1 Introduction

Centuries ago, before the Green Revolution, subsistence farmers relied on diverse cropping systems, such as intercropping, to boost food production. This age-old method enhanced natural plant synergies improving soil health and nutrient availability. A key feature of intercropping was the inclusion of legumes, which could fix atmospheric nitrogen (N) in the soil, thereby supporting the growth of neighboring non-legume crops. However, with the rise of industrial agriculture and the introduction of inorganic N fertilizers during the Green Revolution, this time-tested practice was largely replaced by monocropping systems. While large-scale monoculture significantly increased yields, it also led to the erosion of vital traits such as biodiversity and ecosystem resilience that were inherent in traditional intercropping systems [1]. Modern breeding programs aimed at optimizing plants for monocropping may have inadvertently diminished traits such as N fixation, below-ground N transfer, and natural crop synergies. Though traditional systems often produced lower grain yields per hectare, they offered greater ecological balance and resilience. In contrast, today’s emphasis on maximizing yield comes at the expense of sustainability and biodiversity—underscoring the need to reintegrate traditional practices like crop diversification into modern agricultural systems.

Intercropping, the practice of cultivating two or more crop species simultaneously on the same land area, has been widely recognized for enhancing agricultural productivity and sustainability [2]. Compared to monoculture, intercropped species often utilize resources such as light, water, and nutrients more efficiently due to their complementary growth patterns [3,4,5]. Among these systems, cereal-legume intercropping stands out for its multiple agronomic and ecological benefits. Legumes, when grown alongside cereals, not only contribute to higher overall yields but also improve crop health and end-use quality [6, 7], suppress weeds [8], reduce lodging [9], and optimize land-use efficiency [8]. Additionally, these systems offer vital ecosystem services, including soil and water conservation [10, 11]. A key advantage of cereal-legume mixtures lies in N dynamics: the cereal’s strong competition for soil inorganic N encourages the legume to increase symbiotic N fixation [12, 13]. This process significantly enhances soil N availability and supports the growth of companion crops while reducing dependence on synthetic N fertilizers [14,15,16]. A recent global study estimated that cereal-legume intercropping could reduce the global N fertilizer requirement by 26% compared to cereal monocultures, highlighting its potential to mitigate environmental impacts while supporting sustainable food production [12].

Building on the benefits of cereal-legume systems, pea (Pisum sativum L.) is a versatile legume that is often intercropped with cereals to enhance yield and end-use quality. When paired with crops like barley (Hordeum vulgare L.) [17, 18], oat (Avena sativa L.) [19], triticale (Triticum × Secale) [20, 21], and wheat (Triticum aestivum L.) [20], pea contributes significantly to system productivity. Among these, the wheat-pea combination is particularly notable due to their complementary growth patterns and N use strategies [22, 23]. Wheat aggressively utilizes soil mineral N, while pea relies on symbiotic N fixation, allowing both species to meet their N demands without direct competition [24]. Additionally, pea can supply a portion of wheat’s N requirement through below-ground N transfer facilitated by root contacts, exudate diffusion, and mycorrhizae [25]. These interactions, combined with the legume’s increased N fixation in the presence of a cereal, are central to the enhanced productivity seen in intercropping systems. Identifying pea cultivars with high N fixation and efficient N transfer potential is therefore critical, as these traits directly influence overall grain yield and end-use quality in cereal-legume mixtures.

Despite these advantages, most intercropping systems continue to rely on cultivars bred under monocropping conditions, which may not be well-suited to the interspecific interactions and resource-sharing dynamics of mixed cropping systems [26]. The performance of a cultivar as a sole crop does not necessarily predict its success in an intercrop setting, as intercropping introduces unique selection pressure stemming from interactions with neighboring species [27,28,29]. While plant breeding for intercropping remains underexplored, existing studies suggest that cultivar performance in mixtures can vary significantly, with some performing better and others poor compared to monoculture conditions [30,31,32]. This variability may reflect oversights in conventional breeding programs, where traits such as N fixation and belowground N transfer in pea were not prioritized for intercropping context. In this study, we hypothesized that pea cultivars released over the past six decades differ in their compatibility with wheat in an intercropping system. This study evaluated how cultivar-specific traits of pea influence yield metrics and N fixation inputs in pea-wheat intercropping systems using historical and modern pea cultivars under greenhouse conditions.

