A MODEL BASED ON MEASUREMENT OF SOIL AND PLANT N-15 ENRICHMENT TO ESTIMATE N2 FIXATION BY SOYBEAN (GLYCINE-MAX L MERRILL) GROWN IN POTS

The proportion of N in soybean derived from biological N2 fixation (P(atm)) was estimated in a pot experiment using soil labelled with ammonium chloride (4.67 mg N pot-1 at 22.85, 41.40, 60.00 and 78.50 atom % N-15). A model of total N and N-15 accumulation by soybean and the N-15 enrichment of plant-available soil N was used to estimate P(atm) independent of a non-fixing reference plant, and compared to estimates of P(atm) derived from the use of non-fixing soybean and ryegrass as reference plants. The decline in N-15 enrichment of the plant-available soil N (a(soil)) was described by first-order kinetics of the form, a(soil) = b exp(- Dt). Decline constants (D) for the atom % N-15 excess of KCl-extractable NO3--N were 0.0571, 0.0225 and 0.0211 d-1 for soybean, non-fixing soybean and ryegrass, respectively, and were independent of fertilizer N-15 abundance. By fitting the exponential function to the enrichment of the mineral-N and the NO3--N pools, the N-15 enrichment of soil N being assimilated by the plant at any time was predicted. Total N and N-15 uptake by soybean showed a strong relationship with time of the form, TN(leg) = c exp(NU t) or (TN(leg))-N-15 = c' exp(NU' t), respectively. Using these equations and the N-15 enrichment of the KCl-extractable mineral-N or NO3--N pools, the uptake of soil N was estimated and was then used to calculate P(atm)' This proportion (P(atm)') increased with time after sowing to reach a maximum of 0.6, 90 days after sowing, and was independent of the soil N pool used to estimate the uptake of soil N. Estimates of P(atm) calculated by the isotope dilution method varied from 0.33 to 0.84 depending on the reference plant used. Ryegrass consistently produced higher estimates of P(atm) than non-fixing soybean. Under conditions of rapidly decreasing soil N-15 enrichment, modelled estimates of P(atm) were similar to estimates obtained using non-fixing soybean as the reference plant. There was good agreement between P(atm) calculated from the model (0.46) and by isotope dilution (0.49) at 103 days after sowing using non-fixing soybean as the reference. Failure to correct for the contribution from seed-N, when total N accumulation by the plants was small caused an overestimation of N2 fixation because the unlabelled seed N was included in the estimation of P(atm).