Faba bean productivity, yield stability and N2-fixation in long-term organic and conventional crop rotations

Increasing the production of grain legumes in Europe will contribute to protein self-sufficiency and provide direct and indirect environmental benefits, e.g., delivering ecosystem services such as N input via biological N-2 fixation (BNF). Faba bean (Vicia faba L.) is the main grain legume cultivated in Europe with increasing interest from the organic sector. Agronomic and economic obstacles exist to the inclusion of grain legumes in cropping systems but could be counterbalanced by accounting for the provision of ecosystem services. Thus, variations in productivity and BNF under different management need to be investigated. We assessed productivity, yield stability and BNF in a common faba bean variety (Boxer), grown for four years (2015-2018) in a long-term crop rotation field experiment at Foulum, Denmark. We compared conventional and organic cropping systems with spring cereals and faba bean, with and without long-term use of animal manure and cover crops. N derived from atmosphere (% Ndfa), determined with the 15N isotope dilution method, varied from 78% to 93% with significant effects of year and cropping system. Conventional treatments had the highest %Ndfa and yield, but the lowest yield stability. Organic treatments had problems with pests and diseases, mainly towards the end of the growing season. Quantity of BNF (qBNF) in aboveground biomass was on average 255 kg N ha(-1) in the organic and 334 kg N ha(-1) in the conventional systems, which would have been underestimated by up to 50 and 100 kg N ha(-1) respectively using standard literature %Ndfa values. A correct estimation of N input via BNF has economic and environmental implications (e.g., fertilization of following crops, N losses); thus, we recommend to account for the effect of management on %Ndfa and qBNF.