2 Materials and methods

2.1 Plant growth conditions and experimental design

The experiment was carried out under greenhouse conditions at the University of Alberta, Canada. The planting mix contained a mixture of Sunshine Mix #4 growing mix (composed of perlite, peat moss, and silicon, containing insignificant N or phosphorus nutritional value, physicochemical properties are given in [33]) (Sun Gro Horticultural Canada, Ltd., Vilna, AB, Canada) and sand (Target Products, Ltd., Crippsdale, AB, Canada) homogenized in a 3:1 ratio (v:v). The mixer was filled in 14 L plastic trays (43.2 cm × 28.3 cm × 16.5 cm) (Sterlite Corporation, Townsend, MA, USA), maintaining 5 kg/tray. Five different pea cultivars that were released between 1960 and 2016 were used in this study (Table 1): Century (1960) (developed by Agriculture Canada), Trapper (1970) (developed by Agriculture Canada), CDC Golden (2002) [34], CDC Amarillo (2012) [35], and CDC Spectrum (2016) [36]. Spring wheat cultivar Parata, which was released in 2016 by the University of Alberta wheat breeding program [37], was used as the companion cereal in the intercropping setup. Both pea and wheat seeds were surface-sterilized using 70% (v/v) ethanol for two minutes and subsequently soaked in 4% (v/v) sodium hypochlorite (NaOCl) for three minutes, followed by six rinses with sterile distilled water. Seeds were placed on moistened sterile filter paper in petri dishes. Pea seeds were inoculated with Rhizobium leguminosarum bv. viciae 3841 inoculum at 1 ml/petri dish in which rhizobial cell density was adjusted to OD600 = 0.1 [38]. After three days of pre-germination at room temperature under dark conditions, uniform pre-germinated seeds were selected and transplanted at a depth of 3 cm. Pea-wheat intercropping treatment was established with the five pea cultivars, wherein three monocropping treatments, including pea monoculture, wheat monoculture without N, and wheat monoculture with N, were also established. In all treatments (monoculture and intercrop), twelve seedlings were transplanted per tray. In intercrop treatments, seedlings were transplanted at 1:1 ratio (6 pea and 6 wheat) (7.6 cm plant spacing) in two rows (22.8 cm row spacing) in a mixed intercropping pattern (Fig. 1). Trays were arranged in a randomized complete block design with six replications and maintained at 24 ± 4 °C during the day and 18 ± 4 °C at night under 16 h light/8 h dark photoperiod and light intensity of 500 μmol photons m−2 s−1. After one week of growth, pea seedlings were re-inoculated with R. leguminosarum bv. viciae 3841 inoculum at 2 ml/plant in which rhizobial cell density was adjusted to OD600 = 0.1 [38]. To measure symbiotic N fixation, plants were labeled by adding 100 mL (per tray) of 0.5 mM K15NO3 (10 atom% 15N; 348481-25G; Sigma Aldrich, Oakville, ON, Canada) solution to the potting mix, two and three weeks after planting. Plants of pea-wheat intercropped, pea monoculture, and wheat monoculture (without N) were supplied with 200 mL of quarter-strength N-free Hoagland’s nutrient solution (HOP03-50LT, Caisson Labs, Smithfield, UT, USA) weekly. Wheat monoculture treatment with N was supplied with 200 mL of quarter-strength N containing Hoagland’s nutrient solution weekly (HOP02-50LT, Caisson Labs, Smithfield, UT, USA). Plants were watered every other day to keep the moisture level near field capacity.

Table 1 Morphological characteristics and agronomic traits of pea cultivars
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Fig. 1
figure 1

Planting arrangement of the intercropping (pea-wheat) and monocropping (pea, wheat) in plastic trays. Each symbol represents a plant of a different crop species: pea (X); wheat (O)

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2.2 Determination of plant growth traits

The dry matter of aboveground biomass was evaluated at the seed maturity stage of each species. The number of pea pods and wheat spikes was counted at harvest. All plants were harvested and sorted into pea and wheat. The seeds were threshed manually, and dry weight (per plant) was recorded after drying in a hot air oven at 65 °C for 5 days. Similarly, shoots were separated from the pods/wheat spikes and then dried in a hot air oven at 65 °C for 5 days until reaching a constant weight for dry weight measurements.

