Global warming is becoming more certain and concerning than ever. Increased CO₂ levels from 280 ppm in pre-industrial to 420-425 ppm in recent years were a major driving factor for increased global warming (Choudhary et al., 2025). One-third of the world's land surface is covered by forest and serves as a major carbon sink on Earth. The amount of carbon sequestered and stored in forests varies greatly with factors such as forest type, net primary production, age and overall composition (He et al., 2022). Soil carbon and biomass carbon are two components that contribute to carbon storage in forest ecosystems (Bania et al., 2025). More photosynthesis results in more CO₂ being transformed into biomass, which lowers atmospheric carbon and stores it in plant tissue both above and below ground. Carbon sequestration in trees is significantly influenced by the production of biomass in various forms (Soni et al., 2023). The primary carbon pools in any ecosystem include dead wood, litter, soil organic matter and aboveground and belowground biomass (Sharma et al., 2021).

Degradation of soil fertility is a major limitation to sustainable agricultural productivity, especially in areas affected by erosion, nutrient loss, and poor land management practices. Restoring the productivity of such degraded soils requires environmentally friendly and economically sustainable methods (Leksono et al., 2021). In this context, perennial leguminous tree species have gained more interest due to their ability to enhance soil health through biological processes. Pongamia pinnata is a multipurpose, fast-growing, deciduous tree and is indigenous to the Indian subcontinent (Choudhary et al., 2025) and commonly known as honge in Karnataka, Karanja, or Pongam. Pongamia has a strong root system that helps prevent soil erosion, a leading driver of carbon loss (Chen et al., 2025) and the tree offers carbon storage as an additional benefit (Soni et al., 2023).

By anchoring the soil and reducing erosion, Pongamia helps retain soil organic content, which is vital for long-term carbon sequestration. Pongamia can restore soil fertility by fixing atmospheric nitrogen (Gawali et al., 2015), thereby enriching the soil with nutrients and reducing reliance on synthetic fertilisers. These traits make Pongamia a valuable species for restoring degraded lands while also supporting sustainable agroforestry systems. Since climate change is a global problem, studying Pongamia is crucial for several reasons, including estimating carbon sequestration potential, which can support efforts to mitigate the effects of climate change in India and around the world (Zeng et al., 2024).

RAK Pongamia clones represents a genetically diverse collection of Pongamia pinnata selected from superior trees across varied agro-climatic regions. RAK Pongamia clones plays a crucial role in enhancing soil fertility through biological nitrogen fixation and organic matter addition. Integration of RAK accessions in agroforestry systems contributes to improved soil health, nutrient cycling, and long-term sustainability of degraded lands. Some elite RAK genotypes have shown superior performance in biomass accumulation and carbon sequestration, supporting climate change mitigation efforts. The present study evaluates the performance of different Pongamia clones with respect to growth, biomass, carbon sequestration and soil improvement under an agroforestry system.

In the first year of planting, there was no significant changes in the tree height (Table 2). During the second year after planting, RAK-2015-10 recorded a higher tree height (3.41m), followed by RAK-2015-01, which was superior over the other clones. After the 3^rd and 4^th year of planting, RAK-2015-10 recorded higher tree height, whereas in the 5^th year, RAK-2015-09 recorded higher tree height (4.27 m), followed by RAK-2015-10 and RAK-2015-08. After 6^th year of planting among different clones, RAK-2015-10 recorded higher tree height (4.98 m), followed by RAK-2015-09 and RAK-2015-08. There were variations with respect to the tree increment rate over the years. ( fig 1 )

Diameter at breast height varied significantly among the different clones (Table 3). Significantly higher diameter upto 4 years of planting was recorded by RAK-2015-10. The minimum diameter at breast height was recorded in all years by RAK-2015-07. During 5^th and 6^th year of planting RAK-2015-01 recorded higher diameter (14.25 cm) followed by RAK - 2015-08 and RAK-2015-10.

