At a time when the world is trying to grow forests to offset carbon dioxide emissions, and many companies are looking at options like bamboo and mangrove — which have high potential to suck up atmospheric carbon — news is oozing out about a better candidate for carbon forestry.

Pongamia pinnata — better known in the subcontinent by names like Indian beech, karum tree, mullikulam tree, pongam and pongam oil tree — has attracted the attention of global investors and companies that have committed to net-zero emissions, according to an August 2021 report titled ‘A technical and economic appraisal of Pongamia pinnata in northern Australia’, produced by AgriFutures, Australia. The report says that Qatar Airways and “a large Japanese company” were interested in funding  Pongamia pinnata plantations for carbon offsets. Investancia Holding BV, a leading agroforestry and research company, announced in 2021 that it would plant 50 million  Pongamia pinnata trees over 125,000 ha in Paraguay to deliver 300,000 tonnes of Pongamia ‘reforestation oil’ annually by 2030.

A recent research paper on the tree’s appropriateness for reforestation, titled ‘A critical review of Pongamia pinnata multiple applications: From land remediation and carbon sequestration to socioeconomic benefits’, by scientists at the University of Reading, UK, has gone into the root (pun intended) of the issue. Indeed, the tree’s tap roots go as deep as 10 metres, which means the tree can be grown on marginal lands and won’t compete with food crops.

The paper cites a number of virtues. For one, the seeds are a good source of (non-edible) oil. Though the commercial viability of the oil, as a standalone product, is not certain, it will be viable with carbon credits. “Several companies have recently invested in Pongamia as a source of biofuel, including Investancia, BPA Australia, Tree Oils Limited, Cleanstar Energy, Betterworld Energy, and PHYLA Earth,” the paper says.

Pongamia seeds give out an oil that is yellowish-orange to brown and can be used to produce biodiesel through trans-esterification.

Biodiesel production from Pongamia generates 7.88 kg of biomass waste per kg of biodiesel, mainly in the form of pods and seed cake. Total energy (expressed in megajoules) of the biomass waste has been estimated to be 3.46 times higher than the energy of 1 kg of biodiesel. This provides a great opportunity for an integrated valorisation pathway, as the biomass waste can potentially be used as anaerobic digester feedstock for biogas production. The digestate produced can, in turn, be used as organic fertiliser, given its high nitrogen content.

Cost advantage

“An Indian study concluded that Pongamia residues produce more biogas than other commonly used oilseed trees such as  Jatropha curcas, and that the sale of biogas can lead to economic returns 2 to 3 times higher than the direct sales of residues, potentially reducing biodiesel production costs by 30–80 per cent,” the paper says. This could give Pongamia-derived biodiesel a competitive advantage over diesel. The utilisation of Pongamia waste in biogas digesters can also contribute to a circular bio-economy, benefiting the environment, say the authors.

The tree can grow on a wide range of soil types including rocky, heavy clay, sandy, alkaline, and saline soils; however, drained sandy-loam soil with adequate moisture is ideal for it. The flowers are a good source of pollen and nectar, making bee-keeping viable.

The carbon dioxide sequestration potential of Pongamia during the 10–15 years of its growth has been found to be many folds that of several other tree species. A study in 2006 estimated that, over a 25-year period, one Pongamia tree can sequester 767 kg of carbon. The carbon sequestration ability of Pongamia was calculated for 3,600 trees planted in Adilabad district of Telangana.

The certified carbon emission reduction was sold to ‘500 ppm GmbH’, a German environmental group. The purchase was for ten years’ supply of emission reduction from 140,000 kg of Pongamia oil, worth $4,164.

Santosh Singh, Managing Director—Climate and Agri Solutions, Intellecap, a firm that advises on social and impact investment, told  Quantum that  Pongamia pinnata has emerged as “a favourite for agro-forestry and carbon sequestration projects”.

He said that the “drought-hardy species... seamlessly gets integrated in many agro-forestry models”. It is a good source of biodiesel and “the emergence of carbon market will make it more attractive as this has much better carbon sequestration potential than other species such as mahua and neem,” he said, while also cautioning against monoculture.