Seasonal Soil Quality Gradients from Upper to Lower Kuttanad, an Indian Tropical Agro-Wetland System

Abstract

Soil quality in tropical agro-wetlands is strongly influenced by seasonal hydrology and geomorphological gradients. This study evaluated seasonal and altitude-based soil quality variations across Upper (Ala), Middle (Edathuva), and Lower (Kainakari) Kuttanad, a below-mean-sea-level Ramsar wetland in southwest India. Surface soils (0-15 cm) were sampled during pre-monsoon, monsoon, and post-monsoon seasons and analyzed for pH, electrical conductivity (EC), organic carbon, and available nitrogen, phosphorus, and potassium using standard methods. Mean soil pH indicated predominantly acidic conditions (4.48-5.23), while EC was significantly higher in Lower Kuttanad (0.55 µS m⁻¹), reflecting estuarine influence. Two-way ANOVA showed significant effects of season, altitude, and their interaction on all soil parameters (p < 0.05). Post-hoc analysis confirmed clear altitude-wise differentiation, with organic carbon enrichment in Middle Kuttanad and nutrient instability in Lower Kuttanad. Pearson correlation analysis revealed a strong positive association between nitrogen and potassium (r = 0.78). The Soil Quality Index (SQI) ranged from 0.58 to 0.69 and peaked during the post-monsoon season. The results of this study establish a distinct upper-middle-lower Kuttanad soil quality gradient, underscoring the need for season, and zone-specific soil management strategies to sustain agricultural productivity and ecological integrity in tropical agro-wetlands.