Contribution of di-nitrogen fixation by pea to the productivity and N budget of a wheat-based cropping system

The productivity and N budget of three crop rotations were determined in a field study beginning in 1997. A duplicate experiment was initiated in 1998; each experiment lasted 3 years. The rotations were: continuous wheat (WWW), oilseed rape-wheat-wheat (RWW) and pea-wheat-wheat (PWW). Wheat and oilseed rape received 0 or 60 kg N/ha, apart from fertilized wheat following pea, which received 22-5 kg N/ha, and in the third year of the rotation all plots received 60 kg N/ha. Peak N-2 fixation by pea averaged 86 kg N/ha and grain N offtake was 74 kg N/ha. In the spring following the unfertilized crops the trend for soil NO3-N was: pea > oilseed rape > wheat. The yields of second and third phase wheat followed the order: PWW > WWW > RWW. The superior yield of second phase wheat following pea is attributed to greater availability of N, mainly below the 25 cm depth of soil. Yields of second phase wheat of the other rotations showed no response to N fertilizer, probably because of dry surface soil in those years. Nitrogen deficits of the no-N treatments over the rotation cycle were 31 kg/ha for PWW and RWW and 87 kg/ha for WWW. Application of N fertilizer reduced the N deficit to 13 kg/ha for PWW and resulted in N gains of 22 kg/ha for WWW and 82 kg/ha for RWW. Oilseed rape yields of this study were much lower than normal for the region, thus N removal in the grain was low. It is concluded that pea is superior to oilseed rape as a break crop, and to maintain N balance through the rotation, wheat following pea required about 20 kg/ha less fertilizer N than was applied to wheat following non-legume crops.