Quantifying below-ground nitrogen of legumes. 2. A comparison of 15N and non isotopic methods

Accurate information on below-ground nitrogen (N) of legumes is necessary for quantifying legume effects on soil N pools and on the N economies of crops following legumes in rotation systems. We report a series of glasshouse pot experiments to determine below-ground N (BGN) of the four legumes, fababean (Vicia faba), chickpea (Cicer arietinum), mungbean (Vigna radiata) and pigeonpea (Cajanus cajan) using both 15N shoot-labelling and 15N-labelled soil isotope-dilution methods, a mass N balance approach and the physical recovery of nodulated roots. Data from the 15N shoot-labelling experiment were manipulated in different ways in an attempt to counter errors associated with uneven 15N enrichment of roots and nodules. Values for BGN as percent of total plant N based on the physical recovery of nodulated roots ranged from 4 to 15%. With 15N shoot-labelling, a total of 8.11 mg 15N was supplied to each pot (six plants) as 0.5% 15N urea using either leaf-flap (fababean, mungbean and pigeonpea), petiole (chickpea) or leaf-tip (wheat) feeding. Calculations based on measurement of 15N enrichments of harvested plant parts and root-zone soil suggested that BGN represented 39% of total plant N for fababean, 53% for chickpea, 20% for mungbean and 47% for pigeonpea. The value for wheat was 60%. Adjustment for uneven nodulation patterns on the roots and nodule 15N depletion, resulting in different 15N enrichments between nodulated and unnodulated roots, reduced the fababean value to 37% and chickpea to 42%. Values using the other methods were generally in the same range, viz. 15–57% (simple 15N balance), 11–52% (soil 15N dilution) and 30–52% (mass N balance). We conclude that physical recovery of roots was the most inaccurate method for estimating BGN. Average values for BGN as percent of total plant N using all isotopic and mass N balance methods were 30% for fababean, 48% for chickpea, 28% for mungbean, and 43% for pigeonpea.15N shoot-labelling may be the best method for quantifying BGN of field-grown plants. The methodology is simple, apparently accurate provided care is taken in obtaining representative nodulated root samples and, unlike the soil 15N dilution method, does not require pre-treatment of the soil with 15N enriched material.