Data collated from around the world indicate that, for every tonne of shoot dry matter produced by crop legumes, the symbiotic relationship with rhizobia is responsible for fixing, on average on a whole plant basis (shoots and nodulated roots), the equivalent of 30-40 kg of nitrogen (N). Consequently, factors that directly influence legume growth (e.g. water and nutrient availability, disease incidence and pests) tend to be the main determinants of the amounts of N 2 fixed. However, practices that either limit the presence of effective rhizobia in the soil (no inoculation, poor inoculant quality), increase soil concentrations of nitrate (excessive tillage, extended fallows, fertilizer N), or enhance competition for soil mineral N (intercropping legumes with cereals) can also be critical. Much of the N 2 fixed by the legume is usually removed at harvest in high-protein seed so that the net residual contributions of fixed N to agricultural soils after the harvest of legume grain may be relatively small. Nonetheless, the inclusion of legumes in a cropping sequence generally improves the productivity of following crops. While some of these rotational effects may be associated with improvements in availability ofN in soils, factors unrelated to N also play an important role. Recent results suggest that one such non-N benefit may be due to the impact on soil biology of hydrogen emitted from nodules as a by-product of'N, fixation.

Nitrogen (N) contributed by legumes is an important component of N supply to subsequent cereal crops, yet few Australian grain-growers routinely monitor soil mineral N before applying N fertiliser. Soil and crop N data from 16 dryland experiments conducted in eastern Australia from 1989–2016 were examined to explore the possibility of developing simple predictive relationships to assist farmer decision-making. In each experiment, legume crops were harvested for grain or brown-manured (BM, terminated before maturity with herbicide), and wheat, barley or canola were grown. Soil mineral N measured immediately before sowing wheat in the following year was significantly higher (P < 0.05) after 31 of the 33 legume pre-cropping treatments than adjacent non-legume controls. The average improvements in soil mineral N were greater for legume BM (60 ± 16 kg N/ha; n = 5) than grain crops (35 ± 20 kg N/ha; n = 26), but soil N benefits were similar when expressed on the basis of summer fallow rainfall (0.15 ± 0.09 kg N/ha per mm), residual legume shoot dry matter (9 ± 5 kg N/ha per t/ha), or total legume residue N (28 ± 11%). Legume grain crops increased soil mineral N by 18 ± 9 kg N/ha per t/ha grain harvested. Apparent recovery of legume residue N by wheat averaged 30 ± 10% for 20 legume treatments in a subset of eight experiments. Apparent recovery of fertiliser N in the absence of legumes in two of these experiments was 64 ± 16% of the 51–75 kg fertiliser-N/ha supplied. The 25 year dataset provided new insights into the expected availability of soil mineral N after legumes and the relative value of legume N to a following wheat crop, which can guide farmer decisions regarding N fertiliser use.

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.

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Particulate organic matter nitrogen (POM-N) was evaluated as an indicator of crop residue source (pulse versus cereal) and residue management (no-tillage [NT], stubble burned [SB] or stubble mulched [SM]) on soil quality and subsequent crop productivity in a continuous cropping experiment in northern New South Wales (NSW), Australia. The relative contributions to POM of pulse versus cereal, and shoots versus roots, were studied using in situ 15 N shoot labelling. Under NT, a greater proportion of organic N was found in POM-N > 250 µm (5.5% versus 3.5% [SM] and 2.7% [SB]) and POM-N > 53 µm (10.3% versus 9.7% [SM] and 8.7% [SB]). Pulse residues (particularly roots) contributed 2-7 times more N to POM and 2-4 times more N to non-POM-N than barley (15 N data), but this increased contribution was not detectable with non-isotopic analysis. POM-N was sensitive to residue management, but was not a reliable measure of N inputs from pulse versus cereal residues, nor a useful tool for predicting subsequent crop N uptake. pulse / cereal / crop residue / particulate organic matter / soil nitrogen Résumé-Relation entre la matière organique azotée particulaire et non particulaire et les résidus de culture de légumineuses, leur gestion et le prélèvement d'azote par les céréales. La matière organique particulaire POM-N a été évaluée comme indicateur des sources des résidus de cultures (légumineuses ou céréales) et de la gestion des résidus (non-labour [NT], brûlage des pailles [SB], mulch de paille [SM]) sur la qualité du sol et la productivité induite des cultures dans une expérience continue de culture sans le Nord des New South Wales en Australie. Les contributions relatives des POM des légumineuses par rapport à celles des céréales et des tiges par rapport aux racines ont été étudiées en utilisant des tiges marquées à l'azote 15 N in-situ. Sous NT, une plus grande proportion de l'azote organique a été trouvé dans la fraction POM-N > 250 µm (5,5 % par rapport à 3,5 % pour [SM] et 2,7 % pour [SB]) et POM-N > 53 µm (10,3 % par rapport à 9,7 % pour [SM] et 8,7 % [SB]). Les résidus de légumineuses (particulièrement les racines) ont apporté 2 à 7 fois plus d'azote sous forme POM et 2 à 4 fois plus d'azote sous forme non particulaire que l'orge (données de 15 N) mais cette contribution accrue n'était pas détectable avec une analyse non isotopique. POM-N était sensible à la gestion des résidus mais n'était pas une mesure fiable pour les apports d'azote à partir des légumineuses comparativement aux résidus de céréales ni un outil utile pour prédire le prélèvement induit d'azote par la culture.