Morphological and Physiological Characteristics of Maize Roots in Response to Controlled-Release Urea under Different Soil Moisture Conditions

Controlled-release urea (CRU) is being promoted in Chinese maize (Zea mays L.) planting to improve nitrogen (N) use efficiency, yield and reduce N losses, but its impacts vary widely depending on the soil moisture condition. There has been little study on the effects of CRU on the morphological and physiological characteristics of maize roots under different soil moisture conditions. We conducted three soil moistures (severe stress, mild stress, and adequate condition) and four levels of CRU (0, 105, 210, and 315 kg N ha(-1)) in a specially designed soil column experiment. Results revealed that CRU regulated plant growth by affecting root morphology and activity under different soil moistures, and ultimately influenced yield. Drought limited root and shoot dry matter accumulation, and decreased root length and root length density, which significantly reduced root active absorption area; leaf area index (LAI), chlorophyll content, and net photosynthetic rate (P-n) were also inhibited. Increasing CRU application did not counteract the inhibition of root and shoot growth under severe water stress, but did counteract this effect under mild water stress. An application of CRU beyond the optimal N rate did not consistently promote maize root growth or increase yield under adequate soil moisture. The CRU application of 210 kg N ha(-1) under adequate moisture was the best treatment combination, and was associated with superior root morphology and activity during the grain-filling period, which could transport more water and nutrients to aboveground plant, improved LAI, chlorophyll content, and P-n, ultimately increased yield. Based on the yield and cost, the CRU application of 315 kg N ha(-1) was optimal under mild drought stress, and selecting the lower CRU application of 210 kg N ha(-1) under adequate soil moisture condition is recommended to promote root growth and increase grain yield in maize production.