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A new approach for quantifying the role of legume nitrogen in crop rotation.

D.F. Khan1, D.F. Herridge2, Mark Peoples3, G.D. Schwenke2, S.H. Shah1, S.M. Shah1, T. Khan4 and D. Chen5

1Soil and Plant Nutrition, ARI, Tarnab, Peshawar-Pakistan. Email
N.S.W. Agriculture, Tamworth Center for Crop Improvement, Australia. Email
C.S.I.R.O. Plant Industry, G.P.O. Box 1600, Canberra, Australia. Email
M.Sc. (Hons) Student, NWFP Agricultural University Peshawar-Pakistan. Email
Institute of Land and Food, The University of Melbourne, Parkville Vic 3052. Email


Significant amounts of N2 fixed by legumes are incorporated into the soil as fallen leaves and stems, but nitrogen (N) will also be contributed by roots, nodules and root exudates. Generally, large proportion of the N accumulated during the growth of crop legumes is removed with the harvested seed, it is commonly concluded that the net return of fixed N to the soil is likely to be small. When the amounts of N2 fixed by legumes have been compared with the N removed in seed, most calculations have relied totally on above-ground measures of fixed N. However, recent applications of the 15N-shoot labelling techniques to field grown crops suggest that the below-ground pool of legume N (nodules, roots and their exudates) may represent between 24% (fababean) and 76% (chickpea) of the total plant N. In the past, the values of legume rotations have been greatly underestimated as the root contribution of N has previously been ignored.

Media summary

Applications of the 15N-shoot labelling techniques to field grown crops suggest that the below-ground pole of legume N may represent 24% and 76% of the total plant N in fababean and chick pea, respectively.

Key Words

Legume, Nitrogen fixation, the 15N-labelling.


Nitrogen balances of legume phases of crop and crop-pasture rotations either ignore or underestimate the below-ground fraction. Nitrogen contained in intact and partly decomposed roots and nodules of legumes and in the soil as deposited organic and inorganic materials may represent the largest source of N contributing to soil N fertility. Calculations of N2 fixed by legumes are normally based solely upon shoot-based measures of fixed N, with the below-ground contribution of N ignored. This has major implications for our understanding of the potential role of pulses in the N economies of cropping systems and may adversely affect our management of those systems. The objective of this study was to adapt a 15N shoot labelling technique to quantify the below-ground fraction of legume N in the filed.


A field experiment (commenced in May 1997) with chickpea and faba bean was conducted at the Breeza long-term experimental site in the northern New South Wales, Australia. At the seedling stage, 4 metal microplot frames measuring 0.5 m x 0.64 m were placed in the ground to a depth of about 30 cm in each of 8 plots for the 15N study. Each microplot contained either 7 (chickpea) or 8 (faba bean) plants. All the plants within the microplot were fed with 0.2 ml of 0.5% 15N urea (99% atom% 15N) on five occasions during vegetative and early reproductive growth phases. The microplots were harvested just prior to physiological maturity. Shoots, intact roots and all soil to 25 cm depth (8 soil corse of 25-45 cm) were recovered for dry matter, %N and 15N analyses.


The 15N and N contents of shoots, recovered roots and soil collected at around the time of peak biomass were used to determine the amount of crop N partitioned above- and below-ground. The 15N enrichment of all plant and soil samples was measurably greater than natural abundance. Recoveries of 15N were greater than 80% of the 15N supplied to the plant shoots during growth. Calculations using the 15N excess of the recovered roots and the 15N enrichment of soil suggested that 24 and 76% of total crop N was associated with the roots of faba bean and chickpea, respectively (Table 1). The below-ground value for faba bean (equivalent to 41 kg N/ha) lay similar to previous measurements using 15N-based techniques (Russell and Fillery 1996; Rochester et al. 1998). However, the value derived for chickpea roots seemed excessively high because of very low 15N content in the recovered roots, and a very high in the 0-25 cm soil component (Table 1).

Table 1. Estimates of below-ground plant N (BGN) for field-grown faba bean and chickpea (all data are expressed per 0.32 square meter microplot).



Recovered root

BGN in soil

Total BGN





mg 15N



% crop Nc

Faba bean







24 3.4








76 1.5

aBGNsoil = (15Nsoil / 15NRR /g); bBGNtotal = NRR + BGNsoil
Total BGN as % crop N = 100 x (total BGN)/(shoot N + total BGN). Data indicating mean SE

The shoot-labelling approach used to estimate below-ground N is based on two assumptions, that:

  • All 15N excess detected in soil originated from 15N enriched material.
  • The relationship between mg 15Nexcess/gram root N of the recovered root sample was representative of the N concentration and 15N enrichment of all root derived N present in soil.


Shoot-labelling techniques have been adapted for quantifying below-ground N in legumes. The results suggested that 24% and 76% of total legume N was associated with, or derived from roots in faba bean and chick pea, respectively.


Rochester IJ, Peoples MB, Constable GA and Gault RR (1998). Faba beans and other legumes add nitrogen to irrigated cotton cropping systems. Australian Journal of Experimental Agriculture 38, 253-260.

Russell CA and Fillery IRP (1996). Estimates of lupin below-ground biomass nitrogen, dry matter and nitrogen turnover to wheat. Australian Journal of Agricultural Research 47, 1047-1059.

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