Biological nitrogen fixation and intra- and interspecific competition in two vegetable/legume intercropping systems and their relationship to crop yield

Legume-based intercropping systems are a beneficial alternative for soil functioning, and rhizobacterial community diversity, while can contribute to reduce external inputs. Hence, the aims of this work were to assess: (i) the biological N fixation (BNF) process of cowpea and fava bean under different intercropping patterns compared to monocultures, (ii) the relationship between biologically fixed N and the crop biomass and yields, and (iii) the possible intra- and interspecific competition processes in each intercropping pattern. We compared, for three years, the monocultures of melon and cowpea with three different melon-cowpea intercropping patterns, and the monocultures of broccoli and fava bean with three different broccoli-fava bean intercropping patterns. The different intercropping patterns were: mixed intercropping, row intercropping 1:1 (vegetable:legume), and row intercropping 2:1 (vegetable:legume). Fertilization was reduced by 30 % in the diversified plots. After three years, BNF was higher in cowpea under monoculture (16.55 kg N ha-1), while fava bean showed higher BNF under intercropping row 1:1 (81.49 kg N ha-1). The relative crop yield (g plant-1) and plant biomass (g plant-1) were sensitive indicators to assess the intra- and interspecific interactions. Cowpea showed higher relative yield under mixed intercropping (41.45 g plant-1), coinciding with lower values of N derived from the air (Ndfa = 16.59 %). Fava bean showed the lowest relative yield in the mixed intercropping (195 g plant-1), coinciding with the highest value of Ndfa (92.71 %). The BNF by fava bean was positively correlated with the relative broccoli yield, not observed for cowpea/melon crop. Cowpea showed no effects of inter- or intraspecific competition with similar relative crop yield and plant biomass in monocultures than intercrops. On the contrary, fava bean was more sensitive to inter- and intraspecific competition, with increased biomass and relative crop yields when plant density was low. As a consequence, for fava bean, intercropping row 1:1 and 2:1 contributed to highest relative yield (551 g plant-1 and 613 g plan-1, respectively) and biomass (181 g plant-1 and 101 g plan-1, respectively), just the patterns with lower inter- and intraspecific competition. This research provides insights about how different legumes behave differently when grown intercropped with vegetables, in terms of NBF, growth and yields: competition and facilitation processes are different when comparing fava bean and cowpea grown as intercrops. Both legume species can be recommended for intercropping systems with a reduction in fertiliser rates and no negative effect on cash crops. However, excessive interspecific competition may reduce fava bean biomass and yields, not observed with cowpea intercropped with melon.