Biological nitrogen fixation and nitrogen and phosphorus budgets in farmer-managed intercrops of maize-pigeonpea in semi-arid southern and eastern Africa

Biological nitrogen fixation (BNF), nitrogen (N), and phosphorus (P) imports-exports budgets were estimated at four locations, each with 20 farmer-managed fields for two years in a semi-arid Tanzania and Malawi. The N-15 isotope dilution method was used to quantify BNF by three pigeonpea (Cajanus cajan L. Millspp.) varieties intercropped with maize (Zea mays L.). The N and P accumulation in plant components of sole maize and intercrops of maize-pigeonpea systems were used to estimate the mean exports and imports of N and P. The proportion of N derived from air (%Ndfa) by the pigeonpea varieties ranged from 93.8% to 99.9% in Malawi and 65.6% to 99.3% in Tanzania. The amount of fixed N (BNF; kg N ha(-1) yr(-1)) varied from 37.5 to 117.2 in Malawi and 6.3 to 71.5 in Tanzania. The mean values for BNF during the two cropping seasons were 64.3 for Nyambi, 85.3 for Ntonda, 34.1 for Gairo and -54.3 for Babati sites. The mean N budget (kg ha(-1)) was -26.1 in the sole maize plots and -40.3 for the intercrops at the two locations in Malawi, and -50.1 in the sole crop plots and -51.1 in the intercrops at the locations in Tanzania. In a scenario where all the aboveground material except the edible parts was returned to the soil, a positive value of 30.5 kg N for the intercrops was recorded compared with -8.9 kg N for the sole maize in Malawi. For the same scenario in Tanzania, the budget was more negative (-35.4 kg N) for sole maize compared with intercrops (-5.9 kg N). Including the roots in the calculations, did not change the differences between mono and intercrops. The P budget was negative irrespective of whether the aboveground biomass of maize and pigeonpea was incorporated or exported out of the fields, and the values were similar for intercrops and sole maize. The most negative N and P budgets were recorded in the two study areas where the extractable soil P status of the soils and the maize yields were high. These findings indicate that pigeonpea incorporated into maize-based cropping systems will maintain a very high %Ndfa ( > 90%) in all plant parts and thereby contribute to improved N budgets but not increase the proportion of P mined of the soil.