Nitrogen fixation and soil nitrate interactions in field-grown chickpea (Cicer arietinum) and fababean (Vicia faba)

Soil in which nodulated legumes are growing often contains more nitrate nitrogen (N) than soil in which unnodulated legumes or non-legumes are growing. There is conjecture, however, as to whether the extra or 'spared' N is due to reduced use of soil N by the legume or to net mineralisation of legume root and nodular N. We report results of a field experiment to quantify and compare, at different levels of soil-N supply, N-2 fixation, and soil-N use by chickpea (Cicer arietinum) and fababean (Vicia faba). Wheat (Triticum aestivum) was included as a non-N-2-fixing control. Plants of the 3 species were grown on a low-nitrate Vertosol with fertiliser N rates of 0, 50, and 100 kg/ha (0N, 50N, and 100N), applied 6 weeks before sowing. Samples were collected at sowing and at 64, 100, 135, and 162 days after sowing (DAS) for analysis of soil nitrate, root, and grain dry matter (DM) and N and shoot DM, N, and N-15. The latter was used to estimate the percentage (%Ndfa) and total N fixed by the 2 legumes. Soil nitrate levels to a depth of 1.8 m at sowing were 11-17 kg N/ha (0N), 41-55 kg N/ha (50N), and 71-86 kg N/ha (100N). Grain yields of the 2 legumes were unaffected by soil-N supply (fertiliser N treatment), being 2.0-2.4 t/ha for chickpea and 3.7-4.6t/ha for fababean. Wheat grain yields varied from 1.6 t/ha (0N) to 4.8 t/ha (100N). Fababean fixed more N than chickpea. Values (total plant including roots) were 209-275 kg/ha for fababean and 146-214 kg/ha for chickpea. Corresponding %Ndfa values were 69-88% (fababean) and 64-85% (chickpea). Early in crop growth, when soil N supply was high in the 100N treatment, fababean maintained a higher dependence on N 2 fixation than chickpea (Ndfa of 45% v. 12%), fixed greater amounts of N (57 v. 16 kg/ha), and used substantially less soil N (69 v. 118 kg/ha). In this situation, soil N sparing was observed, with soil nitrate levels significantly higher in the fababean plots (P < 0.05) than under chickpea or wheat. At the end of growth season, however, there were no crop effects on soil nitrate levels. Soil N balances, which combined crop N fixed as inputs and grain N as outputs, were positive for the legumes, with ranges 80-135 kg N/ha for chickpea and 79-157 kg N/ha for fababean, and negative for wheat (-20 to -66 kg N/ha). We concluded that under the starting soil nitrate levels in this experiment, levels typical of many cropping soils in the region, high-biomass fababean and chickpea crops will not spare significant amounts of soil N. In situations of higher soil nitrate and/or smaller biomass crops with less N demand, nitrate sparing may occur, particularly with fababean.