Sustainable Valorization of Wheat Straw: A Dual-Function Biostimulant and Bioherbicide for Cowpea via Allelopathic Hormesis in Iraqi Agroecosystems<b></b>

Background: Open-field burning of wheat straw in Iraq presents a severe environmental and agronomic challenge. Harnessing its allelopathic potential offers a sustainable, circular-economy solution. Objective: This study aimed to evaluate the concentration-dependent effects of Triticum aestivum L. straw aqueous extracts on cowpea (Vigna unguiculata L.) growth, key soil health indicators, and suppression of three dominant Iraqi weeds (Convolvulus arvensis L., Imperata cylindrica L., and Beta vulgaris subsp. maritima L.), and to validate these findings under real-world field conditions. Methods: A complementary field trial was conducted alongside laboratory bioassays. The lab study used aqueous extracts (0%, 2.5%, 5%, 10% w/v) in a randomized complete block design. The field experiment applied chopped wheat straw as mulch at 0, 2, 3, and 5 tons/ha using the same design. Soil health, weed suppression, and cowpea growth/yield were measured in both settings. Results: The 5% extract (lab) and 3 tons/ha mulch (field) were optimal. In the lab, cowpea biomass increased by 55%, soil NO₃⁻-N by 38%, and dehydrogenase activity by 51%. Weed suppression reached 75% (C. arvensis). Critically, the field trial confirmed these effects: cowpea grain yield increased by 58% (2.15 vs. 1.36 tons/ha), weed biomass was reduced by 68–80%, and post-harvest soil organic carbon increased by 28% with improved structure (bulk density ↓8.7%). A regression model showed 93% of cowpea biomass variation was explained by soil N, microbial activity, and bulk density. Conclusion: Wheat straw, at 3 tons/ha as surface mulch, functions as a potent dual-function input: a biostimulant for cowpea and a selective bioherbicide against key Iraqi weeds, mediated through allelopathic hormesis and concomitant soil health improvement. This integrated, field-validated approach provides a robust scientific foundation for recommending wheat straw valorization in Iraqi agroecosystems, directly addressing sustainability, food security, and climate resilience goals.