2.3 Determining the percentage of nitrogen derived from the atmosphere (%Ndfa)

Dry seeds of pea and wheat were ground separately into a fine powder using a coffee grinder. A representative sub sample of each ground sample was further pulverized into fine powder in a 2 mL microcentrifuge tube containing two steel beads, using a bead beater homogenizer (OMNI International, Kennesaw, GA, USA). After homogenization, 5 mg of the finely powdered seed material was weighed into a small tin capsule (8 mm × 5 mm, D1008, Isomass Scientific Inc., Calgary, AB, Canada) using a microbalance (Sartorius Quinti × 35-1S, Goettingen, Germany). The sample was compacted into a small pellet, ensuring it was free of air. The tin capsules were then arranged in a 96-well plate and submitted to the Stable Isotope Facility at Agriculture and Agri-Food Canada, Lethbridge Research and Development Centre, Canada to analyze 15N and total N%. The analysis was performed using an Isotope Ratio Mass Spectrometer (IRMS) coupled with a Flash 2000 Elemental Analyzer (Thermo Fisher Scientific, Voltaweg, The Netherlands) and Conflo IV (Thermo Fisher Scientific, Bremen, Germany) interface between the IRMS [39]. The percentage of N derived from the atmosphere (%Ndfa) was calculated according to the isotope dilution technique using the following formula [40]:

$$\text{\%Ndfa }= \left(1- \frac{\text{atom}{\text{\% }}^{15}{\text{N excess}}_{\left(\text{Pea}\right)}}{\text{atom}{\text{\% }}^{15}{\text{N excess}}_{\left(\text{Wheat}-\text{no N}\right)}}\right)\times 100$$
(1)

where atom% 15N excess = atom% 15N (pea or wheat) − 0.3663.

The amount of fixed N in the seeds was determined by multiplying the total seed N content by the %Ndfa, using the following formula:

Total seed N content of pea × (%Ndfa/100).

2.4 Determining the harvest index and land equivalent ratio (LER)

Harvest index (HI) was calculated according to the following formula:

$${\text{HI}} = \frac{\text{Seed dry weight}}{\text{Total biomass dry weight}}$$
(2)

Land use efficiency was calculated using the land equivalent ratio (LER), which compares the yield achieved by intercropping two species with the yield obtained in a monocrop cultivation. The LER was calculated according to Eq. (3) proposed by Mead and Willey [39]:

$${\text{LER}} = \left(\frac{{\text{SB}}_{\left(\text{Pea IC}\right)}}{{\text{SB}}_{\left(\text{Pea MC}\right)}}\right)+\left(\frac{{\text{SB}}_{\left(\text{Wheat IC}\right)}}{{\text{SB}}_{\left(\text{Wheat MC}\right)}}\right)$$
(3)

where, SBPea IC is the pea seed biomass under intercropping, SBPea MC is the pea seed biomass under monocrop, SBWheat IC is the wheat seed biomass under intercropping and SBWheat MC is the wheat seed biomass under monocrop (with N supplied). Additionally, the LERN was calculated using the same Eq. (3), but with the values of N content accumulated in the seeds.

2.5 Determining nitrogen carryover effect

To evaluate the effect of different pea cultivars on carryover N, canola plants were grown following the harvest of pea and wheat in both monocropping and intercropping stands. Six canola (Brassica napus L.) seeds (variety: A07-26NR) were planted in between the two rows of crop stubble. After two weeks of growth, canola plants were harvested by cutting at the base of the plants. The shoots were dried in a hot air oven at 65 °C for 5 days and dry weight was recorded.

2.6 Statistical analysis

All data were tested for normal distribution (using Shapiro–Wilk test function “shapiro.test” in base R), and with the assumption that the responses were from normal population distribution. The two-way ANOVA was applied to identify the effects of the crop (wheat and pea), cropping pattern (monoculture and intercropping), and their interactions on seed and pod dry weight, seed and pod number, seed N content, N fixation, total N fixed, HI, LER and LERN. Differences between means were determined for two-way interaction with a post hoc Fisher’s least significant difference test (α < 0.05). Each treatment comprised six replicates. Principal component analysis (PCA) was carried out to identify possible interrelations between parameters (traits) and groups (cropping pattern) using FactoMineR in R [41]. All analyses were conducted using R (version 4.4.3 (2025-02-28)) [42].

