Biological remediation of saline-degraded agricultural soil using allelopathic plant residues: Effects on soil quality indicators and wheat productivity in semi-arid Iraq

Abstract

Saline and structurally degraded agricultural soils constrain wheat (Triticum aestivum L.) production across the semi-arid lowlands of Iraq, where electrical conductivity (EC) and sodium adsorption ratio (SAR) frequently exceed crop-tolerance thresholds. This study investigated the capacity of three locally available allelopathic plant residues: Eucalyptus camaldulensis Dehnh. leaves, wheat straw, and freshwater algae (Shamblan; Cladophora sp./Spirogyra sp.) applied as surface mulch or soil incorporation, to remediate a saline degraded soil and restore wheat productivity. A comprehensive suite of soil quality indicators (pH, EC, SAR, organic matter, available N, P, K, cation exchange capacity, and microbial enzyme activities) was measured before and after the 2025-2026 growing season at the University of Wasit research field. Baseline EC was 5.18 dS m-1, and SAR was 15.6, both above critical degradation thresholds. After one season, eucalyptus mulch reduced EC by 18.4% and SAR to 12.1, while organic matter increased by 31.4%. Phenolic acids and terpenoids released during residue decomposition, quantified by HPLC, drove Ca2+/Na+ displacement in the soil exchange complex. Wheat grain weight reached 12.50 g plot-1 under eucalyptus mulch, 212% above the unreclaimed degraded control. Pearson correlation confirmed strong negative relationships between EC and grain weight (r = ?0.891, p<0.01) and between SAR and tiller number (r = ?0.843, p<0.01). Wheat straw mulch achieved the highest return on investment by valorising a zero-cost agricultural by-product. These findings provide a low-cost, ecologically grounded strategy for rehabilitating saline degraded croplands through allelopathy-mediated biological soil reclamation.