Soybean relay-cropped with winter camelina reduces biological nitrogen fixation

The contribution of biological nitrogen fixation (BNF) in soybean [Glycine max (L.) Merr.] to plant nutrition and its N credit for the next crop is significant. The amount of N fixed via BNF in monocropped soybean is well documented, but it is unknown when soybean is relayed into winter camelina [Camelina sativa (L.) Crantz]. Therefore, BNF in soybean relayed into fertilized (WCFerti) and unfertilized (WCUnfert) winter camelina grown for seed, and soybean seeded after winter camelina terminated as a cover crop (WCCC) were compared with a monocrop (control) soybean. The results showed that N-fixing bacteria successfully infected soybean roots and were potentially active in N fixation for all treatments. Ureide-N abundance was in the order of control > WCFerti = WCUnfert > WCCC. The control had 38 and 44% more ureide-N (indicating greater BNF) than WCFerti and WCUnfert, respectively. Relayed soybean had the same grain protein and oil concentrations as the control. However, the control had greater soybean grain, protein, and oil yields than relayed treatments due to grain yield differences. Fertilized winter camelina resulted in greater total (soybean + camelina) oil yield (859 kg ha^–1) than the control (587 kg ha^–1). The lower ureide-N abundance and grain yields for relayed soybean than the control was due to differences in belowground and aboveground biomass. Further research is needed to develop earlier-maturing winter camelina genotypes, better row spacing, and more shade-tolerant soybean cultivars to minimize competition between the two species, enhance BNF, and increase relayed-soybean yield.