The effects of straw mulching combined with nitrogen applications on the root distributions and nitrogen utilization efficiency of summer maize

The Hetao Irrigation District faces growing straw residues with low utilization and air pollution from straw burning. Returning straw to fields can reduce evaporation, regulate soil temperature, improve soil nutrients, and enhance root development—key determinants of water and nutrient uptake and yield. Nitrogen is essential for maize, yet overuse reduces N use efficiency and risks environmental pollution, while appropriate timing and rates can improve yield and N recovery. Few studies in arid regions have examined how N efficiency is regulated in the rhizosphere under straw mulching, particularly considering mulching mode (surface vs. deeply buried) in combination with graded N rates. This study investigates how straw mulching mode and N rate interact to shape root spatial distribution, N uptake and use efficiencies, and yield of summer maize, aiming to identify a tillage–fertilizer strategy that improves N efficiency and productivity in Hetao.

Prior work shows straw return can reduce evaporation and activate soil nutrients; when combined with suitable water management, it improves water productivity and reduces salinity in ploughed layers. Straw mulching cools the soil, with stronger cooling at higher mulch amounts, promoting maize roots, enhancing deep water and nutrient uptake. Typically >85% of roots lie within 0–40 cm, with layers below 20 cm contributing substantially to yield. Root distribution is sensitive to soil water and fertilizer. Excessive N can hinder nutrient transfer to grain and elevate environmental risks; staged or appropriately timed N improves yield and N use efficiency. Straw mulching can increase soil organic carbon and, with suitable N, improve soil fertility, N use efficiency, and economic returns. However, the combined rhizosphere effects of mulching mode and N rate on root distribution and N efficiency in arid systems remain underexplored.

Site: Jiuzhuang demonstration area, Hetao Irrigation District, Inner Mongolia, China (40°42′N, 107°24′E; 1040 m). Semi-arid continental climate; average annual evaporation 2332 mm; surface salt accumulation in spring and winter. Soil: silty loam; 0–100 cm: organic matter 15.33 g kg−1; total N 0.87 g kg−1; available P 14.66 mg kg−1; available K 180.8 mg kg−1; bulk density 1.51 g cm−3. Study period: April 2017–October 2018. Rainfall during maize growth: 75.3 mm (2017), 126.9 mm (2018). Experimental design: Two factors—straw mulching mode and N rate. N rates: N0 = 0; N1 = 135; N2 = 180; N3 = 225 kg N·hm−2 (local rate N3). Mulching modes: Treatment B (surface cover): field tilled to 35 cm, then in year two leveled, shallow raked, rolled, film-coated planting, and rows covered with 5 cm thick straw (1.5 kg·m−2). Treatment S (deeply buried): field tilled to 35 cm; after autumn harvest, a 5 cm straw layer was placed at 35 cm depth; in year two shallow raked and compacted. Control CK: traditional cultivation with no straw, N = 225 kg·hm−2. Nine treatments: CK; BN0, BN1, BN2, BN3; SN0, SN1, SN2, SN3; three replicates in randomized arrangement. Plot size 72 m² with 3 m buffer; plots isolated with polyethylene film to 1.2 m depth (30 cm above surface) to prevent water/fertilizer movement. Fertilization and irrigation: P as DAP at 150 kg·hm−2 (as P2O5), K as KCl at 45 kg·hm−2 (as K2O). P, K, and 50% of N applied as basal; remaining N top-dressed at jointing. Irrigation: Yellow River water (salinity 0.608 g·L−1), 3 irrigations per season, each 135 mm; water metered by gasoline pump. Crop material and management: Summer maize (Junkai 918), sown early May, harvested late September; plant spacing 0.35 m, row spacing 0.45 m; local farmer management practices. Sampling and measurements: Root sampling at elongation (jointing), silking, and maturity stages using Monolith 3D spatial sampling. Aboveground and root samples dried (105 °C then 80 °C to constant weight) for biomass; roots scanned (Epson 4870) and analyzed with WinRHIZO to quantify root length and RLD by depth to 1 m. Yield and components (ear length, ear diameter, 100-grain mass) measured at harvest; five plants sampled for organ biomass; total N concentration by Kjeldahl (H2SO4/H2O2 digestion). Nitrogen use metrics: PFPN = Y/F; AEN = (Y − Y0)/F; REN = (U − U0)/F, where Y = yield with N, Y0 = yield without N, U = plant N uptake with N, U0 = plant N uptake without N, F = N applied. Statistics: Data as mean ± SD (n=3). One-way ANOVA (P<0.05) with Tukey HSD for multiple comparisons (IBM SPSS 20).

