Soil Organic Matter Quality and Carbon Sequestration Potential Affected by Straw Return in 11-Year On-Farm Trials in the Czech Republic

Humic substances affect both soil fertility and carbon sequestration. This study aimed to evaluate the effect of straw return on the quality of soil organic matter on arable land commonly farmed by private farmers at 65 different sites between 2012 and 2022 in the Czech Republic (central Europe). In this study, most of the carbon supply was applied in straw (67% of the carbon input on average). No significant correlation between the total carbon input and both parameters of soil organic matter quality and soil organic carbon content was found. The ratio of optical absorbance at 465 to 665 nm (E4/E6) and humification index correlated most significantly with organic fertilization. However, the E4/E6 ratio was more significantly affected by the altitude of the experimental site compared to the organic fertilization. When the weighted mean C/N ratio of organic fertilizers applied exceeded the value of ca. 65, there was a decrease in the E4/E6 ratio in fluvisols and luvisols and an increase in the humification index in loamy soils, sandy loamy soils, and silt loamy soils compared to the C/N ratio ≤ 50. Leguminous cultivation revealed no significant effect on soil organic matter quality.