Effect of biochar and compost on soil properties and organic matter in aggregate size fractions under field conditions

Aggregation affects soil properties crucial for sustainable soil management and productivity. However, longer-term studies (> five years) of treatments to enhance soil aggregation, such as addition of biochar with labile carbon to derive microbial binding agents, are limited, especially in temperate climates. To fill this gap, we established a field experiment with control, compost only, biochar only, and a mixed compost-biochar application (co-composted and only mixed) at low and high application rates (9-70 t ha(-1)) in southern Germany. After six years of agricultural use, surface (0 -10 cm) and subsurface (10 - 30 cm) soil samples were analyzed for pH, microbial biomass carbon (C-mic), water holding capacity (WHC), and cation exchange capacity (CEC). Particulate organic matter (POM) and aggregate fractions were analyzed for organic carbon (OC) content and characterized using diffuse reflectance infrared Fourier transform spectroscopy. Biochar significantly increased OC and pH, while compost significantly increased OC, pH, C-mic, and CEC six years after application. Biochar also significantly increased OC storage in POM (10-32 times higher) and all aggregate fractions by 56-62 %, 32-47 %, and 29-32 % for the 2 - 0.25 mm, 0.25 - 0.053 mm, and < 0.053 mm fraction, respectively, while compost increased OC storage only in the 10 - 30 cm soil depth in the soil fractions > 0.053 mm. The proportion of reactive C=O groups significantly increased in POM due to biochar and compost application, while only biochar affected the < 0.053 mm fraction. Our results suggest that six years after application, high rates of both biochar and compost are beneficial for soil properties affecting the sustainability of soil agro-ecosystems such as pH and CEC. For long lasting increase in soil C sequestration, our results indicate that only the application of biochar can be considered as a significant measure.