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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 15 N 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 À1 yr À1 ) 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 À54.3 for Babati sites. The mean N budget (kg ha À1 ) was À26.1 in the sole maize plots and À40.3 for the intercrops at the two locations in Malawi, and À50.1 in the sole crop plots and À51.1 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 À8.9 kg N for the sole maize in Malawi. For the same scenario in Tanzania, the budget was more negative (À35.4 kg N) for sole maize compared with intercrops (À5.9 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.

Maize (Zea mays L.) is a major staple food in Sub-Saharan Africa but low soil fertility, limited resources and droughts keep yields low. Cultivation of maize intercropped with pigeonpea (Cajanus cajan L. Millsp.) is common in some areas of eastern and southern Africa. The objectives of this study were (1) to investigate dry matter, nitrogen (N) and phosphorus (P) accumulation in different plant components of maizepigeonpea intercropping systems and (2) to report the effects of the intercrops on soil fertility. Maize-pigeonpea intercrops were compared to sole maize grown using farmers' practices. Intercropping maize and pigeonpea increased (P < 0.05) total system yield compared to sole maize in terms of biomass, N and P accumulation. Pigeonpea planted in maize did not reduce (P < 0.05) the accumulation of dry matter, N nor P in the maize grain. The harvest indices of maize, calculated on basis dry matter, N or P did not differ either (P < 0.05). Total soil C and N contents and inorganic N content, nitrate and ammonium, were not affected by two seasons of maize-pigeonpea intercropping compared to sole maize (P > 0.11). Nitrate and ammonium levels in soil were still not affected by the treatments after the soils were incubated in anaerobic conditions for 8 days at 37°C (P > 0.11). However, pigeonpea added up to 60 kg of N ha )1 to the system and accumulated up to 6 kg of P ha )1 and only 25% of this N and P were exported in the grain. In conclusion, beside the added grain yield of pigeonpea in the intercropped systems, pigeonpea increased the recirculation of dry matter, N and P, which may have a long-term effect on soil fertility. Furthermore, the stems from pigeonpea contributed to household fuel wood consumption. The intercropped system thus had multiple benefits that gave significant increase in combined yield per unit area without additional labour requirements.The main requirement in order to up-scale the maize-pigeonpea intercropping approach is sufficient supply of high-quality pigeonpea seeds.

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