Effects of below-ground nitrogen on N balances of field-grown fababean, chickpea, and barley

Abstract

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.