land use strongly affected the humification level expressed as HD (%) in the soil and the highest HD was determined in the grassland soil in Fluvisol.

The objective of the present study was to estimate the distribution of soil organic carbon (SOC) and its forms (protected and labile organic carbon) and total nitrogen (N tot), plant available phosphorus (P 2 O 5) and pottassium (K 2 O) within 0-30 cm layer of a ploughed horizon in agricultural soils as influenced by tillage and fertilisation. Research was carried out in two regions of Lithuania: in Central (sites I and II) and Northern (site III) parts of Middle Lithuanian Lowland. Tillage systems, including conventional tillage (CT) and no-tillage (NT), straw management methods and mineral fertilisation were investigated on a loam and sandy loam top layer of glacial morain Eutric Endocalcaric Endostagnic Cambisol in site I. Five fertilisation treatments were tested in a grassland with a sandy loam top layer of glacial morain Endocalcaric Albic Brunic Endogleyic Arenosol in site II: without fertilisation and fertilisation with separated liquid and separated solid digestates at rates of 85 and 170 kg ha-1 N. The field experiment on a glaciolacustrine Eutric Endocalcaric Amphistagnic Endogleyic Cambisol with clay loam in site III involved the following tillage systems: conventional tillage (CT), ploughless tillage (PT), ploughless tillage with lime sludge incorporation (PT + LS) and no-tillage with a cover crop for winter mulch (NT + WM). Long-term NT caused pronounced stratification of N tot , available K 2 O and SOC within the arable layer of Eutric Endocalcaric Endostagnic Cambisol. The highest accumulation of SOC and chemically protected-humified carbon in the grassland on an Endocalcaric Albic Brunic Endogleyic Arenosol was determined at the highest digestate rate of 170 kg ha-1 N. High SOC accumulation potential was shown by PT + LS in a Eutric Endocalcaric Amphistagnic Endogleyic Cambisol. This significantly increased the share of humified carbon fraction bound with calcium in 0-10 and 10-20 cm soil layers. Reduction of tillage intensity by applying PT and NT + WM increased the concentration of labile organic carbon in the soil, especially in the upper 0-10 cm layer. The lime sludge application reduced the amount of labile organic carbon by its incorporation with calcium in this amendment, while increasing the stability of carbon compounds. In Eutric Endocalcaric Endostagnic Cambisol, the mean results of 0-30 cm layer revealed that the share of F4 carbon fraction, resistant to degradation, in the SOC was significantly higher under NT than under CT.

Given the growth in the number of biogas power plants and the increase in the generation of waste from energy production, it is relevant to study the sustainable nature of this waste. Digestate is a product of the anaerobic digestion process, and is a valuable bio-fertilizer containing organic matter and nutrients necessary for agricultural plants’ growth. The study showed that different rates of liquid and solid phases of anaerobic digestate influenced the contents of carbon and nitrogen in genetically young soil in alluvial deposits—Fluvisol. The application of solid digestate (SD) considerably increased soil organic carbon content (SOC) in the 0–10 cm soil layer; however, SOC did not reach the 20–30 cm layer. Liquid digestate (LD) significantly increased SOC in the deeper layers. The levels of mineral nitrogen (Nmin) and water extractable organic carbon (WEOC) increased in the 0–10 cm soil layer soon after fertilization with LD and SD. The mobile components of the soil (Nmin and WEOC) were characterized by high variability during the growing season. Within the 2-month period, their concentrations decreased drastically and were close to those of unfertilized soil. The research indicates that anaerobic digestate had a greater effect on mobile forms of carbon and nitrogen in the soil than on their total amounts.

Biogas slurry is widely used to fertilize crops. However, their impact on soil parameters and waxy winter wheat (Triticum aestivum L.) nutrition is poorly understood. The aim of this research was to determine the influence of liquid anaerobic digestate and pig slurry applied to waxy winter wheat on the dynamics of soil organic carbon (SOC) and total nitrogen (Ntot) in different forms on grain yield, and to compare them with the use of ammonium nitrate. The nitrogen rates (kg N·ha−1) used for fertilization were N0, N60, N120, and N120+50. The study showed that the variation of nitrate nitrogen (N-NO3) and water-extractable organic carbon (WEOC) in the soil during the growing season depended on N fertilizer rates, meteorological conditions of the year, and, to a lesser extent, on fertilizer forms. Meteorological conditions were responsible for the demand and supply of nutrients from the soil by the waxy winter wheat variety. This determined the wheat yield and the variation in the soil parameters studied. Over the 2 years, the soil C:N ratio decreased, especially at the medium and high N fertilizer rates. The lowest changes were observed in the unfertilized and fertilized plots at a rate of 60 kg N·ha−1.

Biogas and anaerobic digestion has begun to be considered an important renewable and sustainable energy source. The sustainable development of the anaerobic digestion process depends largely on the ability to manage large amounts of by-products generated during the biogas production process. We hypothesized that the use of digestate increases the accumulation of C in stable forms. We supposed that the effect of digestate on soil depends on the land-use system, leading to different stratifications of C. The main task of our research was to ascertain changes in the amount of stabile organic carbon (SOCstabile) in digestate-treated soils. Two field experiments were performed using the same design in 2019–2020. We studied the fertilization effects of digestate on Fluvisol. Fertilization: control; separated liquid digestate 85 kg ha−1 N and 170 kg ha−1 170 N; separated solid digestate 85 kg ha−1 N and 170 kg ha−1 N. A randomized experimental design with three field replicates was used. In terms of carbon stabilization in Fluvisol, soil used for grassland showed an advantage over the arable soil. The study showed that digestate, especially solid digestate, contributes to C accumulation and stabilization in the soil.