Rice–wheat cropping systems managed on 10 million ha in the Indo-Gangetic Plain region (IGPR) of India are the most important production systems for national food security. Recent reports, however, indicate that the system is under production fatigue and the growth rates of rice and wheat have started declining. We, therefore, conducted field experiments at Modipuram, Meerut, India, for 3 consecutive years (1998–99 to 2000–01), to study the conservation of soil organic carbon, improvement in nitrogen use efficiency and increase in system yields through inclusion of a grain legume (pigeon pea) in place of rice. The wheat yields following pigeon pea crops were significantly (P<0.05) greater than those following rice crops during 1999–2000 and 2000–01, but not during 1998–99. The economic optimum doses of fertiliser N for wheat in the pigeon pea–wheat system were smaller (128–133 kg N/ha) than those in the rice–wheat system (139–173 kg N/ha), owing to increased N supply, greater N use efficiencies and a better crop growth environment in the pigeon pea–wheat system. The post-wheat harvest nitrate-N (NO3-N) at 90–105 cm soil depth in plots fertilised with 120 or 180 kg N/ha was greater for the rice–wheat system (6.5–8.1 mg/kg) than for the pigeon pea–wheat system (5.8–6.0 mg/kg), suggesting that inclusion of pigeon pea may help to minimise NO3-N leaching to deeper soil profile layers. In plots of pigeon pea, soil organic carbon at 0–15 cm and 15–30 cm soil depths was increased at the end of the experiment compared with the initial organic carbon content. With continuous rice–wheat cropping, the bulk density of soil increased over the initial bulk density, at different soil profile depths in general, and at 30–45 cm soil depth in particular. Inclusion of pigeon pea in the system maintained soil bulk density at its initial level, and thus eliminated sub-surface soil compaction. Despite these advantages of pigeon pea over rice as a preceding crop to wheat, permanent substitution of rice with pigeon pea in rice–wheat system is unlikely, because rice is a staple foodgrain crop in India. Nonetheless, decline in wheat productivity owing to puddling-induced soil constraints that arise on continuous rice–wheat systems could be minimised by introduction of pigeon pea into the system at longer time intervals.
Improving winter wheat grain yield and water-/nitrogen-use efficiency by optimizing the micro-sprinkling irrigation amount and nitrogen application rate