3 Results

3.1 Effect of monocropping and intercropping on yield parameters of different pea cultivars and wheat

All the parameters were expressed per plant basis. In general, yield parameters of pea, such as seed weight, number of pods, and number of seeds, were higher when grown in intercropping compared to monocropping (p < 0.05) (Fig. 2a–c). When Century and Trapper varieties were grown in intercropping, they showed an increase in seed weight [88.7% (p < 0.001) and 59.6% (p < 0.05)], number of pods [105% (p < 0.001) and 68.2% (p < 0.001)], and number of seeds [132.2% (p < 0.001) and 23.6% (p = 0.08)] compared to monocropping. For pea varieties, CDC Golden, CDC Amarillo, and CDC Spectrum, an increase in seed weight (56, 21.8 and 23.2%), number of pods (42.9, 23.8 and 26.3%), and number of seeds (47, 26.9 and 25.3%) were observed in intercropping compared to their monocropping, but this increase was not significant (Fig. 2a–c).

Fig. 2
figure 2

Boxplots and bar graphs of the yield parameters of different pea cultivars and wheat grown in monocropping and intercropping. Graphs on the left-side of the panel show the data for different pea cultivars while graphs on the right-side show wheat data. a Seed dry weight (per plant) of pea/wheat intercrop and mono-crop. b Number of pods/spikes (per plant) of pea/wheat intercrop and mono-crop. c Number of seeds (per plant) of pea/wheat intercrop and mono-crop. d Harvest index of pea/wheat intercrop and mono-crop. Bold horizontal lines inside boxplots represent median values. The lower and upper ends of the boxes represent the first and third quartiles, respectively; vertical lines extend to the most extreme data points. Different letters above the boxes/bars represent significant differences among treatments according to two-way ANOVA and Fisher LSD test (p ≤ 0.05). Each treatment comprised six replicates. Means followed by the same letter are not significantly different. CW, TW, GW, AW, and SW are different pea cultivars (Century-C, Trapper-T, CDC Golden-G, CDC Amarillo-A, CDC Spectrum-S) intercropped with wheat-W cultivar Parata. −N = wheat mono-crop without nitrogen; +N = wheat mono-crop with nitrogen

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In comparison to wheat monoculture with N application, a significant reduction in wheat seed dry weight per plant was observed when intercropped with pea varieties Century (−53.5%, p < 0.001), Trapper, (−45.2%, p < 0.01) and CDC Amarillo (−33.9%, p < 0.05) (Fig. 2a). In contrast, when wheat was intercropped with pea varieties CDC Golden and CDC Spectrum, there was no significant difference in seed dry weight compared to wheat monoculture with N application. The number of spikes in wheat when intercropped with older pea cultivars such as Century, Trapper and CDC Golden was reduced by at least 36% compared to wheat monoculture with N application (p < 0.001) (Fig. 2b). However, the number of wheat spikes when intercropped with the newer cultivars CDC Amarillo and CDC Spectrum were not different compared to wheat monoculture with N application. Similarly, the number of wheat seeds per plant showed a significant reduction by at least 38% when intercropped with any of the five pea cultivars (p < 0.001) compared to wheat monoculture with N application (Fig. 2c).

The HI of pea varieties Century and Trapper grown in intercropping was significantly higher [37.8% (p < 0.01) and 24.2% (p < 0.05)] compared to their monocropping, whereas no differences were observed for other pea cultivars (Fig. 2d). For wheat, the HI was higher only when intercropped with CDC Golden as compared to the wheat grown in monoculture without N application (p < 0.05).

3.2 Effect of monocropping and intercropping on nitrogen parameters

The seed N concentration of pea variety Trapper was significantly higher (14.9%, p < 0.05) when grown in intercropping compared to monocropping, with no difference observed in pea varieties Century, CDC Golden, CDC Amarillo, and CDC Spectrum (Fig. 3a). In wheat, seed N concentration was significantly higher by at least 23% when intercropped with any of the pea varieties except CDC Golden (p < 0.01) compared to the wheat grown in monoculture without N application (Fig. 3a). In contrast, the seed N concentration in wheat in intercropping was significantly lower by at least 14% for all pea cultivars than the monoculture wheat with N (p < 0.05) (Fig. 3a).

