Contribution of chickpea nitrogen fixation to increased wheat production and soil organic fertility in rain-fed cropping

Nitrogen balances, i.e. the difference between N ₂ fixation inputs and N in harvested products (outputs), and rotational benefits of chickpea ( Cicer arietinum) on soil organic fertility and wheat ( Triticum aestivum) yields were quantified for rain-fed systems in the northern Punjab, Pakistan. The experiments were conducted during 1995–2000 at three sites. The four treatments were continuous wheat (0 N), continuous wheat (+N), chickpea-wheat (0 N) and chickpea-wheat (+N). The +N fertiliser rate was 100 kg N ha ^-1 applied to the wheat. Grain yields of the wheat with 0 N varied in the range 1.0–3.0 t ha ^-1, compared with 2.0–3.2 t ha ^-1 for the N-fertilised wheat. Chickpea grain yields were in the range 0.6–2.0 t ha ^-1. Chickpea N ₂ fixation was assessed using the natural ¹⁵N abundance method. Percentage of chickpea N derived from N ₂ fixation (%Ndfa) estimates were 58% (Mandra), 65% (Taxila) and 86% (Islamabad). The overall mean %Ndfa was 78%. Crop N fixed by the chickpea varied between sites (87–186 kg N ha ^-1) and essentially reflected crop biomass. The overall mean N balance for chickpea (crop N fixed minus N removed in grain and above-ground residues) was +28 kg N ha ^-1. Wheat grain yields responded to chickpea (19–73% increase for the three sites), to fertiliser N (99–136% increase) and to the combination of chickpea and fertiliser N (106–145% increase). Chickpea in the rotation increased soil organic C by 30% and soil N by 38%, relative to the continuous wheat with 0 N. These experiments indicated that chickpea could have a positive N balance, even when shoot residues were removed, and confirmed the rotational benefits of chickpea for improving soil organic fertility and yield of a following wheat crop.