Does nitrogen fixation of commercial, dryland chickpea and faba bean crops in north-west New South Wales maintain or enhance soil nitrogen?

Abstract

Summary. Nitrogen (N2 ) fixation accords pulse crops the potential to sustain or enhance total soil nitrogen (N) fertility. However, regional field experiments have shown that this potential is often not realised because N2 fixation is inhibited by the supply of nitrate N in the root zone (0–90 cm) coupled with a low demand for N during plant growth. The objectives of this study were to establish whether commercially grown chickpea and faba bean crops in the northern grain belt of New South Wales were depleting, maintaining or enhancing soil N fertility, and whether current farm management practices were maximising the N2 fixation potential of the crops. Fifty-one rainfed crops of chickpea (Cicer arietinum L.) and faba bean (Vicia faba L.) were surveyed in the Moree, Walgett and Gunnedah districts of north-west New South Wales during the winters of 1994 and 1995. Nitrogen fixation was measured using the natural 15N abundance technique. Net N balance was calculated for each crop by subtracting grain N harvested from fixed N2. Soil, plant and fallow conditions with potential to influence N2 fixation were also documented. The percentage of crop N derived from N2 fixation (Pfix) ranged from 0 to 81% for chickpea and 19 to 79% for faba bean. Nitrogen fixation of chickpea was uniformly low in the 1994 drought. Total N2 fixed ranged from 0 to 99 kg/ha for chickpea and 15 to 171 kg/ha for faba bean. Net N balance ranged from –47 to +46 kg N/ha for chickpea crops, and –12 to +94 kg N/ha for faba bean crops. About 60% of the difference in Pfix between chickpea and faba bean at the average level of soil nitrate (65 kg/ha) was explained by the higher N demand of the latter. The remaining 40% could be due to greater tolerance of the faba bean symbiosis to nitrate effects. In addition, faba bean had a lower N harvest index than chickpea, which meant that proportionally less N needed to be fixed by faba bean to offset removal of grain N. On average, Pfix needed to exceed 35% for chickpea and 19% for faba bean to balance soil N. The equivalent soil nitrate levels were 43 kg nitrate N/ha for chickpea and 280 kg/ha for faba bean (extrapolated from the relationship between measured Pfix and soil nitrate). Double-cropping chickpea into summer cereal or grass pasture stubble provided the most consistent strategy for achieving the low levels of soil nitrate.