NBPT improves root morphology and yield of cotton under salinity through changes in root nitrogen-metabolizing enzymes.

Rapid urea hydrolysis often limits nitrogen use efficiency (NUE) in cotton, especially under salinity. The urease inhibitor NBPT reduces ammonia loss, yet its effects on root morphology, nitrogen-metabolizing enzymes and exudation under salinity remain unclear. We conducted a greenhouse pot experiment, complemented by a two-year field trial, to test cotton responses to urea with or without NBPT under low (LS) and moderate (MS) salinity. NBPT increased total root length, surface area, volume and dry mass under both salinity levels. Root exudation of total organic carbon, organic acids, sugars and amino acids declined. At the same time, root urease activity fell, root urea content rose, activities of GS, GOGAT and AS increased, GDH activity decreased, and the GS/GDH ratio rose. These changes were accompanied by higher nitrogen uptake and by higher seed cotton yield and NUE in the pot experiment. Relative to urea alone, pot yield was 12.1% higher under LS and 20.5% higher under MS; the MS contrast was significant. A salinity × treatment interaction was detected for root urea content and pot NUE, but not for root morphology or yield. Redundancy analysis summarized the joint root-trait pattern. Across treatment means, larger roots and higher urea content co-occurred with higher pot yield. Field data followed the same direction for root dry mass, root urease activity, root urea content and seed cotton yield. NBPT therefore improved cotton performance under salinity through coordinated changes in root morphology and nitrogen-metabolizing enzyme activities.