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

Stable carbon and nitrogen isotope signatures (delta C-13 and delta N-15) o f Cannabis sativa were assessed for their usefulness to trace seized Cannab is leaves to the country of origin and to source crops by determining how i sotope signatures relate to plant growth conditions. The isotopic compositi on of Cannabis examined here covered nearly the entire range of values repo rted for terrestrial C-3 plants. The delta C-13 values of Cannabis from Aus tralia, Papua New Guinea and Thailand ranged from -36 to -25 parts per thou sand, and delta N-15 values ranged from -1.0 to 15.8 parts per thousand. Th e stable isotope content did not allow differentiation between Cannabis ori ginating from the three countries, but delta C-13 values of plantation-grow n Cannabis differed between well-watered plants (average delta C-13 of -30. 0 parts per thousand) and plants that had received little irrigation (avera ge delta C-13 of -26.4 parts per thousand). Cannabis grown under controlled conditions had delta C-13 values of -32.6 and -30.6 parts per thousand wit h high and low water supply, respectively. These results indicate that wate r availability determines leaf C-13 in plants grown under similar condition s of light, temperature and air humidity. The delta C-13 values also distin guished between indoor- and outdoor-grown Cannabis; indoor- grown plants ha d overall more negative delta C-13 values (average -31.8 parts per thousand ) than outdoor-grown plants (average -27.9 parts per thousand). Contributin g to the strong C-13-depletion of indoor- grown plants may be high relative humidity, poor ventilation and recycling of C-13-depleted respired CO2. Mi neral fertilizers had mostly lower delta N-15 values (-0.2 to 2.2 parts per thousand) than manure-based fertilizers (7.6 to 22.7 parts per thousand). It was possible to link delta N-15 values of fertilizers associated with a crop site to soil and plant delta N-15 values. The strong relationship betw een soil, fertilizer, and plant delta N-15 suggests that Cannabis delta N-1 5 is determined by the isotopic composition of the nitrogen source. The dis tinct delta N-15 values measured in Cannabis crops make delta N-15 an excel lent tool for matching seized Cannabis with a source crop. A case study is presented that demonstrates how delta C-13 and delta N-15 values can be use d as a forensic tool.