Table 2: Tree height of different Pongamia pinnata clones from 2020-21 to 2025-26

The inherent genetic variability among the different clones, which controls their growth potential, resource efficiency, and environmental adaptability, is the main cause of the observed variation in growth and increment rates among Pongamia pinnata clones across different years (Chen et al., 2025). A similar observation was also made by Ravi et al., 2025 states that RAK-2015-10 performed better in terms of height compared to other clones.

Table 3: Diameter at breast height (DBH) of Pongamia pinnata clones

Table 4: Canopy cover (m) of Pongamia pinnata as influenced by different clones

Clones of Pongamia performed differently with respect to canopy cover due to variations in their genetic characteristics. However, the changes were significant in the initial two years of planting. After 2 years, there were no significant changes in the crown cover. Clonal selection RAK-2015-10 recorded better spread in North-South and East-West directions (3.47 m, 3.17 m and 4.15 m and 3.71 m) during the 2 years of establishment. RAK-2015-01 recorded maximum canopy cover during the 3^rd, 4^th and 5^th years of establishment (Table 4). Variations in the canopy cover in N-S and E-W directions were monitored to evaluate the clones for their growth and performance over the years. Different Pongamia clones exhibit inherent genetic variations that regulate growth habit, crown architecture and branching pattern. Canopy spread increases as Pongamia trees mature, driven by increased branching and crown expansion (Danu et al., 2024).

Different clones of Pongamia differed significantly over the years on volume of biomass produced year after year (Table 5). During the 1^st year, RAK-2015-01 produced more biomass and volume of 7.41 m³ compared to other clones, with a minimum biomass of 0.89 m³ produced by RAK-2015-07. Further, with the incremental rate, RAK-2015-10 performed better with volume in the 2^nd, 3^rd, 5^th and 6^th year except during the 4^thyear. Clonal selection RAK-2015-07 recorded a lesser volume per hectare over the years. Superior clones convert assimilates more efficiently into the stem, branches and root biomass, resulting in higher wood volume. Age-related accumulation, genetic potential, effective photosynthesis, canopy growth, and favourable climatic conditions are the main causes of increased volume and total biomass throughout time (Sahoo et al., 2021). Clones with superior growth features and more effective resource utilisation result in higher biomass and volume production (Chen et al., 2025).

Table 5: Volume (m³ ha^-1) of Pongamia pinnata as influenced by different clones

Total biomass produced by pongamia clones over the years was measured periodically and the results showed significant differences (Fig. 1). During the 1^st year of RAK-2015-01, it recorded significantly higher biomass (7.66 t ha^-1) over all the other clones. In the second and third year, RAK-2015-10 recorded higher biomass (12.96 and 15.77 t ha^-1). During the 3^rd year, RAK-2015-08 recorded significantly higher biomass (17.04 t ha^-1), followed by RAK-2015-01 and RAK-2015-10. There was non-significant biomass production by the pongamia clones during the 5^th year. In the recent year (2025-26), RAK-2015-10 produced significantly higher biomass (41.77 t ha^-1), followed by RAK-2015-01 and RAK-2015-08. However, the performance of RAK-2015-07 clone was found to be the least biomass production and not comparable to any of the clones in all the years.

Carbon sequestration potential of all the clones under the study was calculated based on growth characteristics and biomass production (Table 6). During the first year, RAK-2015-01 sequestered significantly higher carbon compared to all other clones (14.04 t ha^-1). In the 2^nd and 3^rd year, RAK-2015-10 recorded higher carbon sequestration potential (23.76 and 28.54 t ha^-1) over other clones, and a similar trend was followed during 5^th and 6^th year (47.13 and 76.58 t ha^-1) except during 4^th year, wherein RAK-2015-08, which recorded the maximum carbon sequestration potential (31.24 t ha^-1). The C sequestration potential by RAK-2015-07 was non-significant in all the years of study.

