The interest in growing pulse-oilseeds together as intercrop combinations in western Canada is increasing, but little is known about the nutrient dynamics operative that drive the observed synergies. Two promising pulse-oilseed combinations (chickpea-flax and dry pea – white mustard) were grown as intercrops in mixed and alternate row configurations in two contrasting soil zones (Brown Chernozem and Black Chernozem) in southern Saskatchewan in 2019 and 2020 without added fertilizer and compared to traditional monocrop system. Comparison was made of grain and straw yields and N and P uptake, proportion and amount of biologically fixed nitrogen (BNF) contributed from the pulse crops, and the transfer of fixed nitrogen to the oilseed crops to their corresponding monocrops. Total land equivalent ratios (LER) were calculated for grain yield and grain N and P uptake for the intercrops to determine benefit to intercropping versus monocrops. Partial LERs for each crop of the intercrops were calculated to evaluate dominance of each crop within the intercrop system. The two pulse-oilseed intercrops grain yield LERs, nitrogen uptake LERs, and phosphorus uptake LERs were at or above 1 for the four site-years, indicating benefit from intercropping in increasing total yield, N and P uptake from a land area.. The proportion of N derived from BNF was not enhanced in the two intercrop combinations, but significant biologically fixed N (9% – 41%) was transferred from the legume to the oilseed crops that would significantly reduce depletion of soil N and contribute additional N in the intercrops compared to monocrops. The partial LERs showed that flax was dominant and chickpea subordinate in chickpea-flax intercrops for all four site-years. In contrast, dry pea was dominant and mustard subordinate for the two Redvers, SK site-years while mustard was more competitive at the two Central Butte, SK site-years. This was attributed to pea as a nutrient scavenger having a competitive advantage under the low P fertility conditions in Redvers soil. Early establishment of chickpea appeared to be an important factor in preventing flax dominance.

Supporting evidence for synergy in nutrient availability in the intercropping systems was found in consistently greater concentrations of soluble organic carbon and nitrogen found in the root zone of the legume crop partners (chickpea, pea) alone and in mixes with the non- legume partners (flax, mustard) at both Redvers and Central Butte sites in both years. Soluble nitrogen that is released in the legume rhizosphere serves as substrate for microorganisms to decompose and mineralize to plant available N, transferring inorganic N to the non-legume where it may be taken up and utilized as an additional N source in the system derived from fixation. Greater soluble organic carbon may enhance solubilization of P through complexation reactions and increase P availability. Supporting the enhancement of greater nutrient availability by the legume partner, greater PRS nitrate supply rates were observed in the root zone of the legume mixes over the season and occasionally greater P supply rates as well. The colonization of roots by AMF fungi overall was high (>90%) and not greatly affected by the intercropping system or arrangement. However, there were one or two incidences where the colonization was slightly but significantly enhanced in the flax when it was present in combination with the chickpea. Less N and P drawdown observed with legume partners, especially chickpea, supported the ability of this crop to help maintain soil N and P supplies and provide some additional nutrient to the non-legume partner when grown together.

Overall, the results of this study show benefits can be realized from intercropping of chickpea-flax and pea-mustard in yield and nutrient utilization efficiency from a land area, improved nutrient use efficiency, and synergies. The transfer of N derived from fixation by the legume to the non-legume partner appears to be an important process supplying additional N, and the solubilization of P through microbial processes may also be slightly enhanced in the legume-oilseed intercropping systems that were evaluated.