Fig. 3
figure 3

Boxplots of symbiotic nitrogen fixation parameters of different pea cultivars, and seed nitrogen concentration and seed nitrogen content of wheat grown in monocropping and intercropping. The left-side graphs in panels A and B show the data for different pea cultivars while the graphs on the right side of the panel show data for wheat. a Seed nitrogen concentration of pea/wheat intercrop and mono-crop. b Seed total nitrogen content (per plant) of pea/wheat intercrop and mono-crop. c Percentage nitrogen derived from the atmosphere in pea intercrop and mono-crop. d Seed total nitrogen fixed (per plant) under pea intercrop and mono-crop. Bold horizontal lines inside boxes represent median values. The lower and upper ends of the boxes represent the first and third quartiles, respectively; vertical lines extend to the most extreme data points. Different letters above the boxes represent significant differences among treatments according to two-way ANOVA and Fisher LSD test (p ≤ 0.05). Each treatment comprised six replicates. Means followed by the same letter are not significantly different. Pea cultivars are C = Century, T = Trapper; G = CDC Golden; A = CDC Amarillo; S = CDC Spectrum; CW, TW, GW, AW, and SW are pea-wheat (cultivar; Parata) intercropping combinations; −N = wheat mono-crop without nitrogen; +N = wheat mono-crop with nitrogen

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The total seed N content per plant in different pea varieties was higher in intercropping compared to monocropping, but a significant difference was observed only in Century (106.7%, p < 0.001) and Trapper (38%, p < 0.05) (Fig. 3b). In wheat, the total seed N content per plant was found to be significantly higher when intercropped with CDC Golden and CDC Spectrum (62.5%, p < 0.05 and 59.5%, p < 0.05, respectively), whereas no significant difference was observed when intercropped with other pea cultivars compared to the monoculture wheat without N application (Fig. 3b). However, in comparison to wheat grown in monoculture with N, total seed N content of wheat was found to be significantly lower when intercropped with pea varieties Century (73.4%, p < 0.001), Trapper (63.5%, p < 0.001), CDC Golden (47.8%, p < 0.001), CDC Amarillo (58.6%, p < 0.001), and CDC Spectrum (48.8%, p < 0.001) (Fig. 3b).

The percentage of N derived from the atmosphere (%Ndfa) was significantly higher in pea varieties Century CDC Golden, CDC Amarillo, and CDC Spectrum (p < 0.001) when grown in intercropping, compared to those grown in monoculture (Fig. 3c). Seed total N fixed by different pea varieties was also higher when grown in intercropping compared to monocropping, but a significant difference was observed only for Century and Trapper (p < 0.05) (Fig. 3d).

3.3 Effect of monocropping and intercropping on land equivalent ratio (LER) and nitrogen-based LER (LERN)

The land equivalent ratio was above one when wheat and peas were grown in intercropping (Fig. 4a). Although there was no significant difference among different pea cultivars for LER and LERN, there was a clear trend that the oldest pea cultivar Century had lower LER and higher LERN than all other cultivars, suggesting there is better use of land due to intercropping with newer cultivars but new cultivars are not capturing as much N as the oldest one (Fig. 4b).

Fig. 4
figure 4

Bar graphs of land equivalent ratio (LER) and nitrogen-based LER (LERN) under pea-wheat intercropping. a Land equivalent ratio (LER). b Land equivalent ratio for N (LERN). The relative contribution of pea and wheat in the LER and LERN are also shown. Different letters in the stacked bars represent significant differences in partial LER and LERN among treatments according to two-way ANOVA and Fisher LSD test (p ≤ 0.05). Each treatment comprised six replicates. Pea cultivars are C = Century, T = Trapper; G = CDC Golden; A = CDC Amarillo; S = CDC Spectrum; CW, TW, GW, AW, and SW are pea-wheat (cultivar; Parata) intercropping combinations

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3.4 Nitrogen carryover effect by pea cultivars

When canola was grown following the harvest of different pea cultivars under monocropping, it showed a significant increase in shoot dry weight [Century (216.8%, p < 0.05), Trapper (426%, p < 0.001), CDC Golden (187%, p < 0.05), and CDC Spectrum (243.2%, p < 0.001)] compared to the canola plants grown in soil where the previous crop was wheat monoculture with N (Fig. 5). However, when canola plants were grown following the harvest of different pea cultivars from intercropping, only the cultivar Century showed a significant increase in shoot dry weight (170.2%) compared to the plants grown in trays where the previous crop was wheat monoculture with N (Fig. 5).