The potential of tree carbon sequestration is determined by the location and method of tree planting. The results clearly state that older trees sequester more carbon when compared to younger trees. Genetic traits, environment, density and age of the trees were also attributed to variations in carbon sequestration in Pongamia clones (Soni et al., 2023). The trees support a large number of leaves, have a high woody biomass, and enhance carbon fixation, which can eventually sequester more carbon and serve as a long-term carbon sink. This helps in reducing the greenhouse effect and climate change (Norby et al. 2024). In the present study, RAK-2015-10 noticed highest height and biomass, which helps in sequestering more carbon and these results are in line with the findings of Ravi et al. (2025) wherein, RAK-2015-10 performed better than other clones in terms of carbon sequestration under an agroforestry system.

Table 6: CO₂ sequestration (t ha^-1) potential of Pongamia pinnata at different growth stages as influenced by clones

Carbon stock in soil was assessed after 5 years of study. There were significant changes with respect to different soil depths. Carbon stock varied with depth among the various clones however, RAK-2015-10 recorded higher C stock in all the depths (10608, 6936 and 5100 kg ha^-1, respectively). Significantly lower C stock was noticed in RAK-2015-09 clone at 0-15, 15-30 and 30-45 cm depths (Fig. 2). The higher accumulation in the top soil might be due to higher accumulation of organic carbon by the decomposition of leaf litter by the Pongamia clones (Negash and Starr, 2021). Similar variations were also noticed by Giweta (2020) that decomposition of leaf litter results in the formation of soil organic matter and nutrient cycling, contributing significantly to topsoil carbon accumulation.

Pongamia growth over the years influenced a few soil chemical properties (Table 7). There was improvement in soil properties due to pongamia clones, with respect to soil pH, electrical conductivity and organic carbon status, there was a slight positive variation recorded. However, Primary nutrient status in soil differed significantly among the clones when compared to initial values. Significantly higher availability N was recorded under RAK-2015-10 (278.18 kg ha^-1) over the other clones. Soil under RAK-2015-10 clone recorded significantly higher availability of P₂O₅ and availability of K₂O over the other clones (29.65 and 211.09 kg ha^-1), followed by RAK-2015-09 and RAK-2015-08 clones. Irrespective of clones, major nutrient availability was improved due to the litter fall, carbon sequestration and nodulation potential of different clones. Pongamia is a fast-growing tropical legume tree that accumulates biomass quickly and is ideal for the sequestration of carbon. The crop has its ability to form symbiotic associations with beneficial soil microbes, such as rhizobia and mycorrhiza that provide the tree with critical nutrients (Kumar et al., 2017). The increased availability of nutrients resulted in increased production of photosynthates and their translocation into leaves and branches. The higher amount of nitrogen in the soil might be due to the addition of organic, manures, fertilizers. However, Pongamia pinnata is nitrogen fixing tree species, which helps to accumulation of more nitrogen; this may be one of the reasons for increased available nitrogen under Pongamia pinnata plantation. These results are in conformity with the findings of Parasuram, 2013. The litter fall and its decomposition over the years might have accounted for the improved availability of carbon and other major nutrients in soil.

Table 7: Soil chemical properties and soil nutrient status as influenced by Pongamia clones (2025-26)

The present study was conducted to assess the growth, biomass and carbon sequestration potential of various Pongamia clones along with soil carbon stock, including nutrient status of soil. The study found that Pongamia pinnata clones vary significantly in growth, biomass and carbon sequestration within an agroforestry system. RAK-2015-01 performed better in the early growth stage, but RAK-2015-10 consistently outperformed the other clones in height, biomass accumulation and carbon sequestration over time. Carbon sequestration was increased with age of the tree and biomass, emphasizing the significance of choosing high-performing clones for long-term climatic benefits. Pongamia also increased soil carbon storage and nutritional status, showing its importance in soil fertility. Overall, Pongamia pinnata is a good species for sustainable agroforestry and climate change mitigation, as RAK-2015-10 is shown to be the most promising clone for maximizing production of biomass and carbon sequestration.

We thank ICAR-CAFRI, Jhansi for the financial and continued support.