Assessment of Physico-Chemical and Biological Properties of Mangrove Soils in Mundrothuruth, Kollam District, Kerala, India

Abstract

Introduction: Mangrove soils, common in wetland ecosystems, are typically anoxic and rich in organic matter due to waterlogging that slows decomposition. These soils, often comprising less than 35% sand, 40% silt, and 45% clay, influence mangrove forest structure, composition, and productivity through their nutrient content. Despite generally low nutrient levels, soil characteristics vary significantly across ecosystems and within mangrove stands. The present study examines the physico-chemical and biological properties of rhizosphere and non-rhizosphere soils from three sites in the Munrothuruthu wetland, Kerala, India.

Objectives: The present study aims to compare the soil characteristics of rhizosphere and non-rhizosphere soil associated with mangrove vegetation. Also aims to enhance the importance of conserving mangrove ecosystems for their ecological and environmental benefits.

Methods: The present study was conducted in the wetlands of Munroe Island (also known as Munrothuruthu) in Kollam District, Kerala. Covering an area of approximately 13.4 km², Munroe Island is a Panchayath situated at the confluence of Ashtamudi Lake and the Kallada River. Over 70% of the Panchayath lies within the delta formed by the Kallada River, and the local population primarily depends on prawn farming for their livelihood. For the present study, soil samples were collected from the selected three locations within Munroe Island. At each location, three soil samples were obtained from non-rhizosphere and rhizosphere by excavating pits to a depth of 30 cm using a spade. Soil samples were collected from two distinct layers; the top layer (0–15 cm) and the sub-surface layer (15–30 cm). Care was taken to prevent any mixing of soil between the layers. The parameters studied included soil pH, electrical conductivity, soil moisture content, water-holding capacity, organic carbon, available phosphorus, available potassium, micronutrients and VAM colonisation studies were carried out employing standard methods.  

Results: The soils exhibited acidic conditions, with pH averaging 6.1 in topsoil and 5.6 in subsoil. Electrical conductivity ranged from 0.24 to 0.75 mmhos, while subsoil moisture content exceeded that of topsoil. Organic carbon was highest in non-rhizosphere topsoil (2.7%) and lowest in rhizosphere topsoil (0.4%). Macro and micronutrient availability varied notably. Phosphorus peaked in non-rhizosphere subsoil and was lowest in rhizosphere subsoil. Potassium levels were relatively stable across samples. Calcium ranged from 44.8–51 ppm, magnesium from 22.5–93.5 ppm, and sulphur was highest in non-rhizosphere topsoil (60.57 ppm). Iron and boron concentrations were greatest in rhizosphere subsoil, while zinc and copper were highest in non-rhizosphere soils. Manganese was most abundant in rhizosphere topsoil (44 ppm). Root colonization by Vesicular Arbuscular Mycorrhizal (VAM) fungi was also observed.

Conclusions: The results indicate that soil physico-chemical properties significantly influence the ecophysiology, vegetation, species composition, and forest structure of mangroves.The findings of the study emphasize the vital role of soil properties and mycorrhizal associations in mangrove ecosystem functioning and call for conservation strategies that prioritize soil health and microbial interactions under increasing anthropogenic stress