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