Biochar-Associated Shifts in Nitrogen Status, Microbial Communities and Dissolved Organic Matter in Salt-Affected Maize Soil

Salt-affected soil constrains maize establishment, and rhizosphere and non-rhizosphere responses to soil biochar remain difficult to establish. We evaluated soil physical and nitrogen properties, qPCR marker genes, 16S and ITS communities, and molecular profiles of dissolved organic matter (DOM) in a 90-day randomized complete block pot experiment (four biochar rates; five blocks; 20 pots). Control maize did not survive; control pots therefore yielded only non-rhizosphere soil, whereas rhizosphere and non-rhizosphere samples from biochar pots were paired. Block-adjusted comparisons showed higher water content and porosity and lower bulk density in all biochar non-rhizosphere groups than in the control (Holm-adjusted p < 0.05). Biochar-associated shifts in mineral-N partitioning and selected DNA-level marker-gene abundances accompanied design-aware community differences for NR treatment, R dose and pooled compartment (16S R2 = 0.360, 0.435 and 0.218; ITS R2 = 0.720, 0.712 and 0.304; all p < 0.001); fungal differences also included heterogeneous dispersion. D90 FT-ICR MS profiles described relative DOM molecular variation, but cross-layer residual associations did not survive false-discovery rate correction. Thus, biochar-associated physical and nitrogen changes coincided with microbial and relative DOM restructuring under the tested pot conditions. Because control survival was confounded with treatment, these are observed-group associations rather than pure causal biochar effects.