ASSESSMENT OF REFERENCE CROPS FOR THE QUANTIFICATION OF N2 FIXATION USING NATURAL AND ENRICHED LEVELS OF N-15 ABUNDANCE

In studies with the N-15-isotope dilution method for quantifying N2 fi xation, the importance of selecting a reference crop with a similar pa ttern of soil N uptake as the N2-fixing crop has been emphasized. Beca use temporal variation in the N-15 enrichment of soil inorganic N will be different following addition of N-15-enriched and natural abundanc e sources of N, we hypothesized that only a valid reference crop would provide similar estimates of N2 fixation in both cases. Barley (Horde um vulgare L.), flax (Linum usitatissimum L.), and a non-N2-fixing pea (Pisum sativum L.) cv. were grown as reference crops for N2-fixing pe a under greenhouse conditions. The soil was amended with either N-15-e nriched plant material or natural abundance (NH4)2SO4. The amount and N-15 enrichment of soil inorganic and total plant N were determined pe riodically between 6 and 84 days after planting. Barley and non-N2-fix ing pea assimilated soil inorganic N earlier than N2-fixing pea, while flax assimilated soil inorganic N later. The deltaN-15 of soil inorga nic N increased between days 14 and 36 due to isotopic fractionation d uring plant N uptake. By day 84 the deltaN-15 of plant N was the same as initial soil inorganic N for barley and non-N2-fixing pea, while fl ax had a lower deltaN-15 due to incomplete depletion of soil inorganic N. In the N-15-enriched treatment, barley had a lower N-15 enrichment at the final sampling date than the other reference crops, even thoug h it had obtained a higher proportion of its N earlier. This was attri buted to a greater root-induced turnover of soil N in the barley treat ment. Estimates of N2 fixation were only similar in natural abundance and N-15-enriched soils when barley was used as the reference crop. It was concluded that comparison of estimates of N2 fixation in natural abundance and N-15-enriched soils may provide a useful criterion for s electing a valid reference crop.