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Improved management of nitrogen (N) in low N soils is critical for increased land productivity and economic sustainability. We report results of a rainfed rotation experiment, conducted in the Northwest Frontier Province (NWFP), Pakistan, during 1995-1999 to evaluate effects of residue retention and fertiliser N on N2 fixation inputs and yields of a mungbean (Vigna radiata)-wheat (Triticum aestivum) sequence, and a lentil (Lens culinaris)-summer cereal sequence. Mungbean and sorghum (Sorghum bicolor) or maize (Zea mays) were grown in the summers and lentil and wheat in the winters. Immediately after grain harvest, above-ground residues of all crops were either completely removed (−residue), or chopped into 5-20 cm pieces, spread across the plots and incorporated by chisel plough (+residue). Fertiliser N rates were nil or 120 kg N/ha for wheat and nil or 150 kg N/ha for sorghum/maize. The percentage of mungbean N derived from N2 fixation (%Ndfa) ranged from 47% to almost 100% (mean of 75%). On average, mungbean fixed 112 kg N/ha (+residues) and 74 kg N/ha (-residues), with N balances of +64 kg N/ha (+residues) and +9 kg N/ha (-residues). Lentil %Ndfa ranged from 50 to 87% (mean of 73%). Values for crop N fixed were 42-85 kg N/ha, with a mean of 68 kg N/ha. Average N balances for lentil were +27 kg N/ha (+residues) and +16 kg N/ha (-residues). Grain yields of the 0N wheat responded to the previous mungbean (36% increase over the 0N sorghum), but showed an even greater response to fertiliser N applied to the previous sorghum (150% increase). Highest yields were recorded for the N-fertilised wheat (average of 2.27 t/ha). Shoot biomass yields of the 0N sorghum and maize responded strongly to the previous lentil crop (49% average increase over the 0N wheat) and fertiliser N, applied either to the crop itself (140%) or to the previous wheat crop (32%). Residue retention increased shoot biomass yields of both the summer (average of 20%) and winter crops (average of 9%). Grain yield benefits of residues were 13% for mungbean, and 8% for wheat and lentil. Soil organic N and total organic C, labile C and C management index (CMI), were all increased by N inputs, from both fertiliser and N2 fixation, and by retention of residues. We concluded that retention of residues improves the N economy of the cropping system and enhances crop productivity through the additional N and other soil effects. The question of whether farmers who traditionally remove residues for fodder and fuel would change practices and return the residues to the soil will depend to a large extent on the relative profitability of both options.
