Jenkinson, David Stewart (1928–2011), soil scientist, was born on 25 February 1928 in Hollywood Hospital, Los Angeles, California, USA, the eldest of three sons of Hugh McLoughlin Jenkinson, investor and businessman, and his wife Isabel Frances (née Glass). His father, an Irish immigrant, met Isabel on one of his trips to Ireland, where they married, before settling in California. The family, affected by the stock market crash of 1929, returned to Ireland in 1932, buying Hollyfield House and its accompanying farm, between Armagh town and Portadown, near Isabel's family. Enraptured by the bird life and flora he found on the family farm, David was a boy scout and a keen visitor to the Armagh County Museum (whose curator, T. G. F. Paterson (qv) (1888–1971), nurtured interests in botany, geology and archaeology). He grew flowers and vegetables at home and, with his brother Donald (who became a geophysicist), built short-wave radios.
After attending the Armstrong primary school (1933–40), he won a scholarship to the Royal School, Armagh (1940–46). Despite suffering from asthma and spinal tuberculosis as a teenager, the latter inducing spinal curvature which contributed to an inferiority complex, he was awarded a sizarship and the Louis Purser entrance prize to study experimental science at TCD (1946–50). Revelling in the lectures of Ernest Walton (qv), left-wing student politics (which added to his rationalist and humanist worldview), and Dublin's dramatic and musical life, he graduated with a BA and first-class B.Sc. (1950). Invited to undertake research into organic chemistry by Wesley Cocker (1908–2007), relying on cutting edge ultraviolet spectrography, he was awarded his Ph.D. in 1954. Cocker arranged for a job with Hedley's Soaps, a subsidiary of Procter and Gamble, in Newcastle upon Tyne, where Jenkinson synthesised new detergents; he was offered a permanent position, but refused, finding the work uninteresting.
He joined the department of agricultural chemistry, University of Reading, as assistant lecturer (1955–7), and undertook research into the chemical structure of organic matter in soil, assessing the infrared spectra of humic substances at the nearby National Institute for Research in Dairying. Having witnessed the heavy labour required to work the land, he was determined to use science to make farming easier. Appointed a researcher (1957) in the department of soils and plant nutrition at Rothamsted Experimental Station, Harpenden, Hertfordshire, he collaborated with Jack Bremner (1922–2007) on two papers in 1960. Interested in the decomposition of carbon found in plant material in soil, Jenkinson was one of the first to utilise radioactive isotopes as tracers to measure the quantity of carbon in living cells. The thermonuclear tests undertaken in the 1950s and 1960s infused soils with atmospheric carbon-14, the indicative value of which facilitated comparative calculations of how much carbon goes into the soil each year from plants. Jenkinson's novel conceptualisation of what he termed the soil microbial biomass (SMB) as a collective ecosystem, and the required methodology to investigate its biochemistry, were the foundations of his transformative work in soil science. He regarded the SMB, which governed soil biology and biochemistry, as the fulcrum of the soil ecosystem.
Measurement of the quantity of carbon in living soil organisms contributed to a broader methodology, first outlined in Jenkinson's seminal paper in the Journal of Soil Science in 1966 ('Studies on the decomposition of plant material in soil', vol. xvii, no. 2, 280–302), later developed in a series of co-written papers in the late 1970s. Jenkinson became a pioneer in mathematically modelling the dynamics of soil organic carbon, deploying emergent computing techniques in collaboration with James Rayner, and laid the foundations of the Rothamsted Carbon (RothC) model. During a year in Australia (1979), Jenkinson firstly was Hannaford research fellow at the Waite Research Institute, University of Adelaide, where he absorbed the institute's exemplary soil measurement techniques, and then worked for three months with the division of soils at the Commonwealth Scientific and Industrial Research Organisation.
