Estimation of N2-fixation by sugarcane,Saccharum sp., and soybean,Glycine max,grown in soil with15N-labelled organic matter

Summary Two varieties of sugarcane, and nodulated and non-nodulated soybean isolines, were planted in a soil previously mixed with¹⁵N-labelled plant material. 45 days was allowed to elapse before planting, to permit initiation of organic matter mineralization. Plants were grown for 60 days, then harvested, dried, weighed and analysed for total N. Analysis of soil samples pre-incubated in the laboratory was carried out to evaluate ammonium and nitrate from added organic matter. Dry weights of the soybean isolines were similar, but total N was higher for the nodulated line. Both sugarcane varieties showed similar weight and total N. Nitrogen derived from applied organic matter (NdfOM) was higher in non-nodulated soybean than in all other plants. Although there is the possibility of different¹⁵N availabilities between species, nitrogen derived from fixation (Nfix) was calculated based on the¹⁵N enrichment of the non-nodulating soybean. Nfix was 72% for nodulating soybean and ranged from 19 to 39% for different parts of sugarcane plants, despite high levels of available-N. Nitrogen derived from soil was calculated by difference. NdfOM was lower in roots than in upper parts (leaves+stalks) of plants. Use of¹⁵N labelled organic matter seems a useful approach to the longer term measurement of N₂-fixation.IAEA Project BRA/5/009-CENA.