Natural abundance of stable carbon and nitrogen isotopes in Cannabis sativa reflects growth conditions

Abstract

Stable carbon and nitrogen isotope signatures (δ¹³C and δ¹⁵N) of Cannabis sativa were assessed for their usefulness to trace seized Cannabis leaves to the country of origin and to source crops by determining how isotope signatures relate to plant growth conditions. The isotopic composition of Cannabis examined here covered nearly the entire range of values reported for terrestrial C₃ plants. The δ¹³C values of Cannabis from Australia, Papua New Guinea and Thailand ranged from –36 to –25‰, and δ ¹⁵N values ranged from –1.0 to 15.8‰. The stable isotope content did not allow differentiation between Cannabis originating from the three countries, but δ¹³C values of plantation-grown Cannabis differed between well-watered plants (average δ¹³C of–30.0‰) and plants that had received little irrigation (average δ¹³C of –26.4‰). Cannabis grown under controlled conditions had δ¹³C values of –32.6 and –30.6‰ with high and low water supply, respectively. These results indicate that water availability determines leaf ¹³C in plants grown under similar conditions of light, temperature and air humidity. The δ¹³C values also distinguished between indoor- and outdoor-grown Cannabis; indoor-grown plants had overall more negative δ¹³C values (average –31.8‰) than outdoor-grown plants (average –27.9‰). Contributing to the strong ¹³C-depletion of indoor-grown plants may be high relative humidity, poor ventilation and recycling of ¹³C-depleted respired CO₂. Mineral fertilizers had mostly lower δ¹⁵N values (–0.2 to 2.2‰) than manure-based fertilizers (7.6 to 22.7‰). It was possible to link δ¹⁵N values of fertilizers associated with a crop site to soil and plant δ¹⁵N values. The strong relationship between soil, fertilizer, and plant δ¹⁵N suggests that Cannabis δ¹⁵N is determined by the isotopic composition of the nitrogen source. The distinct d15N values measured in Cannabis crops make δ¹⁵N an excellent tool for matching seized Cannabiswith a source crop. A case study is presented that demonstrates how δ¹³C and δ¹⁵N values can be used as a forensic tool.