Strip width ratio expansion with lowered N fertilizer rate enhances N complementary use between intercropped pea and maize

Closing global yield gaps while minimizing environmental costs requires improving nitrogen (N) use efficiency. Intercropping, particularly cereal-legume systems, can enhance resource use through complementary canopy and root traits but may also intensify competition for limited resources. In maize/pea intercropping in northwest China, pea is sown earlier and tends to be more competitive during co-growth, potentially improving N use efficiency but temporarily restricting maize growth. After pea harvest, maize may recover via compensation growth. This study investigated how modifying strip width ratio (maize:pea 80:80 cm vs 120:80 cm) and reducing N fertilizer rates affect N competition (pea vs maize), N compensation (maize after pea harvest), N use efficiency, and productivity. Hypotheses: (1) widening the maize:pea strip ratio with reduced N intensifies N competition and improves N compensation; (2) intensified competition and improved compensation increase N use efficiency and yields of both crops.

Prior work shows intercropping improves productivity, pest and disease control, ecological services, and profitability. Mechanisms include improved radiation interception and root spatial-temporal differentiation, enhancing nutrient and water use. However, intercropped species can strongly compete, often reducing yield of subdominant crops. Earlier-sown species (e.g., faba bean, winter pea) commonly outcompete later-sown cereals during co-growth. In maize/pea systems, pea sown earlier exhibits higher competitive ability. Cereal-legume intercropping can stimulate legume biological N2 fixation due to cereal competition for soil N and enable N transfer to cereals (direct during co-growth or indirect after legume harvest). Optimizing interspecific interactions (competition vs facilitation) is key to maximizing N utilization.

Design: Three-year (2009–2011) split-plot field experiment at Oasis Agricultural Research Station, Hexi Corridor, Wuwei, NW China (37°30′N, 103°5′E; 1776 m). Aridisol soil; long-term mean temp 7.2°C; annual rainfall ~156 mm (mostly Jun–Sep); evaporation >2400 mm; high solar radiation. Initial soil: total N 0.78 g kg⁻¹; available N 25.6 mg kg⁻¹, P 33.8 mg kg⁻¹, K 134.2 mg kg⁻¹. Main plots (4 systems): sole maize; sole pea; intercropping Int-1 (maize:pea strip width 80:80 cm in 160-cm strips; 2 maize rows at 40-cm interrow; 4 pea rows at 20-cm interrow); intercropping Int-2 (maize:pea 120:80 cm in 200-cm strips; 3 maize rows at 40-cm interrow; 4 pea rows at 20-cm interrow). Three replicates. Subplots (N rates): N0 (0 kg N ha⁻¹), N1 (pea 90; maize 300 kg N ha⁻¹), N2 (pea 135; maize 450 kg N ha⁻¹). For maize (N1/N2): 30% pre-sowing incorporated; 60% top-dressed pre-tassel; 10% at grain filling via banded furrow (5–6 cm deep, 4–5 cm from plants) before irrigation. For pea: all N basal pre-sowing. All plots received 150 kg P2O5 ha⁻¹ basal; Ca superphosphate used where N=0; diammonium phosphate where N>0. Crop residues removed. Crops and management: Pea cv. Long-wan No.1: sown early April, harvested early July. Maize cv. Wu-ke No.2: sown late April, harvested late September. Plot sizes: Int-1 38.4 m² (4.8 × 8 m), Int-2 45.6 m² (5.7 × 8 m), sole 48 m² (6 × 8 m). Three pairs of maize–pea strips per intercropped plot. Maize strips mulched with plastic film. Target plant populations: pea ~1,800,000 plants ha⁻¹; maize ~90,000 plants ha⁻¹; intercropped seeding based on land share. Irrigation: 120 mm in late fall pre-freeze; in-season drip irrigation quotas by crop and system (e.g., pea 66–82.5 mm; maize 202.5–243 mm), applied area-equivalently across systems. Measurements: Grain yield (GY) measured at maturity for each crop and system. Biomass and N accumulation (NA): Sampling every 15 days before pea harvest (from 15 d after maize sowing); for maize, continued every 25 days post-pea harvest to maize maturity. Per sample: 10 maize plants and pea from four 30-cm row segments (4 rows × 30 cm), separated into organs, oven-dried (80°C), weighed; N concentration via Elementar vario MACRO cube; NA = N% × dry matter. Derived indices: Nitrogen competitive ratio (N-CR) of pea vs maize: N-CR = (NAip/NAsp × Fm) / (NAim/NAsm × Fp), where NAip/NAim are NA of intercropped pea/maize, NAsp/NAsm are NA of sole pea/maize, and Fm/Fp are land proportions of maize/pea. N-CR >1 indicates pea advantage. Nitrogen compensation effect (N-CE) for maize during recovery (post-pea harvest) periods: N-CE = NARim / (NARsm × Fm), where NAR is NA rate between sampling dates; assessed over 75–105 (V16–R1), 105–135 (R1–R4), 135–165 d (R4–R6) after maize sowing. N-CE >1 indicates compensation. Apparent N recovery (ANR, %): ANR = (NAwith N − NAwithout N) / Nrate × 100. Nitrogen utilization efficiency (NutE, g g⁻¹): NutE = GY / NA. Statistics: ANOVA (SPSS 17.0) with year, cropping system, and N rate as fixed effects; replication random; means separated by LSD at P≤0.05. Pearson correlations assessed relationships among N-CR/N-CE and NA, ANR, NutE.

