A simple urea leaf-feeding method for the production of ¹³C and ¹⁵N labelled plant material

The use of ¹³C isotope tracer techniques in terrestrial ecology has been restricted by the technical requirements and high costs associated with the production of ¹³C enriched plant material by ¹³CO₂ release in labelling chambers. We describe a novel, simple and relatively inexpensive method for the small-scale production of ¹³C and ¹⁵N labelled plant material. The method is based on foliar feeding of plants with a urea solution (97 atom% ¹³C, 2 atom% ¹⁵N) by daily misting. Maize was grown in a greenhouse in a compost-soil mixture and enclosed in clear polythene bags between urea applications. Final enrichment in 27 d old maize shoots was 211 ‰ δ¹³C (≈1.34 atom% ¹³C) and 434 ‰ δ¹⁵N (≈0.52 atom% ¹⁵N). Enrichments of hot-water extractable fractions (289 ‰ δ¹⁵N) were only slightly higher than those observed in plant bulk material, which suggests that daily urea applications ensured fairly uniform labelling of different biochemical fractions and plant tissues. Recovery of applied excess ¹³C and ¹⁵N in plant shoots was 22% and 42%, respectively. Roots were less enriched (21 ‰ δ¹³C and 277 ‰ δ¹⁵N), but no attempts were made to recover roots quantitatively.