Estimates of N accumulated below-ground by grain legumes derived using leaf or stem ¹⁵N-feeding: in search of a practical method for potential use at remote field locations

Below-ground (BG) N of N₂ fixing grain legumes is an important N input to farming systems, likely underestimated as N solely in coarse roots. ¹⁵N methodology can improve measures of BG N accumulation. Our objective was to identify a ¹⁵N method for potential use at remote field sites. We hypothesised that method and frequency of ¹⁵N feeding may result in different estimates of BG N.

Glasshouse-grown grain legumes, leaf or stem fed ¹⁵N once or twice, were sampled three weeks after feed and at physiological maturity. Three BG fractions were isolated using 2 mm sieving; recovered cleaned roots>2 mm, unrecovered roots >2 mm remaining on sieve with adhering soil, and bulk soil that passed through sieve along with fine roots <2 mm. Fractions were measured for N/¹⁵N to estimate BGN. Inorganic, total soluble organic and microbial N/¹⁵N were also assessed for bulk soil.

Estimates of BG N were not influenced by method or frequency of ¹⁵N feeding. Recovered root N was 33–55% of estimated plant BG N at physiological maturity. Low amounts of fed ¹⁵N detected as inorganic or soluble organic N (0.1–0.7%) and microbial biomass N (0.2–2.5%) were attributed to rhizodeposition. A large proportion of fed ¹⁵N in bulk soil (51–67%) was present as ‘insoluble’ N attributed to fine roots.

A single ¹⁵N stem-feeding at remote field sites should suffice to provide a measure of BG N larger than that N measured in recovered roots on a 2 mm sieve. Little evidence for direct leakage into soil labile N pools of highly labelled ¹⁵N post-feed.