CO2 evolution and N mineralization after biogas slurry application in the field and its yield effects on spring barley

The objective of this study was to investigate the effects of biogas slurry derived from straw-rich farmyard manure on the soil microbial biomass, on the mineralization in the field and on the related crop yield. The experiment was carried out in the following four treatments: (1) fallow, (2) fallow+biogas slurry, (3) spring barley, and (4) spring barley+biogas slurry. The CO₂ evolution rate ranged between 15 and 120mgCm⁻² h⁻¹ in both fallow treatments and showed a significant exponential relationship with the soil temperature at 5cm depth. According to the extrapolation of the CO₂ evolution rates into amounts per hectare, approximately 200kgCha⁻¹ or 27% of the biogas slurry derived C were mineralized to CO₂ during a 50 days' period to 18 June in the fallow treatment with biogas slurry. An additional amount of up to 29.5kg inorganic Nha⁻¹ could be calculated as the sum of NH₄-N already present in biogas slurry at the time of amendment and from the amount of biogas slurry mineralized in the soil to NO₃-N. A good agreement between measured and modelled stocks of inorganic N at 0-60cm depth was obtained after having five-fold increased soil organic C turnover compared to the default values of the model DNDC. The mineralization data are in line with an amount of up to 21kgha⁻¹ more N transferred by the barley plants to their aboveground biomass in biogas slurry treatment. The N not accounted for by the aboveground plant biomass could be explained by the belowground plant-derived N. CO₂ evolution from the soil surface, inorganic N content at 0-60cm depth and N transfer into barley aboveground biomass lead apparently to similar results after the application of biogas slurry. The soil ATP content after harvest of the barley was significantly larger in the two treatments with biogas slurry, especially in the fallow treatment indicating a positive effect on the soil microbial community.