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The objectives of this study were to quantify below-ground nitrogen (BGN) of rainfed fababean (Vicia faba), chickpea (Cicer arietinum), and barley (Hordeum vulgare) and to use the values to determine N balances for the 3 crops. The BGN fraction of legumes in particular represents a potentially important pool of N that has often been grossly underestimated or ignored in calculating such balances. A field experiment was conducted at Breeza on the Liverpool Plains, New South Wales, in which BGN of fababean, chickpea, and barley was estimated using 15N methodologies. Plants were grown in 0.32-m2 microplots and labelled with 15N on 5 occasions during vegetative growth with a total of 1.0 mL of 0.5% 15N urea (98 atom% 15N) using leaf-flap (fababean), leaf-tip (barley), or cut petiole (chickpea) shoot-labelling procedures. At peak biomass (146–170 days after sowing), all plant material and soil to 45 cm depth was sampled from one microplot in each replicate plot and analysed for dry matter (DM), %N, and 15N. At plant maturity, the remaining 3 microplots in each replicate plot were harvested for shoot and grain DM and N. With fababean, 15N enrichments of intact roots and shoots were reasonably uniform at 537‰ and 674‰, respectively. Microplot soil at 0–25 cm depth had a 15N enrichment of 18‰ (natural abundance of 6.1‰). The 25–45 cm soil enrichment was 8.7‰ (natural abundance of 6.3‰). In contrast, 15N enrichment of chickpea shoots was about twice that of recovered roots (685‰ v. 331‰), and the soil enrichment was relatively high (30‰ and 8.8‰ for the 0–25 and 25–45 cm depths, respectively). The 15N enrichments of barley shoots and recovered roots were 2272‰ and 1632‰, respectively, with soil enrichments of 34‰ and 10.7‰ for the 0–25 and 25–45 cm depths, respectively. Estimates of BGN as a percentage of total plant N, after adjusting the 15N shoot-labelling values of fababean and chickpea for uneven distribution of 15N-depleted nodules, were 24% for fababean, 68% for chickpea, and 36% for barley. The BGN values were combined with N2 fixation (fababean and chickpea only) and shoot and grain yield data (all 3 species) to construct N budgets. The inclusion of BGN in the budgets increased N balances by 38 kg N/ha to +36 kg N/ha for fababean and by 93 kg N/ha to +94 kg N/ha for chickpea. As there was no external (N2 fixation) input of N to barley, the inclusion of BGN made no difference to the N balance of the crop of –74 kg N/ha. Such values confirm the importance of BGN of N2-fixing legumes in the N economies of cropping systems.

Functions quantifying relationships between N2 flxation by legumes and other factors would be useful to farmers in the management of legumes and nitrogen in their production systems. The 2 most critical factors regulating N2 flxation are legume yield and soil nitrate and both should be included as independent variables in the functions. Data from 9 experiments in northern New South Wales on soil nitrate at sowing, yields of shoot and grain dry matter (DM) and N of chickpea, and - 15 N of shoots of chickpea and non N2-flxing reference crops (wheat, barley, and uninoculated chickpea) were used to determine the percentage of chickpea N derived from N2 flxation (Pflx), total N2 flxed, and N balance (flxed N2 minus grain N). Data were then subjected to simple and multivariate regression analyses with Pflx and total N2 flxed as the dependent variables and soil nitrate, shoot N, and grain yield as the independent variables. Simple regression coe-cients (r 2 ) for Pflx were 0¢26 with shoot N as the independent variable, 0¢59 with soil nitrate, and 0¢62 with grain yield. Coe-cient values were increased in the 2-factor (multiple) regressions to 0¢74 (P

Chickpea has potential as a rotation or break crop in the northern grains region of New South Wales and Queensland. Definition of that potential requires information on chickpea N2 fixation and on effects of chickpea on maintenance of soil N fertility and delivery of mineral N for use by a following cereal crop. Results from 6 experiments in northern NSW in which chickpea and wheat in one season were followed by wheat in subsequent seasons indicated variable N2 fixation by chickpea (mean 73 kg/ha; range 4-116 kg/ha), associated with variable Pfix (percentage of chickpea N derived from N2 fixation) (mean 57%; range 4-79%). There were no consistent differences between chickpea and wheat in effects on soil water, either pre-harvest or after the summer fallow. Chickpea ‘spared’ nitrate, relative to wheat (mean 15 kg/ha; range 1-35 kg/ha), and mineralised additional nitrate during the summer fallow (mean 18 kg/ha; range 5-40 kg/ha). Nitrate-N in the top 1·2 m of the soil profile at sowing of the following wheat crop was on average 89 kg/ha after chickpea (range 63-113 kg/ha) and 56 kg/ha after wheat (range 33-94 kg/ha). Nitrogen mineralisation rates during the summer fallow at 2 sites of 0·17 and 0· 21 kg N/ha · day (after chickpea), although greater than the rates after wheat (0· 07 and 0·12 kg N/ha · day), were not sufficient to meet the N requirements of a moderate to high yielding cereal crop. We concluded that chickpea did not fix sufficient N2 or mineralise sufficient N from residues either to maintain soil N fertility or to sustain a productive chickpea{wheat rotation without inputs of additional fertiliser N.

Rotational effects of chickpea, an important N2-fixing pulse legume of the northern grains region, on subsequent wheat require quantification of the contribution of the legume to soil N and the N status of the wheat, and of suppression of soil and stubble-borne pathogens, such as crown rot (Fusarium graminearum Schwabe Group 1). Results from selected treatments of 10 experiments in northern New South Wales in which chickpea and wheat in one season were followed by wheat in following seasons indicated generally higher dry matter (DM) and grain yields of wheat after chickpea than after wheat. Responses to chickpea were -0·8 to 3·3 t/ha (shoot DM) and -3 to 39 kg N/ha (shoot N). Responses in wheat grain yields were -0·1 to 1·7 t/ha (mean 0·85 t/ha); grain N responses were -2 to 33 kg/ha (mean 19 kg/ha). Grain protein responses were small (0·6%) and variable. Although these productivity responses could be explained largely in terms of additional nitrate-N following chickpea, we measured reduced incidences of crown rot in wheat after chickpea (range 1-36%, mean of 12%), compared with wheat after wheat (range 5-52%, mean 30%). Modelling the incidence of crown rot indicated highly significant interactions between prior crop and total water (pre-plant soil water plus in-crop rainfall). When wheat followed chickpea, incidence of the disease declined sharply with increasing water. When wheat followed wheat, there was a marginal decline in disease incidence with increasing water. Our results support the strategy of using legumes in rotation with wheat in the northern grains region for enhanced soil-N supply and disease-break effects.