The effect of straw and wood gasification biochar on carbon sequestration, selected soil fertility indicators and functional groups in soil: An incubation study

Abstract Annual removal of crop residues may lead to depletion of soil organic carbon and soil degradation. Gasification biochar (GB), the carbon-rich byproduct of gasification of biomass such as straw and wood chips, may be used for maintaining the soil organic carbon content and counteract soil degradation if applied to soil. This study investigated the effect of straw removal and GB addition on soil biological, chemical and physical properties in a 22-months soil incubation study with a temperate sandy loam soil. Soil application of wood and straw GB (WGB and SGB) resulted in very low CO 2 emissions, confirming the stability of the material against microbial degradation. Both GBs increased total organic carbon, cation exchange capacity and pH of the soil. The application of SGB and WGB did not affect aggregate stability, whereas SGB did not affect and WGB decreased clay dispersibility. In contrast, the addition of straw resulted in a high soil respiration rate, and about 80% of the added carbon was respired at the end of the incubation. However, the addition of straw increased aggregate stability and decreased clay dispersibility. Results from Fourier transformed infrared photoacoustic spectroscopy revealed a lower content of O–H and aliphatic C–H together with a higher content of aromatic groups in soils amended with GB compared to soils amended with straw. This suggested that the improvement in aggregate stability in straw treatments could be related to microbial derived aliphatics and simple sugars, and that increased stability against microbial degradation in biochar amended soil was related to highly condensed aromatic groups. Addition of nutrients (N, P and S) together with straw resulted in higher soil respiration compared to the straw treatment, but did not cause differences in other soil processes. Results from this study suggest that GB has a potential for increasing soil carbon sequestration, CEC and pH. However, the straw turnover process plays a vital role for aggregate stability and clay dispersibility. Thus, annual straw removal on loamy soil needs to be considered with care in order to avoid soil degradation and risk of soil compaction or erosion.