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Biogas is an important energy source produced by the anaerobic fermentation of raw faecal slurries and plant residues. Separation of the total digestate increases the fertilizer quality of the liquid fraction and the carbon sequestration potential of the solid fraction. A 12-day incubation study was carried out to investigate the relationships between the chemical composition of different digestate qualities and the immediate response of soil microbial activity and biomass indices. The highest cumulative (Σ) CO2-C efflux was observed after adding the solid fraction and lowest after adding the liquid fraction to soil, which was even lower than that of the control. The ΣCO2-C efflux showed the strongest negative correlation with the raw ash and strong positive correlations with the raw fibre concentration and the C/N ratio of the different digestate qualities. The highest and similar ΣN2O-N efflux was observed after adding the total digestate or the liquid fraction, which were equivalent to approximately 1% of added N. This relatively low percentage indicates a possible origin from nitrifier denitrification. Total digestate and its liquid fraction exhibited considerable net-N mineralisation rates, which could mainly be predicted by the C/N ratios of the different digestate qualities. Microbial biomass C did not respond to the application of any digestate quality, whereas the fungal ergosterol content increased after applying the solid and the composted solid fractions. This raw fibre–induced fungal growth led to strong net-N immobilisation in soil after applying these two digestate qualities.

Biogas slurry is increasingly used as fertilizer. Earlier research was focused on plant growth and soil chemical properties, with only little information available regarding the effects of biogas slurry on soil and root microbial indices. For this reason, a 70 d pot experiment was conducted in which biogas and raw slurries obtained from six biodynamic farms were added to a soil. Italian ryegrass (Lolium multiflorum Lam.) was cultivated to investigate the effects on plant yield, N uptake (two harvests), soil microbial biomass, soil fungi, and root-colonizing microorganisms. Biogas slurries increased the mean total above-ground plant biomass by 66% and raw slurries by 35% in comparison to the control. The mean plant N-uptake increased under biogas and raw slurry application by 166% and 65%, respectively, compared with the unfertilized pots. The effects of biogas and raw slurry application on soil microbial indices were similar except for the lower fungal biomass after biogas slurry amendment. In contrast to biogas slurries, the raw slurries significantly increased microbial biomass C and N by roughly 25% in comparison to the control. The application of biogas slurries significantly decreased the soil ergosterol content in comparison with raw slurry and control treatment, leading to a significantly lower ergosterol : microbial biomass C ratio. In the roots, biogas and raw slurry application significantly decreased the concentrations of the amino sugars galactosamine and glucosamine by 39 and 27%, respectively, but not that of ergosterol in comparison with the control. This was most likely due to a reduced colonization with arbuscular mycorrhizal fungi in the presence of highly available plant nutrients.

Biogas slurry, the secondary product of the anaerobic digestion process, is increasingly being used as fertilizer. Information is available on its chemical and physical properties and their effects on plant growth. However, there is a demand to characterize the microbial quality of slurries, which may control further mineralization processes after application to soil. In this study, biogas and raw slurries obtained from six farms were analyzed for their ergosterol and amino sugar concentrations as indices for microbial biomass. A reliable, precise method for determining ergosterol in slurries is presented. Biogas slurries contained significantly less ergosterol (−34%), muramic acid (MurN; −42%), galactosamine (GalN; −32%), and fungal glucosamine (GlcN; −40%) than raw slurries. The mean fungal GlcN to ergosterol ratio (50) and also the mean fungal carbon (C) to bacterial C ratio (0.29) did not significantly differ between the slurry types. The mean microbial C concentration in the biogas slurries was significantly lower than in the raw slurries. Consequently, the contribution of microbial C to slurry organic C was 3.6% in the biogas slurries and 5.7% in the raw slurries. Microbial C revealed significant nonlinear relationships with the fiber and ash concentration, pH, as well as the C/N ratio of the slurries.

