Objectives In order to ascertain the interaction on nitrogen fixation and transfer from peanut to oat in the intercropping system of oat and peanut, field experiments were conducted for understanding the characteristics of nitrogen cycle under oat and peanut monocropping and intercropping system.

Methods The field experiments with the randomized block design were conducted in 2011 and 2012. Three planting patterns were designed including oat monoculture (TO), peanut monoculture (TP) and oat and peanut intercropping (TOP). Peanut roots were washed to investigate the nodule growth, the ¹⁵N-isotope labeling was used to quantify N fixation and subsequent transfer in the intercropping system. Dry matter accumulation, N uptake efficiency, nitrogen fixation amount of peanut, nodule growth characteristics at the peanut podding stage and content of N transfer from peanut to oat were investigated.

Results Compared to the monoculture, the aboveground dry matter accumulation of oat was significantly higher under intercropping system, and the accumulated N in oats increased significantly in 2011 and 2012 (P < 0.05). At the mature stage, the average dry matter accumulation of intercropped oats was increased by 40.6% in 2011 and 2012, respectively, and the average nitrogen accumulation amounts of oat were increased by 49.0%. The aboveground dry matter and the amounts of accumulated N of intercropped peanut were not significantly affected in the early growing stage compared with monoculture, but at mature stage, the average dry matter accumulation was significantly decreased by 20.6%. The average nodule numbers of intercropped peanut were decreased by 21.3% in 2011 and 2012, respectively, and the average weight of root nodules decreased by 16.8% in 2011 and 2012, respectively, the nitrogenase activities per unit weight of root nodules at the podding stage were significantly decreased by 26.2% (P < 0.05). Although the nitrogen fixation efficiencies of intercropped peanuts at the physiological maturity stage were increased by 10.3% and 37.1%, the biological nitrogen fixation amounts were decreased by 52.3% and 26.3%, respectively. The N transfer rate from peanut to oat was 21.4% in the intercropping system, with N transferred amount of 15.3 mg/plant in 2012.

Conclusions This study demonstrated that intercropping of oat and peanut was an efficient strategy to achieve higher nitrogen utilization for oat. Although the oat and peanut intercropping system decreased the nitrogenase activity of peanut root nodules and nitrogen fixation amount of peanut, N-transfer from peanut to oat was evident in this experiment. The oat and peanut intercropping system can also benefit dual directional regulation between above ground and below ground for crops simultaneously, and is one of the most significant ways to optimizing nitrogen management in the farmland ecosystem in Northeast China.