Synergistic Effects of Biochar and Algae-Transformed Organic Waste from the Dairy Industry on Soil Organic Matter and Soil Sorption Properties

Biochar and algal extracts are promising organic soil amendments, but their synergistic effects on soil organic matter and the sorption complex are still insufficiently understood. Therefore, a 30-day laboratory incubation experiment was conducted using a Haplic Luvisol to evaluate the effects of biochar (S+B), an N-rich algal extract (S+AGN), and their combined application (S+AGN+B) in comparison with the untreated control soil (S). The results showed that biochar led to a substantial increase in soil organic carbon (Corg) by 49% in S+B and by 50% in S+AGN+B treatments compared to S. Labile carbon (CL) increased by 48% in S+B and by 40% in S+AGN+B. The algal extract alone did not significantly affect either CL or Corg. Non-labile carbon increased by 2.22 g kg−1 in S+B but slightly decreased in the combined treatment (−2.00 g kg−1), indicating different dynamics of stable carbon fractions when both amendments are applied simultaneously. The combined treatment S+AGN+B, however, had the strongest effect on soil sorption properties. Specifically, the sum of basic cations was the highest among all treatments (189 mmol(+)kg−1, i.e., +18–28 mmol(+)kg−1 compared to S, S+B, and S+AGN), while the cation exchange capacity (CEC) reached the highest values (198 mmol(+)kg−1, representing an increase of 7–27 mmol(+)kg−1 compared to the other treatments). The base saturation remained high across all treatments, and the highest value was observed in S+AGN+B (95.6%). PCA confirmed that the combined treatment produced the most pronounced shifts in the multivariate parameter space and demonstrated a synergistic effect exceeding the effects of the individual organic amendments. Overall, the results indicate that biochar is the dominant factor contributing to the accumulation of stable carbon and the improvement of CEC, whereas the algal extract enhances the accumulation of labile carbon fractions and synergistically promotes the saturation of the sorption complex. The combined application of biochar and algal N effectively increases soil organic matter and sorption capacity.