In the present field study, the effects of applying biogas slurry for 25 years on soil microbiological and soil chemical properties were investigated in comparison with the application of raw cattle slurry. An on-farm research approach was used with pairwise comparison of neighboring fields, taking samples at a high spatial resolution from two biodynamic farms down to 10 cm. Texture, soil organic C (SOC), total N and P, microbial biomass C, N, and P as well as P fractions were determined by different extraction procedures. Application of biogas slurry had no significant effects on SOC, total N, or microbial biomass C, N, and P. Clay and pH effects outweighed site-specific differences for most properties. Biogas slurry increased the fractions of plant-available inorganic phosphate (+66 % more Olson-Pi) and of Ca-associated phosphates (+40 % 1 M HCl-Pi) at the expense of hardly soluble Fe and Al phosphates (-25 % 6 M HCl-Pi). This is in line with the observation that biogas slurry contains significantly more H2O-Pi and 1 M HCl-Pi than raw slurry. The long-term application of biogas slurry had no negative impacts on any of the soil properties analyzed.

Residues from biogas production contain essential plant nutrients such as nitrogen (N), phosphorus (P), and potassium (K) but also organic matter, and should be recycled in crop production. For efficient re-use as fertilizers, the availability of nutrients for crops and the effect of the residues on soil fertility need to be evaluated. Focusing on the element P, we compared effects of codigested slurry with dairy slurry, highly soluble mineral NPK fertilizer, and a control without any P supply (NK). Codigested slurry used in this experiment was based on anaerobic digestion of dairy slurry, maize silage, and wheat grain. The fertilizing effects were tested in an 8-week pot experiment on a sandy and a loamy soil using two crop species (Zea mays L., Amaranthus cruentus L.). The plant P uptake was up to 64% greater in the slurry treatments than in the treatment without P. The effect of codigested slurry on P uptake was comparable to that of dairy slurry and mineral P. Plant N uptake from codigested slurry was lower than that from mineral N (NK, NPK), but tended to be higher than from dairy slurry. The water-soluble and double lactate–soluble P content of the soil was lower in the slurry treatments than in the mineral-P treatments and accompanied by higher contents of microbial-bound P. Differences between both organic fertilizers were detected for dehydrogenase activity which was up to 32% lower in soils fertilized with codigested slurry than it was in soils fertilized with dairy slurry. Our results indicate that codigestion of slurries in biogas plants does not substantially alter their fertilizer value as P and N sources for crops.

The long-term effects of biogas slurry application on soil fertility indices were compared with raw slurry in biodynamic organic farming systems. An on-farm soil and slurry sampling was carried out to quantify the effects on stocks of soil organic matter, microbial biomass and microbial residues. Five fields with biogas slurry and five neighbouring fields with raw slurry amendments were selected at 5 different sites in the north-east of Baden-Württemberg, Germany. The application of biogas slurry ranged from 15 to 25 years and did not affect SOC, total N stocks or the soil C/N ratio. Biogas slurry application decreased the soil microbial biomass to SOC ratio, which indicates a reduced availability of the biogas slurry C input to soil microorganisms compared with raw slurry. At some sites, differences in clay content masked any slurry effects on the microbial activity, biomass, and residue indices. There were no general effects of biogas slurry on the ratios of ergosterol to microbial biomass C or amino sugar-based fungal C to bacterial C, whereas an increasing clay content caused a significant shift towards bacteria according to the latter ratio. Since the soils had been farmed organically in diverse crop rotations for at least 40 years, chemical differences in slurry composition were not great enough to result in different biochemical properties. The consistency in the data of all approaches strongly indicates the validity of the current on-farm study by comparing neighbouring fields. The long-term effects of biogas slurry application on soil fertility indices were compared with raw slurry in biodynamic organic farming systems. An on-farm soil and slurry sampling was carried out to quantify the effects on stocks of soil organic matter, microbial biomass and microbial residues. Five fields with biogas slurry and five neighbouring fields with raw slurry amendments were selected at 5 different sites in the north-east of Baden-Württemberg, Germany. The application of biogas slurry ranged from 15 to 25 years and did not affect SOC, total N stocks or the soil C/N ratio. Biogas slurry application decreased the soil microbial biomass to SOC ratio, which indicates a reduced availability of the biogas slurry C input to soil microorganisms compared with raw slurry. At some sites, differences in clay content masked any slurry effects on the microbial activity, biomass, and residue indices. There were no general effects of biogas slurry on the ratios of ergosterol to microbial biomass C or amino sugar-based fungal C to bacterial C, whereas an increasing clay content caused a significant shift towards bacteria according to the latter ratio. Since the soils had been farmed organically in diverse crop rotations for at least 40 years, chemical differences in slurry composition were not great enough to result in different biochemical properties. The consistency in the data of all approaches strongly indicates the validity of the current on-farm study by comparing neighbouring fields.