A new approach to quantifying the N benefit from pasture legumes to succeeding wheat

Vegetative subterranean clover (Trifolium subterraneum L.) and serradella (Ornithopus compressus L.), growing in 1-m soil columns under glasshouse conditions during 1994, were fed N-15 tracer by immersion of individual leaves (5 per plant) in a 0.4% (w/w) solution of labelled urea (99.6 atom% N-15). Four replicate soil-plant systems were harvested in late October 1994 at legume peak biomass (41 days after labelling) and in early December 1994 at maturity (90 days after labelling). The shoots were removed and the soil columns fractionated into clean macro-root, residual (root/soil) fraction, and bulk soil; the shoots from the remaining replicates were also harvested at maturity leaving the labelled soil columns intact. These intact columns were kept dry for 5 months during the summer then rewetted and planted with wheat in June 1995. Four replicate soil-plant systems were harvested at planting, tillering, anthesis, and maturity of the wheat and fractionated as before. Mean recovery of fed N-15 by the plant-soil systems was 42% for subterranean clover and 64% for serradella. Proportional distribution of the recovered N-15 was similar for both plant-soil systems: 67-69% recovered above-ground and 31-33% recovered below-ground for subterranean clover compared with 71-75% above-ground and 25-29% below-ground for serradella. Uniform labelling of below-ground nitrogen (BG N) enabled estimation of total BG N accumulation, under undisturbed conditions, for the two pasture species. Less than 60% of the total legume BG N for both species was recovered as macro-root, With UP to 17% recovered in the residual fraction and 33-51% in the bulk soil. Subterranean clover increased its total amount of BG N from 174 to 218 mg/plant between peak biomass and maturity with >65% of this located in the top 10 cm of the soil. Total BG N for serradella was similar at peak biomass (172 mg/plant) and not only decreased slightly by maturity (160 mg/plant) but was also redistributed to depth between the 2 sampling times. The ratio of shoot N to total BG N at peak biomass was 1:0.68 for subterranean clover and 1:0.60 for serradella. Recovery of labelled legume BG N at harvest by wheat following subterranean clover was 25% and after serradella was 18%. Root residues from subterranean clover appeared to decompose more rapidly than those from serradella, manifest by rapid uptake by the succeeding wheat so that 66% of the total N benefit had accrued by tillering, whereas only 44% of the N benefit from serradella roots had accrued by tillering and 72% by anthesis.