Phosphorus availability and soil microbial activity in a 3 year field experiment amended with digested dairy slurry

The application of biogas residues to agricultural fields is important for nutrient cycling. A 3 year field experiment was conducted to assess the phosphorus (P) fertilizer value of digestate from biogas production, taking into account soil microbial activity. The input substrate (inputS) and digested substrate (digestS) from a biogas plant using dairy slurry, maize silage and wheat corn, were applied at a rate of 30 m³ ha−1 annually. For control, mineral N and K, but no P, were applied in equal amounts with the biogas substrates. Maize was cultivated every year, and the biomass yield and P and N uptake were determined. Soil samples were collected on different sampling dates, and the P contents, pH, organic matter contents and enzyme activity were analyzed. The CO2 efflux was measured biweekly using a portable soil respiration chamber (EGM 4). After 3 years, the P and N uptake increased by 25% in the digestS treatment compared with that of the control but did not differ from that of the inputS treatment. The plant-available P contents were also higher in the inputS- and digestS-amended soil. The fertilizer application did not influence the organic matter content but did influence the enzyme activity in soil. Averaging of all the sampling dates in 2010 and 2011, the activities of dehydrogenase and alkaline phosphatase were 50% lower in the soils that were amended with digestS compared with inputS. However, the CO2 efflux from the soil surface was the same for the inputS and digestS treatments. Our results indicate that the anaerobic digestion of substrates does not affect the plant P uptake but the performance of soil microorganisms. •The fertilizer values of a biogas digestate and an undigested substrate were compared.•Attention was also given to the effects of the substrates on soil microbial activity.•Both substrates had positive effects on maize P nutrition and P availability in soil.•Activities of soil enzymes were relatively low after application of the digestates.•The CO2 efflux from the soil surface did not depend on the type of substrate applied.