Soil microbial dynamics and enzymatic activity under boundary plantation of green manure trees in wetland paddy system of Kerala

Abstract

Integrating appropriate tree species on paddy boundaries is one of the key strategies to enrich soil health by sustaining microbial communities and enhancing nutrient cycling for long-term soil fertility replenishment. The present study investigated the impact of twelve green manure tree species (Terminalia arjuna, Neolamarckia cadamba, Gmelina arborea, Cassia siamea, Hibiscus tiliaceus, Gliricidia sepium, Garuga pinnata, Pongamia pinnata, Bridelia retusa, Thespesia populnea, Acrocarpus fraxinifolius, and Macaranga peltata) planted along wetland paddy field boundaries following randomised block design (RBD) with three replications on soil microbial properties in Kerala, India. After two years of establishment, soil samples (0-20 cm depth) were collected from base of each tree species and from treeless control plots. Microbial populations (bacteria, fungi, actinomycetes) were quantified using serial dilution techniques, and dehydrogenase activity was measured as an indicator of microbial oxidative metabolism. Results revealed a significant increase in microbial populations and dehydrogenase activity in soils under boundary plantations compared to treeless controls. The highest bacterial count was observed under Neolamarckia cadamba (22.17 cfu g-1×105) while Cassia siamea (16.67 cfu g-1×104) supported the highest fungal population and Thespesia populnea (5.17 cfu g-1×104) recorded the maximum actinomycete count. Hibiscus tiliaceus (3.99 µg TPF g⁻¹ 24 hr⁻¹) and Cassia siamea (3.90 µg TPF g⁻¹ 24 hr⁻¹) exhibited the highest dehydrogenase activity, indicating enhanced microbial metabolic potential. These improvements were attributed to increased organic matter inputs, root exudates, and improved microclimatic conditions under tree cover. The findings underscore the potential of tree-based boundary interventions for enhancing soil biological health in paddy agroecosystems.