Effects of combined application of acidified biogas slurry and chemical fertilizer on crop production and N soil fertility

Abstract Field studies evaluating the efficiency of anaerobic digestates (AD) as fertilizers often present variable results, mainly due to the effects of weather, site, soil conditions, and differences in the application technique. To better understand the soil influence on the effects of AD application on crop growth, we excluded site-specific effects of weather conditions and application technique and their interaction. A two-year experiment was conducted in southeast Germany, where pits were excavated in the field and re-filled with soils from seven Bavarian regions. Winter wheat followed by spring wheat was grown on the plots. The fertilized treatments were (i) mineral fertilizer alone (MIN), (ii) acidified anaerobic digestate plus mineral fertilizer application (ADM), and (iii) an unamended control treatment. Fertilized plots received nitrogen split into three dressings, amounting to 40-100−40 N kg ha−1 based on the NH4+ content. In the ADM treatment, the AD was applied in the second dressing, interspersed with mineral fertilization in the first and third dressings. Treatments were applied to the same plots in both years. Results showed that in the 1st-experimental year, the soil influence on crop parameters was more pronounced compared to the second year. The principal component analysis revealed that yield and grain number per spike were higher related to soil characteristics. The apparent nitrogen utilization based on the NH4+ content negatively correlated with the soil’s initial amount of mineral nitrogen. In the 2nd-experimental year, the effect of different fertilizer sources was more evident, and a trendy higher global effect on the crop parameters could be perceived in soils amended with the ADM treatment compared to those receiving only mineral fertilizers. Overall, the application of acidified and injected anaerobic digestate combined with mineral fertilizer was as effective as solely mineral nitrogen fertilizer application, with applied nitrogen based on the ammonium content, and the effect of different soils in the actual year of anaerobic application varied not markedly.