A greenhouse study was conducted at Seibersdorf, Austria to simulate sorghum-cowpea cropping systems to quantify biological nitrogen fixation (BNF), estimate the nutrient budgets and yield advantages under low-P (Hungary) and medium-P (Waldviertel) soils. The soils were collected from Kompolt in Hungary and Waldviertel in Austria. 15N isotope dilution method was used to quantify BNF while a simple input/output model was used for budgeting. Hungarian soil produced significantly (P< 0.001) higher biomass than Waldviertel soil for both cowpea and sorghum in all cropping systems. However, crops grown in Waldviertel soil accumulated more N than on Hungarian soil. Despite the
apparent variations in total N fixed under the sole cowpea and intercrop cropping systems, cowpea derived 14 to 73% of their N from fixation. Intercropping cowpea with sorghum in both soils significantly (P < 0.001) reduced the biomass of cowpea by 30 to 50% and of sorghum by ~70% while increased %N derived from fixation by 20% in Hungarian soil only. Intercropping had > 1.0 land equivalency ratio than sole cropping systems. Exporting crop residues in all cropping systems led to nutrients mining while incorporating cowpea residues gave positive N balance. This study demonstrated the potential of intercropping to produce a sustainable cropping system through BNF and sparing of P within the systems.