Maize Growth, Soil Properties and Microbial Communities Across Biochar Application Rates in Saline–Alkali Soil

Biochar adds carbon and mineral ash to soil, while crop responses can differ among application rates. We compared 0.5%, 1% and 2% maize-straw biochar in a greenhouse experiment with five blocks, comprising 15 planted amended pots and five control (CK) soil pots that contained no plants. Plant measurements and paired non-rhizosphere (NR) and rhizosphere (R) soil were obtained from the amended pots; 16S rRNA and ITS2 amplicons characterized soil bacterial and fungal communities. CK was sampled over 0–20 cm, whereas amended NR soil was sampled at 15–20 cm. The CK–amended differences are therefore descriptive; treatment inference uses the amended rates. The mean grain yield was highest at 1% (55.6 g plant−1), but the 1–2% difference was uncertain (7.32 g plant−1; Bonferroni 95% CI −3.52 to 18.16; BH q = 0.114). The photosynthetic rate was 3.16 µmol m−2 s−1 higher at 1% (Bonferroni 95% CI −0.92 to 7.24; BH q = 0.0475). At 2%, NR total organic carbon exceeded CK by 12.31 g kg−1, including added biochar carbon. The bacterial and fungal composition differed among amended rates after adjustment for block (R2 = 0.299 and 0.653; both p = 0.00077). Neither environmental db-RDA model passed corrections across the two markers, and none of 33 soil–plant associations survived correction after treatment and block were accounted for. These differences do not establish a field optimum or microbial mediation.