Fig. 5
figure 5

Boxplots comparing the shoot dry weight of canola grown after pea/wheat intercropping versus monocropping. Bold horizontal lines inside boxes represent median values. The lower and upper ends of the boxes represent the first and third quartiles, respectively; vertical lines extend to the most extreme data points. Different letters above the boxes represent significant differences among treatments according to two-way ANOVA and Fisher LSD test (p ≤ 0.05). Each treatment comprised six replicates. Means followed by the same letter are not significantly different. Pea cultivars are Century, Trapper; CDC Golden; CDC Amarillo; CDC Spectrum; −N = wheat mono-crop without nitrogen; +N = wheat mono-crop with nitrogen

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3.5 Interrelationship between yield attributes influenced by monocropping and intercropping

In the PCA biplot analysis, the first two components captured 80% of the variance across the eight traits of pea cultivars grown in both monocropping and intercropping systems (Fig. 6). The intercropped pea cultivars formed a distinct cluster, clearly separated from those grown in monocropping. Several traits, including seed weight, seed N fixed, seed N content, number of pods, and number of seeds, showed a strong positive correlation in intercropped pea (Fig. 6a). The HI and %Ndfa also showed a positive correlation; however, both traits had a weak correlation with total seed N concentration (Fig. 6a). In the PCA biplot for wheat, the first two components captured 84% of the variance across the six traits. Notably, monoculture wheat supplied with N formed a separate cluster, while intercropped wheat partially overlapped with monoculture wheat that was not supplemented with N (Fig. 6b). Similar to the findings for peas, the wheat biplot revealed positive correlations among seed weight, number of seeds, and seed N content, while the HI displayed a weak correlation with seed N concentration.

4 Discussion

Intercropping offers a promising pathway to more sustainable agriculture by improving land-use efficiency, enhancing resource capture, and reducing the reliance on external inputs. In this study, we evaluated five pea cultivars released over the past six decades to determine whether breeding history influences intercropping performance with wheat. Our findings highlight cultivar-specific differences among the selected pea cultivars in both yield components and N status, which reflect underlying physiological and phenotypic traits that affect resource sharing and competition dynamics in intercropping systems.

4.1 Yield components

Yield characteristics such as seed dry weight, number of pods, and number of seeds per plant collectively illustrate the cultivar-specific responses of peas to intercropping. In this study, older pea cultivars like Century and Trapper demonstrated superior yield attributes compared to newer cultivars (CDC Golden, CDC Amarillo, CDC Spectrum), especially when intercropped with wheat. This aligns with previous research showing that older, normal-leaf pea cultivars often perform better in intercropping systems, likely due to their greater competitiveness against cereals, which reflects traits selected before the widespread shift toward monoculture breeding [43, 44]. In pea-dominated intercrops, the large leaf area of normal-leafed pea cultivars can cause shading, reducing light availability and photosynthesis in intercropped wheat, thereby hindering its growth and resource use efficiency [45]. However, the semi-leafless trait, selected to improve monocrop performance in newer cultivars, appears to reduce pea dominance by limiting the competition for light and other resources. In mixed-cropping systems involving cereals and semi-leafless peas, cereals tend to be more competitive than peas, often resulting in reduced overall yields primarily due to a decline in pea productivity [46, 47]. This trend was also observed in the present study, where newer, semi-leafless cultivars such as CDC Golden, CDC Amarillo, and CDC Spectrum have exhibited reduced yield attributes compared to the older, normal-leaf cultivars such as Century and Trapper (Fig. 2). These contrasting performances highlight the role of niche complementarity as a key mechanism driving cultivar-specific outcomes in intercropping systems. Older cultivars may occupy different ecological niches by fixing more N or intercepting light differently over time, while newer cultivars with reduced leaf area may enable better light penetration and resource sharing with wheat. Such complementary use of space and time can improve total system efficiency, though it is highly dependent on the specific trait combinations of the cultivars involved.

Despite the observed yield reductions, the HI of peas remained consistent across cultivars (Fig. 2d). Although semi-leafless cultivars are generally considered less competitive than their normal-leafed counterparts, largely due to reduced leaf area [48], this study found no significant differences in HI between monocropping and intercropping setup, with the exception of Century. This suggests that, regardless of cropping system, most cultivars maintain a similar efficiency in partitioning biomass to seed production. Leaf area is a key driver of cultivar performance in intercropping, largely due to its role in light interception and canopy structure [49, 50]. Normal-leaf cultivars possess a more complex leaf morphology expansive and complex leaf, including stipules, petioles, leaflets, and tendrils, all of which enhance their ability to intercept light and outcompete neighboring plants [51,52,53]. This structural advantage may explain their superior performance in intercropping systems where light becomes a limiting resource.