• Grain yield (GY): • Pea: Intercropping Int-2 (120:80 cm) increased GY vs Int-1 (80:80 cm) by 12.7% (N0), 11.9% (N1), 11.1% (N2); no significant difference between N1 (90 kg N ha⁻¹) and N2 (135 kg N ha⁻¹) in either sole or intercropping. • Maize: Int-2 yielded 7.7% more than Int-1. In intercropping, N1 (300 kg N ha⁻¹) vs N2 (450 kg N ha⁻¹) showed no significant GY difference; in sole maize, N1 reduced GY by 7.4% vs N2.
• Nitrogen accumulation (NA): • Pea: Int-2 increased total NA by 5.7% vs Int-1. In sole pea, NA with N1 was 4.8% lower than N2; in intercropping, NA did not differ between N1 and N2. • Maize: Before 75 days (co-growth), intercropped NA < sole; after 75 days (post-pea harvest), intercropped NA > sole, indicating compensation. At maturity, Int-1 and Int-2 increased NA by 27.5% and 34.6% vs sole, respectively; Int-2 exceeded Int-1 by 5.5%. In sole maize, NA with N1 was 12.0% lower than N2; in intercropping, NA did not differ between N1 and N2.
• Nitrogen competitive ratio (N-CR, pea vs maize): Peaked at 45 days. Int-2 vs Int-1 increased N-CR by 7.1% (45 d), 4.5% (60 d), 4.1% (75 d). Lower N (N1) vs higher N (N2) increased N-CR by 5.8% (45 d), 7.5% (60 d), 5.0% (75 d). Average intensification from strip widening ~5.2%; from lower N ~6.1% (since 45 d).
• Nitrogen compensation effect (N-CE, maize): • 75–105 d: Int-2 +8.2% vs Int-1; N1 +5.7% vs N2. • 105–135 d: Int-2 +10.3%; N1 +5.5% vs N2. • 135–165 d: Int-2 exceeded Int-1 by ~11.5% (2009), 9.6% (2010), 9.5% (2011); N1 exceeded N2 by 5.9–11.2% depending on year/system.
• Apparent nitrogen recovery (ANR): Intercropping > sole for both crops. • Pea: N1 vs N2 increased ANR by 8.0% (sole), 10.0% (Int-1), 20.0% (Int-2). Int-1 and Int-2 exceeded sole by 15.9% and 25.2%, respectively. Int-2 exceeded Int-1 by 8.0%. • Maize: N1 vs N2 increased ANR by 7.0% (sole), 7.9% (Int-1), 11.4% (Int-2). Int-1 and Int-2 exceeded sole by 16.2% and 24.0%. Int-2 exceeded Int-1 by 6.8%.
• Nitrogen utilization efficiency (NutE): Intercropping > sole; maize > pea. • Pea: Int-1 +3.5%, Int-2 +12.7% vs sole; Int-2 +8.9% vs Int-1. N1 had the highest NutE, exceeding N0 by 19.7% and N2 by 6.0%. • Maize: N1 exceeded N2 by 4.9% (sole), 10.9% (Int-1), 12.0% (Int-2). Int-1 and Int-2 exceeded sole by 5.3% and 10.7%. Int-2 exceeded Int-1 by 5.1%.
• Correlations: N-CR positively correlated with pea NA (at 45 d), and with ANR and NutE of both crops; N-CE positively correlated with maize ANR and NutE. Indicates pea competition triggers improved fertilizer N uptake and conversion to grain, and maize compensation enhances N use and yield post pea-harvest.

The study addressed how altering spatial configuration and N inputs modulates interspecific interactions and N dynamics in maize/pea strip intercropping. Widening the maize:pea strip ratio intensified early N competition from pea (earlier-sown, dominant), which enhanced pea N accumulation—likely via stimulated N2 fixation under reduced soil mineral N—and improved pea ANR and NutE. After pea harvest, maize exhibited stronger N compensation, particularly in widened strips and under reduced N, allowing recovery from early suppression. This compensation, linked to root and canopy expansion into vacated pea strips and potential indirect N transfer, increased maize ANR, NutE, and final NA, translating into higher yields. Lowering N fertilizer rates maintained or improved N use metrics and yields compared to higher N, indicating that optimizing strip geometry combined with moderate N inputs can enhance N complementary use and system productivity while potentially reducing environmental N burdens.

Expanding the maize:pea strip width ratio from 80:80 cm to 120:80 cm and reducing N fertilizer rates intensified N competition during co-growth and improved post-harvest N compensation in maize. These interspecific interaction changes increased N accumulation, apparent N recovery, N utilization efficiency, and grain yield for both pea and maize. Specifically, compared with 80:80 cm, the 120:80 cm configuration increased pea NA by 5.7%, ANR by 8.0%, NutE by 8.9%, and GY by 11.9%; for maize it increased NA by 5.5%, ANR by 6.8%, NutE by 5.1%, and GY by 7.7%. Lower N rates further improved ANR and NutE without compromising yields. Future research should directly trace N flows (e.g., 15N labeling) to quantify N transfer and fixation contributions and test the approach across environments and management regimes to refine recommendations.

• The study did not directly trace nitrogen sources or transfers (e.g., via 15N isotopic techniques), limiting mechanistic attribution of increased N accumulation to biological N2 fixation vs soil/fertilizer N.
• Results are from a single semi-arid site in northwest China with specific management (plastic mulch on maize, drip irrigation, residue removal), which may limit generalizability to other climates, soils, and management systems.
• While conducted over three years, year × treatment interactions for some variables were not always significant; broader multi-site, multi-year validation would strengthen external validity.