Wheat production can be improved and soil organic fertility enhanced through involvement of legumes in crop rotation, application of fertilizer N and P, and management of tillage practices. These hypothesis were tested in two, each 4-years long, field experiments during 1998-2002. One experiment was conducted under irrigated condition at the Research Farm of NIFA, Tarnab (Peshawar) to assess the influence of soybean and fertilizer N on wheat yield and build up of soil organic fertility. The second experiment was carried out under rain-fen conditions at farmers’ field at Urmar (Peshawar) to assess the influence of legumes (lentil, chickpea), fertilizer P and tillage on wheat yield. The irrigated rotation experiment at NIFA involved soybean and maize in the summer followed each by wheat with three N levels (0. 60, 120 kg ha-1) in the winter. The rain-fed experiment involved lentil/chickpea and wheat as wheat-wheat, wheat-legume or wheat intercropped with legume in the winter with no crop in summer. Data on crop yields, legumes N2 fixation and soil organic fertility were recorded and reported in this manuscript.
The amount of N derived by soybean from atmosphere (%Ndfa) varied greatly over the years ranging from 10% (27 kg N ha-1) in 1998 to 50% (67 kg H ha-1) in 2001. We observed that both soybean and maize responded to fertilizer N applied to wheat in the previous season. The yields and N2 fixation (soybean) were generally greater for treatments following non-N fertilizer than the N fertilized wheat.
Application of fertilizer N significantly increased the grain, straw and N yields of wheat in each season, the increase was linear with increasing level of fertilizer N. Wheat also responded significantly to fertilizer N applied to maize in the previous season. On average such responses were 67% (1939 kg hq-1) in shoot biomass, 71% (688 kg ha-1) in grain yield, 64% (1250 kg ha-1) in straw yield and 90% (23.6 kg ha-1) in crop N. Both the non-N fertilized and N-fertilized wheat treatments responded significantly to the previous soybean in each year. On average, the responses were 210% (2029 kg ha-1) in grain yield for the non-N fertilized, and 163-167%(1569-1605 kg ha-1) for the N-fertilized treatment. Shoot biomass and straw yield increased in similar pattern.
Crop rotation of application of fertilizer N had no considerable effect on soil organic C or total N. However, the average mineral N content of soil, was relatively greater in soybean (22.03 ug g-1) than in maize (15.43 ug g-1) treatments.
Tillage treatment did not improve the yield of rain-fed wheat, lentil and chickpea. Crop yields (wheat, lentil, chickpea) were generally grater at higher P (60 kg ha-1) of its N from atmosphere, 6% from applied fertilizer and 12% from soil. Similarly, chickpea derived 52-64% of its N from atmosphere, 9% from applied fertilizer and 39% from soil I.egumes (lentil, chickpea) involvement in cropping system substantially improved (19-32%) the yields of rain-fed wheat and increased the farm income by Rs.6232 ha-1.
Based on the above findings we recommend to regularly involve legumes in crop rotation to reduce reliance on fertilizer N for sustaining higher wheat yield both in irrigated and rain-fed environments. The residual value of fertilizer N applied to previous maize must be taken into account while fertilizing the following wheat. We also recommend to encourage zero or minimum tillage for rain-fed wheat.