Following the analysis of pea yield dynamics, the performance of wheat within these intercropping systems also revealed cultivar-specific interactions. In wheat, seed dry weight, number of spikes, and number of seeds increased when intercropped with newer pea cultivars, compared to intercropping with older ones (Fig. 2a–c). This pattern reflects the lower competitive ability of the newer pea cultivars, allowing wheat, a naturally strong competitor, to capitalize on available resources in the intercropping setup [54, 55]. Such outcomes reflect a common pattern in intercrops where cereals outperform legumes due to competitive asymmetry, particularly when legume partners exhibit weaker resource acquisition strategies [56,57,58,59]. While intercropping is often driven by niche complementarity, such interactions rely on a delicate balance that can easily shift toward competition [60]. In this study, wheat exhibited significantly improved yield attributes when grown alongside newer pea cultivars, highlighting its capacity to benefit from reduced interspecific competition and potentially more effective resource partitioning in these pairings.

Beyond the immediate interactions within the intercropping phase, the legacy effects of legumes such as pea on subsequent crops also play a critical role in cropping system performance. In this study, canola grown following pea monocultures exhibited higher seedling shoot dry weight after two weeks of growth compared to those following wheat monocultures or pea–wheat intercrops (Fig. 5). This observation aligns with earlier findings that highlight the positive effect of preceding pea crops on the early growth of canola [61]. These benefits are largely attributed to enhanced soil N availability through symbiotically fixed N by pea plants. Due to the lower C:N ratios in pea residues, they decompose more rapidly, contributing to faster nutrient turnover and residual N availability. Legumes also conserve soil mineral N by relying on symbiotic fixation, a phenomenon known as the N-sparing effect [62]. Together, these processes improve soil fertility and support stronger early growth in follow-up crops like canola.

4.2 Ndfa, seed nitrogen content, and total fixed nitrogen

Previous research has demonstrated that symbiotic N fixation tends to increase in pea–wheat intercropping systems compared to pea monocultures [54, 56, 63]. A similar pattern was observed in this study, where intercropping significantly enhanced %Ndfa in peas (Fig. 3c). Although seed N concentration remains relatively stable across different treatments, seed N content and total N fixed were significantly higher in older pea cultivars compared to newer semi-leafless types under intercropping (Fig. 3b–d). This trend closely mirrors the differences in dry matter yield, suggesting that the older cultivars not only produce more biomass but also contribute more fixed N to the system. These results are consistent with previous studies reporting reduced seed and straw N content in semi-leafless cultivars [64, 65]. The observed increase in N fixation can be attributed to the competitive nature of wheat for soil N, which likely forces the legume partner, peas, to meet its N demand by enhancing symbiotic N fixation [8, 66, 67]. This pattern of N dynamics was further influenced by the growth medium used in this experiment. The 3:1 (v/v) sand to Sunshine mix created a relatively N-deficient environment, intensifying the competition for soil N. Under these conditions, wheat exhibited a clear response to N fertilization in both monocrop and intercrop systems, whereas unfertilized wheat monocrops showed markedly limited growth. This outcome reinforces the competitive dominance of wheat for soil N and illustrates how nutrient availability can shape interspecies interactions, drive shifts in N fixation by legumes, and ultimately impact the overall productivity of the cropping system.

In this experiment, wheat grown in intercropping had a significantly higher seed N concentration than wheat monoculture without N (Fig. 3a). This increase likely reflects both the partial transfer of fixed N from peas [63], and the improved N use efficiency typical of intercropping systems compared to monocultures [54, 68]. However, wheat monocrops with added N still had the highest seed N concentration, regardless of the pea cultivar used in intercropping (Fig. 3a). Notably, when intercropped, wheat accumulated more seed N when grown with newer pea cultivars compared than with older cultivars (Fig. 3b). This indicates that newer pea cultivars may enhance N transfer to wheat, even though their N fixation did not differ significantly. N transfer occurs primarily through three mechanisms: (i) mineralization: wherein N is released during the decomposition of legume roots and nodules; (ii) root exudation: where N-containing compounds secreted by legume roots are directly taken up by non-legumes; and (iii) mycorrhizal facilitation: in which symbiotic fungi transfer N across plant species [69, 70]. In this experiment, it is possible that N transfer from pea to wheat occurred through the above three routes. Wheat’s advantage in these systems may also stem from its extensive root system and enhanced soil exploration capacity for N [68]. These results align with previous studies showing cereals to be dominant competitors for N in cereal-legume mixtures [67, 71].