Research with his Ph.D. student David Powlson resulted in a methodology to measure microbial biomass and carbon dioxide release. Combining data from experiments in England and Nigeria in the decay of plant material in soils, from long-running experiments (some over 100 years) at Rothamsted assessing the rates of accumulation and decay of organic matter in soil, from radiocarbon measurements of organic matter in soil, and from recent biomass measurement methodologies, Jenkinson and Rayner published what became known as the RothC carbon turnover model in Soil Science (1977). Accounting for soil type, temperature, moisture content and plant cover to measure organic carbon turnover in soil, Jenkinson and his Rothamsted colleagues collaborated with the UK Met Office Hadley Centre for Climate Research, resulting in a revision (2008) of the RothC model to incorporate weather variations, especially rates of evapotranspiration.
Other important work assessed the fixation of phosphorous and nitrogen in soils, especially the flux of nitrogen through the SMB, as industrial agriculture increasingly adopted chemical fertilisers. Jenkinson assessed, with regard to their cost and environmental impact, how efficiently nitrogen fertilisers were absorbed alongside ensuing leakage into the water table, developing measurement methodologies to assess their path through the SMB. Critical of aspects of the organic farming movement as being anti-scientific, Jenkinson recognised the benefits of organic manures, but stressed their utility was limited owing to productivity and economic constraints. His research on nitrates influenced UK and EU policy-making to limit nitrate pollution, and the annual nitrogen workshop he co-founded with Powlson became a major international forum.
Jenkinson was the first to assess the impact of carbon held in soils on climate change (either by releasing carbon dioxide with warming, or capturing it in soil by sequestration techniques, of which Jenkinson was sceptical). His co-written paper in Nature (1991) confirmed the hypothesis that global warming, increasing the rate of decomposition of organic matter in soil (soil decomposition always creating heat), generates a positive feedback (by releasing more carbon dioxide), inducing further global warming. Jenkinson's recognition of the issue's importance led him to write the readable and authoritative Climate Change: a brief introduction for scientists and engineers – or anyone else who has to do something about it (online ebook, 2010).
Although retiring from Rothamsted in 1988, he continued to be active professionally for another two decades as a senior fellow there and as a visiting professor at the University of Reading. Elected fellow of the Royal Society (1991), he was awarded the Massey Ferguson National Agricultural Award (1993), and was an honorary member of the Soil Science Society of America (1995) and of the British Society of Soil Science (2007), which later established a fellowship in his memory. An international conference, 'Soil Organic Matters' (2009), held at Rothamsted (where the Jenkinson Building was opened in 2008), marked his achievements and assessed their impact; the proceedings were collected in a special issue of the European Journal of Soil Science (2011).
Highly regarded for his clarity and precision, and the elegance of his scientific logic, Jenkinson wrote over 100 peer-reviewed scientific papers (a co-written paper from 1987 had, by 2017, over 7,000 citations). He was supportive of students and colleagues, who recognised his kind, courteous and entirely unassuming manner. While uninterested in rank or promotion, and disdainful of bureaucratic manoeuvring and academic politics, he served on a select number of editorial boards and specialist scientific committees. Perhaps his greatest skill, alongside his supreme analytical rigour and patience with experiments, was his capacity to recognise salient findings in the mass of data produced by results, often helping others to interpret data in their own research findings. He gained an international reputation in soil, agricultural and environmental research circles, and his work influenced key elements of agriculture and climate change analyses. Advocating rigorous long-term field experiments, he firmly believed that modelling and experimental work should proceed in tandem. He was, though, concerned by the tendency of scientific modelling to become divorced from experimental science, and recommended modelling that 'is as simple as one can get away with whilst still giving results that are somewhat near the truth' (Powlson, 14). A polymath, he read voraciously across history, the arts and sciences.
At St Albans Registry Office on 16 March 1958, Jenkinson married Moira O'Brien, a professional musician; they had met in Dublin while attending a play at the Abbey theatre. They had four children, raised in Harpenden, near Rothamsted. Among the couple's many shared interests were music, theatre and walking. Jenkinson died from pneumonia on 16 February 2011 at the Royal Free Hospital, Hampstead, London. He was a member of the British Humanist Association; after a humanist funeral, his body was cremated at Garston crematorium on 7 March 2011.