Biomass production, root distribution and yield responses of green leaf manure trees on wetland paddy bunds

Abstract

India's agriculture sector, which accounts for 18.2 per cent of the GDP and employs nearly half of the workforce, is heavily dependent on imported fertilizers, placing a significant fiscal burden on the economy. To mitigate this dependency and enhance soil fertility, the practice of green leaf manuring offers a sustainable solution. Traditionally, green leaf manuring in paddy fields was a popular practise among Kerala paddy farmers by gathering green leaves from nearby trees, but changes in cropping systems, reduced tree availability, land constraints, and labour shortages hindered this practice. Planting green leaf manure trees directly on bunds can address these issues. This study assesses the growth and green biomass production of selected green leaf manure tree species on wetland paddy bunds, as well as root distribution patterns and interactions with adjacent paddy crops. This study was conducted at paddy fields of State Seed Farm, Mannuthy, Kerala, on existing bund-grown trees comprising of nine different species such as Sesbania grandiflora, Terminalia arjuna, Gliricidia sepium, Cassia siamea, Gmelina arborea, Calliandra calothyrsus, Morus indica, Neolamarckia cadamba, and Cajanus cajan. The study was designed using a Randomized Block Design (RBD) with three replications. Trees were planted at 1-meter spacing and maintained as hedgerows. Growth, yield and root distribution parameters were measured during the second year after establishment. The results revealed significant variation in growth and green biomass production among the bund-grown tree species. N. cadamba excelled in collar diameter, while Morus indica underperformed. C. cajan exhibited complete mortality after one-and-half years, resulting in a survival percentage of zero by the end of the study. N. cadamba and T. arjuna showed notably high annual green biomass yields, at 7.03 kg and 4.54 kg per tree, with survival rates of 96 and 90 per cent, respectively. Other species produced moderate levels of green biomass, with the exception of G. arborea and M. indica, which yielded minimal to no biomass. Root distribution analysis revealed that T. arjuna demonstrated a pronounced rooting depth of 106 cm and a taproot length of 75.67 cm. The maximum lateral root spread was observed in N. cadamba (1.96 m) and G. arborea (1.94 m), both exhibiting a high number of roots with substantial diameter, enhancing bund stability and resource utilization. Root angle analysis showed a preference for lateral spread in G. sepium (82.62°) and C. siamea (80.57°). Conversely, M. indica and C. cajan underperformed across most of the root parameters. 124 Paddy beneath S. grandiflora showed highest grain yield of 5.31 Mg ha-1 within 1m distance from the tree and 5.22 Mg ha-1 at 1-2 m, which indicates the complementary interactions. Moreover, paddy yield under all other tree species were comparable to the control plot except M. indica, T. arjuna and C. calothyrsus, which exhibited competitive interactions with paddy up to a distance of 2 m from the tree base. Paddy nutrient content analysis revealed that paddy fields adjacent to C. cajan exhibited higher concentrations of nitrogen, phosphorus, and potassium compared to other treatments. Soil pH significantly influenced nutrient availability, with C. cajan showing the highest pH (6.06) and C. siamea the lowest (4.31). Electrical conductivity (EC) was highest in treeless bunds (0.17 dS m-¹) and T. arjuna (0.13 dS m-¹), while C. cajan and N. cadamba exhibited lower EC (0.06 dS m-¹). Soil organic carbon (SOC) was highest in open fields (1.21%). Soil beneath T. arjuna had the highest nitrogen (172.20 kg ha-¹) and potassium (41.44 kg ha-¹), while C. cajan had the highest phosphorus (62.09 kg ha-¹). In conclusion, the study highlights N. cadamba and T. arjuna as the most productive and adaptable species, demonstrating superior growth and green biomass production on wetland paddy bunds. S. grandiflora and C. siamea also produced considerable green biomass with minimal interactions with paddy crops. T. arjuna and M. indica were associated with reduced paddy yields, likely due to competition and pest pressures. G. sepium is also a potential species, however, it may perform optimally under conditions that include seedling planting and specific management practices. However, long-term studies are required to get more concluding results on crop interactions and soil fertility changes