Diversification of rice with pigeonpea in a rice-wheat cropping system on a Typic Ustochrept: effect on soil fertility, yield and nutrient use efficiency

Continuous adoption of rice-wheat cropping system (RWCS) has led to depletion of inherent soil fertility resulting in a serious threat to its sustainability in the Indo-Gangetic plain region (IGPR) of India. The inclusion of legumes in RWCS assumes a great significance to restore soil fertility. But farmers in the IGPR rarely grow legumes in the system. We, therefore, carried out farmers' participatory diagnostic survey in the Upper Gangetic plain zone (UGP) to understand farmers' fertilizer management practices for wheat (Trititicum aestivum L. Emend Fiori & Paol) following rice (Oryza sativa L.) or pigeonpea (Cajanus cajan (L.) Millsp). The survey indicated that most of the farmers in UGP grew pigeonpea in place of rice under RWCS as only a break crop at a 2-3 year interval. The farmers applied, on average, 11 kg N ha-1 and 24 kg P ha-1 to wheat sown after rice, and 12 kg N ha-1 and 19 kg P ha-1 to wheat sown after pigeonpea. Wheat yields, however, were lower (3.3 t ha-1) when sown after pigeonpea than after rice (3.7 t ha-1). The survey was followed by a field experiment at Modipuram (29°4′N), Meerut, India that continued during the three consecutive years (1998-1999 to 2000-2001) to examine the effect of inclusion of pigeonpea in place of rice on soil fertility, N and P use efficiency and yields of wheat. In 1998-1999, wheat yields after pigeonpea were lower than after rice, but improved significantly (p<0.05) by 11.4-15.1% in pigeonpea plots compared with those in rice plots during 1999-2000 and 2000-2001, respectively. The use efficiency of applied N and P fertilizers in wheat, measured as agronomic efficiency and apparent recovery, was increased with combined use of fertilizer N and P at recommended rate, and also with inclusion of pigeonpea in place of rice. The post-wheat harvest NO3-N in soil profile beyond 45 cm depth was significantly greater under rice-wheat system than under pigeonpea-wheat system, suggesting that inclusion of pigeonpea may help in minimizing NO3-N leaching to deeper profile layers beyond root zone. Similarly, in the treatments receiving both 120 kg N and 26 kg P ha-1, NO3-N beyond 45 cm soil depth was lower compared to those receiving N or P alone. Inclusion of pigeonpea in place of rice enhanced carbon accumulation in the soil profile. The available P content was, however, invariably low under pigeonpea plots as compared to that under rice. With continuous rice-wheat cropping, the bulk density (BD) of soil was increased, especially in the 30-45 cm soil profile. Inclusion of pigeonpea in the system not only helped maintaining soil BD at initial level in the surface (0-15 cm) soil layers, but also in decreasing (p<0.05) BD in sub-surface layers (15-30 cm and 30-45 cm). Compared to rice, a statistically significant (p<0.05) positive effect of pigeonpea on root volume (58%) and root weight (99.5%) of succeeding wheat was also recorded. The net economic returns under pigeonpea-wheat system were greater compared with rice-wheat system