Cultivation strategy shapes the short-term soil amendment performance of Chlorella sp. biomass under nitrate, urea, and recycled medium conditions

The use of microalgal biomass as a soil amendment is a promising strategy for reducing dependence on synthetic fertilizers, but the soil amendment responses to microalgal biomass may depend on the cultivation strategy used for biomass production. In this study, Chlorella sp. was cultivated under four treatments: nitrate-based BG11 medium, full-urea medium, half-urea medium, and recycled spent medium. The resulting biomass was applied to soil at an equal dry-weight rate to evaluate short-term changes in soil physicochemical properties, enzyme activities, and microbial functional profiles. The half-urea treatment produced the highest biomass yield, while biochemical characteristics differed among cultivation treatments. Dry-weight-normalized protein content differed significantly among treatments, whereas dry-weight-normalized TN did not. After soil amendment, soils receiving biomass from the NS and BN cultivation treatments showed higher urease activity and ammonium-N concentrations, while the BN treatment also showed the highest available phosphorus. Descriptive metagenomic profiling of one composite sample per treatment showed numerical differences in microbial community composition and nitrogen-metabolism-related functional markers, including NarGHI , NxrAB , and NirK ; no inferential statistical tests were performed. However, because nitrogen form, nitrogen input level, and medium composition were not fully disentangled in the present experimental design, these results should be interpreted as cultivation-treatment-associated responses rather than as evidence of isolated nitrogen-source effects. Overall, under the present conditions, the half-urea treatment combined the highest biomass yield and reduced nitrogen input during cultivation with distinct short-term soil responses after biomass application.