Symbiotic N2 fixation by soybean in organic and conventional cropping systems

Summary in the original language of the document

In organic cropping systems nitrogen (N) often limits agricultural production. N2 fixing
crops present an important option to improve N supply and to maintain soil fertility. We
investigated N2 fixation of soybean in conventional and organic cropping systems. The
study was carried out on a long term field experiment, characterized by unequal
fertilization rates and soil microbial activity for the different systems. We assessed the
proportion of N derived from atmosphere (Ndfa) and the total amount of N
symbiotically fixed (Nfix) using the 15N natural abundance method. Ndfa for soybean
was low, ranging from 24 to 54%. The lowest Ndfa was reported for the exclusively
mineral fertilized, and the highest for the bio-organic cropping system. However, there
were no differences between the farming systems in total amount of N symbiotically
fixed. Irrespective of the cropping system, N withdrawal by harvest was higher than N
input by N2 fixation, meaning that soil N stock was not preserved.