Optimized straw incorporation depth can improve the nitrogen uptake and yield of rapeseed by promoting fine root development

Abstract

Nitrogen deficiency in early growth stages is known to enhance crop nitrogen uptake and yield by stimulating root growth. However, whether straw incorporation can induce similar effects by intensifying nitrogen competition between microorganisms and crops remains unclear. A six-year field experiment (2016–2022) investigated three straw management practices—shallow tillage (tillage depth of 10–15 cm) without straw incorporation (STS-), shallow tillage with straw incorporation (STS+), and moderate deep plowing with straw incorporation (MPS+, tillage depth of 20–25 cm)—on soil properties of clay loam, root structure, nitrogen uptake, and rapeseed (Brassica napus L.) yield. STS+ and MPS+ treatments significantly reduced soil bulk density and increased porosity, with MPS+ showing improvements across the 0–30 cm layer. Straw incorporation enhanced root length, surface area, and volume, particularly in the 0–0.5 mm diameter category, while reducing root diameter. STS+ and MPS+ increased root auxin and cytokinin levels and decreased abscisic acid content, enhancing root vitality. Nitrogen accumulation at maturity was significantly higher with straw incorporation, but accumulation patterns differed by incorporation depth. STS+ delayed the onset and termination of rapid nitrogen uptake, resulting in slow accumulation during the seedling stage and rapid uptake post-bolting, while MPS+ achieved a consistently higher nitrogen accumulation rate throughout the growth period. MPS+ increased nitrogen uptake by 18.5 %, leading to a 50.7 % yield increase, whereas STS+ showed no significant nitrogen uptake enhancement but increased yield by 21.2 %. Structural equation modeling suggested that straw incorporation enhanced nitrogen accumulation by promoting fine root growth in the 0–0.5 mm category, which in turn improved yield. Conversely, straw incorporation depth increased nitrogen accumulation by promoting root growth in the 0.5–2 mm and 0–0.5 mm diameter categories. These results indicate that MPS+ creates a moderately nitrogen-deficient environment during early growth stages, promoting fine root development, nitrogen uptake, and yield in rapeseed.