Assessment of reference crops for the quantification of N 2 fixation using natural and enriched levels of 15N abundance

Abstract

In studies with the 15N-isotope dilution method for quantifying N 2 fixation, the importance of selecting a reference crop with a similar pattern of soil N uptake as the N 2-fixing crop has been emphasized. Because temporal variation in the 15N enrichment of soil inorganic N will be different following addition of 15N-enriched and natural abundance sources of N, we hypothesized that only a valid reference crop would provide similar estimates of N 2 fixation in both cases. Barley (Hordeum vulgare L.), flax (Linum usitatissimum L.), and a non-N 2-fixing pea (Pisum sativum L.) cv. were grown as reference crops for N 2-fixing pea under greenhouse conditions. The soil was amended with either 15N-enriched plant material or natural abundance (NH 4) 2SO 4. The amount and 15N enrichment of soil inorganic and total plant N were determined periodically between 6 and 84 days after planting. Barley and non-N 2-fixing pea assimilated soil inorganic N earlier than N 2-fixing pea, while flax assimilated soil inorganic N later. The δ 15N of soil inorganic N increased between days 14 and 36 due to isotopic fractionation during plant N uptake. By day 84 the δ 15N of plant N was the same as initial soil inorganic N for barley and non-N 2-fixing pea, while flax had a lower δ 15N due to incomplete depletion of soil inorganic N. In the 15N-enriched treatment, barley had a lower 15N enrichment at the final sampling date than the other reference crops, even though it had obtained a higher proportion of its N earlier. This was attributed to a greater root-induced turnover of soil N in the barley treatment. Estimates of N 2 fixation were only similar in natural abundance and 15N-enriched soils when barley was used as the reference crop. It was concluded that comparison of estimates of N 2 fixation in natural abundance and 15N-enriched soils may provide a useful criterion for selecting a valid reference crop.