Short term effects of bioenergy by-products on soil C and N dynamics, nutrient availability and biochemical properties

The shift towards a biobased economy will probably trigger the application of bioenergy by-products to the soil as either amendments or fertilizers. However, limited research has been done to determine how this will influence C and N dynamics and soil functioning. The aim of this work was to investigate the effects of different bioenergy by-products on C and N mineralisation, nutrient availability and microbial content and activity of amended soil and compare them to other more commonly used organic amendments. Two agricultural soils were amended (0.5% w/w) with four different bioenergy by-products (anaerobic digestate, rapeseed meal, bioethanol residue, biochar) and three other commonly used organic amendments (sewage sludge and two composts) and incubated at 20 degrees C in the laboratory for 30 days. During incubation, soil CO2 and N2O evolution were measured every 4 h by an automatic chromatographic system. After 2, 7 and 30 days of incubation, soil samples were analysed for K2SO4-extractable C, N, NO3-, NH4+ and P, microbial biomass C and three enzymatic activities (beta-glucosidase, alkaline phosphatase and leucine aminopeptidase). Soil amendment led to a general increase in soil respiration, available N and P and microbial content and activity, but with remarkably different dynamics and values. Particularly, rapeseed meal and the bioethanol by-product led to N2O emissions and the greatest increases in soil respiration, N availability and enzymatic activity compared with the other amendments. The exception was represented by biochar that did not cause any significant variation with respect to the control, but promoted C accumulation. According to their impact on soil biochemical properties, the materials can be ranked as follows: rapeseed meal, bioethanol residue > anaerobic digestate, sewage sludge > composts > biochar. For each measured parameter, soil properties did not affect the response pattern found for the different treatments, but modified the magnitude of the response. In particular, soil respiration and enzymatic activity were higher in the slightly acidic soil, while greater values of available P were found in the alkaline soil. This study clearly indicates that the impact on GHG emissions and soil functioning of bioenergy byproducts needs to be taken into account for a correct life cycle assessment of the bioenergy chain. Moreover, when properly managed, they may represent an effective alternative to usual amendments to improve the quality and nutrient balance of amended soils.