Pollinators and intercropping help faba beans fix more nitrogen

Faba beans, like other legumes, can draw nitrogen directly from the air and convert it into a form they can use. They do this with thanks to a partnership with nitrogen-fixing bacteria that live in nodules on their roots. Farming practices and biotic interactions might affect this natural process.

Exploring how strip edges shape bean growth and nitrogen fixation

SLU researchers recently examined how strip-intercropping, the practice of planting different crops side by side, might influence the performance of faba beans. They wanted to know whether beans planted at the edges of intercropped strips behave differently from those growing in sole-cropping systems. Laura Riggi is one of the authors of the study:

– Edges are special places in agriculture: they often have more sunlight and airflow, they may encounter different soil conditions, and they are more accessible to insects such as bees. The question that we asked was: can these edge effects make a measurable difference in how well beans grow and fix nitrogen?

The answer, at least in this first round of experiments, appears to be yes. The researchers found that faba beans growing at the edge of the strip, neighbouring a strip of pumpkin plants, developed more root nodules, and the nodules were larger and more active than those in the sole crop. In fact, strip-intercrop plants had nearly half as much total nodule mass as sole crop plants. Yields were also higher in the strip-intercrop plants, pointing to a synergy between planting layout, pollination, and nodulation.

– We found that faba bean yield depends not just on pollination but also on how many of the plant’s root nodules are active in fixing nitrogen. In self-pollinated plants, having more active nodules helped close the yield gap by increasing the number of seeds per pod, says Chloë Raderschall, another author of the study.

Pollination and intercropping interact in unexpected ways to influence bean productivity

The researchers also observed more complex patterns depending on whether beans were grown in sole crop or strip-intercrop systems. In the sole crop, insect-pollinated plants showed a trade-off: higher nodulation sometimes coincided with fewer seeds. In intercropped systems, though, this trade-off disappeared. The authors caution that plant development stage and other factors might have influenced the results, so more detailed studies are needed. Still, these findings suggest that both pollinators and spatial crop diversity play important roles in boosting faba bean yield.

– While interactive effects between pollination, intercropping and nodulation on faba bean yield need to be confirmed, faba bean intercropping has been found to support crop yields and nodulation in other studies as well. In addition, increasing crop diversity has been shown to benefit several ecosystem services across systems such as pest control and biodiversity, concludes Laura.

Read more

The scientific study in the journal Agriculture, Ecosystems & Environment:
https://pub.epsilon.slu.se/27330/1/riggi_l_a_g_et_al_220317.pdf