Abstract

Media summary

Key Words

Legume, Nitrogen fixation, the 15N-labelling.

Introduction

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.

Methods

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.

Results

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).

^aBGN_soil = (¹⁵N_soil / ¹⁵N_RR /g); ^bBGN_total = N_RR + BGN_soil
^cTotal 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.

Conclusion

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.

References

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.