Crop straw residue management is crucial for sustainable agriculture, particularly in regions like the Hetao Irrigation District of Inner Mongolia, China, where straw accumulation is high. Burning straw poses environmental risks, while its efficient utilization can improve soil fertility and crop productivity. Previous research indicates that returning straw to the soil reduces evaporation, improves soil nutrient availability, and regulates soil temperatures, positively influencing crop root growth. Roots are vital for water and nutrient uptake, and their distribution significantly affects crop yields. Nitrogen (N) is an essential nutrient, but excessive N application leads to environmental pollution and reduced N utilization efficiency. While studies have examined the effects of straw mulching or N fertilization individually, limited research has explored their combined effects on root system regulation and N use efficiency in arid areas. This study addresses this gap by investigating the impact of various straw mulching methods combined with different N application rates on root distribution, N utilization efficiency, and summer maize yield in the Hetao Irrigation District.

Existing literature highlights the multifaceted benefits of straw mulching, including reduced evaporation, improved soil nutrient availability (especially when combined with water application), and soil temperature regulation. These factors positively affect root growth, which is directly linked to water and nutrient absorption and subsequently, crop yields. Research shows that the majority of crop roots are concentrated in the upper soil layers (0-40cm), with deeper roots contributing substantially to yield. However, root distribution is sensitive to soil environmental factors, especially soil water and fertilizer levels. Nitrogen (N) is critical for crop growth and yield, and while N fertilizer application enhances yield, excessive use causes environmental pollution. Studies suggest that timed N application, such as single application at the seedling stage or staged application during the growing period, improves N utilization efficiency and yield. Conversely, excessive N application negatively impacts nutrient transfer to seeds, reduces yield improvements and leads to environmental pollution. Previous work demonstrates that straw mulching improves soil organic carbon content and that combining straw mulching with suitable N fertilizer applications enhances soil nutrient availability, N utilization efficiency, and economic benefits.

Field experiments were conducted in the Hetao Irrigation District (40° 42′ N, 107° 24' E, 1040 m altitude), Inner Mongolia, China, from April 2017 to October 2018. The study area has a semi-arid continental climate with high annual evaporation. The soil was silty loam. Two factors were studied: straw mulching depth (surface coverage and deep burial) and N application rate (0, 135, 180, and 225 kg·hm⁻²). Nine treatments (including a traditional cultivation control) were replicated three times. Each plot (72 m²) was separated by polyethylene film to prevent water and fertilizer channeling. Junkai 918 summer maize was planted mechanically in early May and harvested in late September. Root samples were collected at three growth stages using the Monolith 3D spatial sampling method. Root length density (RLD) was analyzed using a root scanner and Win RHIZO software. Above-ground plant parts were collected to measure dry matter yield and total nitrogen content using the Kjeldahl method. Nitrogen use indicators (partial factor productivity of applied nitrogen (PFPN), agronomic efficiency of applied nitrogen (AEN), and apparent recovery efficiency of applied nitrogen (REN)) were calculated using standard formulas. Statistical analysis using one-way ANOVA and Tukey's HSD test was conducted to assess treatment effects (P < 0.05).

Treatment S (deeply buried straw) significantly increased RLD in soil layers deeper than 40 cm, while Treatment B (surface straw coverage) increased RLD in the 0-30 cm layer. RLD increased with N application rate, but the yield response varied. Treatment B showed minimal yield increase with increasing N levels; only BN3 showed a slight increase (0.4%). In contrast, Treatment S showed an initial yield increase followed by a decrease, with SN2 (medium N application) yielding a 9.3% increase. SN2 also exhibited significant increases in REN (66.8%), N uptake (20.4%), and PFPN (34.1%). The study also found that Treatment S substantially increased N uptake compared to Treatment B at the same N application rate. The highest N uptake in Treatment B was observed with BN3, showing only a 0.21% increase, while the highest N uptake in Treatment S was in SN2, resulting in a significant 20.4% increase compared to the control. Analyses of ear length, ear thickness, and 100-grain mass showed that Treatment S, especially at medium N levels, resulted in improved yield components. The Harvest Index (HI) was also significantly higher in Treatment S, especially with medium N application. In Treatment S, the increase in deep RLD corresponded to the yield and nitrogen use efficiency improvements. Treatment B showed no significant improvement in deep root growth.

The findings demonstrate the synergistic benefits of deeply buried straw mulching and optimized N fertilization for improving summer maize production in arid conditions. The significant increase in deep root length density (RLD) under Treatment S enhanced nutrient and water uptake, leading to higher yields and N use efficiency. The contrast between the effects of surface coverage and deep burial highlights the importance of promoting deep root systems for efficient water and nutrient acquisition. This supports the findings of other studies that indicate the crucial role of deeper roots in contributing to overall crop yield. The study's results suggest that the optimal strategy for summer maize cultivation in similar environments involves deeply buried straw to stimulate deep root growth, combined with medium N application to maximize nutrient use efficiency and yield while minimizing environmental impacts associated with excessive N use.

This study revealed the interactive effects of straw mulching and N application on summer maize growth and N use efficiency. Deeply buried straw mulching combined with medium N fertilizer application (SN2) proved most effective, increasing deep root length densities by 67.5%, improving N use efficiency by 66.8%, and boosting yields by 9.3%. Future research should focus on optimizing N application depth and placement under straw mulching conditions, as well as exploring the coupled effects of water and fertilizer management within this system.

The study was conducted in a specific location with a particular soil type and climate. The generalizability of the findings to other regions may be limited. Further investigation is needed to explore the effects of different straw types, decomposition rates, and the long-term impacts of this management practice on soil health and sustainability. The analysis focused primarily on macronutrients; future research should evaluate the effects of this management strategy on other essential nutrients.