According to the PCA analysis, intercropping enhanced both seed yield and seed N content in pea and wheat (Fig. 6). Moreover, traits such as number of seeds, number of pods, and seed yield showed positive correlations with parameters related to N fixation, indicating that N fixed by pea through the symbiotic N fixation was transferred to wheat when grown in the intercropping system.

Fig. 6
figure 6

Principal Component Analysis (PCA) -biplot analysis representing the clustering of dry matter production, yield components, and nitrogen-related traits. The biplot on the left side (a) of the panel shows the data for different pea cultivars, while the biplot on the right side (b) of the panel shows data for wheat. The magnitude of the vectors (lines) shows the strength of their contribution to each PC. Vectors pointing in similar directions indicate positively correlated variables, vectors pointing in opposite directions indicate negatively correlated variables, and vectors at proximately right angles indicate low or no correlation. Colored concentration ellipses (size determined by a 0.95 probability level) show the observations grouped by mark class

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4.3 LER and LERN

The dry matter yield is reflected in the LER, which was highest with newer pea cultivars, especially reaching 1.72 in CDC Golden (Fig. 4a). Consistent with previous studies, LER values greater than one indicate greater resource use efficiency in intercropping systems compared to monocultures [29, 72, 73]. Partial LER (pLER) revealed cultivar-specific dynamics; CDC Golden, CDC Amarillo, and CDC Spectrum showed a decline in pLER under intercropping when compared to Trapper, while wheat pLER increased when paired with newer pea cultivars. This suggests that wheat becomes more competitive and contributes more to total LER index, as reflected in its shoot dry matter gains. Some studies have found that intercropping can reduce overall yield due to interspecies competition, though that was not the case in our study [2, 74]. While modern pea cultivars generally showed higher LER values and produced the higher yields in some previous studies [75, 76], our findings also reveal that traditional cultivars can outperform them under certain conditions.

Values of LERN also exceeded one in most treatments (Fig. 4b), supporting prior findings that cereal-legume intercrops are more efficient at acquiring N than monocultures [71, 77, 78]. This advantage is likely due to complementary N acquisition, where wheat relies on soil N while peas fix atmospheric N. While LERN values were above 1 with older cultivars, most newer cultivars (except CDC Golden) had LERN values near or below unity, suggesting cultivar-specific variation in N dynamics despite improved biomass productivity. Additionally, studies have reported higher LER and greater N transfer from legumes to cereals in such systems, reinforcing the efficiency of these complementary interactions [63].

Our findings highlight cultivar-specific differences in yield and N status, reflecting traits that influence resource sharing and competition in intercropping systems. Older pea cultivars, with prolonged vegetative growth and delayed senescence, likely enhanced symbiotic N fixation by providing a longer N-fixing window, benefiting both the pea plant and the system. In contrast, modern cultivars, optimized for monocultures, often have more compact architectures and earlier maturity, reducing competition with wheat but potentially limiting pea biomass and total N fixation in shoots and seeds. This trade-off emphasizes the importance of matching cultivar traits to both productivity and functional roles in intercropping systems.

A key limitation of this study is that N fixation was measured only in seeds, potentially underestimating total N fixation. This could affect N management recommendations in intercropping systems. Additionally, the use of a horticultural growing mix combined with sand in a greenhouse study may not fully replicate real-world field soil conditions, which could affect nutrient availability and N fixation. Lastly, the use of a single wheat variety limits the applicability of the findings. Future studies should track N fixation at multiple stages to better capture its dynamics, incorporate a wider range of wheat and pea cultivars to enhance generalizability, and use field soils to represent field soil conditions. Field validations under diverse agroclimatic conditions are also needed to confirm the practical applicability of these findings.

5 Conclusion

In conclusion, this study emphasizes the importance of pea cultivar selection in pea-wheat intercropping systems, revealing distinct differences between older and newer cultivars. Older pea cultivars excelled in terms of yield and total N fixation, while newer cultivars improved the yield and seed N content of the companion wheat crop. The improved quality of wheat in intercropping with newer pea cultivars may provide economic benefits for farmers by increasing seed protein content. Overall, the findings suggest that newer pea cultivars offer better complementarity in intercropping systems, enhancing overall productivity and resource efficiency.