- Root distribution: Surface mulching (B) increased RLD in 0–30 cm by 12.8% and 14.2% vs. S and CK, respectively. Deep mulching (S) increased RLD in 30–40 cm by 11.7% and 15.8% vs. B and CK, and in >40 cm by 41.7% and 46.5% (P<0.05). In >40 cm, SN2 and SN3 increased average RLD by 67.5% and 68.1% vs. CK; deep root percentage in 1 m profile under S (except SN0) reached 6.3–10.1% vs. <2.1% overall under CK/B. - Response to N: Across depths, RLD increased with N, with strongest deep-root gains under S at N2–N3; low or zero N reduced RLD significantly vs. CK. - N uptake: Increased with N rate but with diminishing returns. For equal N, S exceeded B in plant N uptake by 9.2%, 15%, 32%, and 21% (for N0–N3) across years (P<0.05). Maximum N uptake under S occurred at SN2 (+20.4% vs. CK); under B, BN3 increased by only 0.21% vs. CK. N uptake in 2018 exceeded 2017 by 6.7–19.9%. - N use efficiency: PFPN, AEN, REN increased then decreased with N rate. Relative to CK(B), PFPN increased by 20.9%, 23.5%, 0.9% for BN1, BN2, BN3; relative to CK(S), PFPN increased by 30.8%, 34.1%, 2.7% for SN1, SN2, SN3. Under S, AEN and REN peaked at SN2; AEN up to +52.8% and REN up to +66.8% vs. CK. Despite similar deep RLD at N2 and N3 under S, SN2 had substantially higher PFPN (e.g., +30.6%, +41.2%, +20% vs. SN3 across metrics). - Yield and components: Under B, yield rose with N but gains plateaued; BN3 was only +0.4% vs. CK on average. Under S, yield first increased then decreased with N; SN2 had the highest yield, averaging +9.3% vs. CK. Example yields (kg·hm−2): 2017—CK 7385, SN2 8103, SN3 7633, BN3 7315; 2018—CK 7600, SN2 8268, SN3 8035, BN3 7731. Ear length and diameter under S were significantly higher than CK at medium–high N (+11.5% and +8.3% on average). 100-grain mass under S increased by 8.8–9.3% at N2–N3 vs. CK; under B, only BN3 matched CK. - Harvest index: HI ranged 0.32–0.42 and was sensitive to mulching and N. Under S, HI increased first then decreased with N, peaking at SN2 (+10.7% vs. CK on average). Under B, HI increased with N, peaking at BN3 (+4.5% vs. CK on average). - Correlations: Under S, yield tracked deep (30–40 cm) RLD closely (R2=0.97, P<0.05); no significant relation with total RLD. Findings highlight the importance of deep roots for N uptake and yield in arid systems.

Deeply buried straw mulching shifted root distribution downward, enhancing RLD in 30–40 cm and >40 cm layers. This deep rooting increased plant N uptake and N use efficiencies (PFPN, AEN, REN), translating into higher yields, particularly at a medium N rate (N2). In contrast, surface straw primarily enhanced shallow roots (0–30 cm), limiting access to deeper soil water and nutrients and providing minimal yield benefit even at high N. High N (N3) ensured root growth but reduced N use efficiency and did not improve yield relative to N2, indicating luxury N supply and potential environmental loss. The strong positive association between deep RLD and both N uptake and yield under S demonstrates that rhizosphere regulation through mulching depth, coupled with moderate N, is critical to maximize resource capture and productivity in arid irrigated maize. Thus, optimizing straw placement to promote deep roots, rather than increasing N rate, is the more effective pathway to improve N efficiency and yield.

The interaction between straw mulching mode and N rate governs root spatial distribution, N uptake and use efficiency, and yield in summer maize. Deeply buried straw mulching combined with a medium N rate (SN2, 180 kg N·hm−2) was optimal, increasing deep RLD by about 67.5%, plant N uptake by 20.4%, N use efficiency (REN) by 66.8%, and yield by 9.3% relative to traditional practice. Surface mulching mainly enhanced shallow rooting and provided limited yield benefits even at high N. For the Hetao Irrigation District, the recommended tillage–fertilizer strategy is deeply buried straw with medium N to improve N efficiency and productivity while mitigating environmental risks. Future work should refine N placement depth and synchronization with irrigation under mulching and develop water–fertilizer coupling models that incorporate soil nutrient dynamics after straw return.

- Single region and soil type (silty loam) over two seasons; generalizability to other climates/soils requires validation. - Variability in rainfall between years (notably higher in 2018) may confound treatment effects on N uptake and efficiency. - N application was split only as 50% basal and 50% at jointing; alternative timing/placement strategies were not tested. - Environmental N losses (leaching, volatilization, N2O emissions) were not measured; conclusions on environmental benefits are indirect via efficiency metrics. - The study did not experimentally vary N placement depth/location relative to straw layers; authors note this as a target for further research alongside water–fertilizer coupling analyses.