Land use effects on soil organic matter in Manali watershed of Thrissur district, Kerala

Abstract

Soil Organic Matter is fundamental to the vitality of terrestrial ecosystems, underpinning nutrient cycling, soil fertility, and the equilibrium of microbial communities. Its composition and abundance are intricately shaped by environmental variables and anthropogenic activities, notably land-use practices. A nuanced comprehension of these influences is indispensable for formulating effective soil management strategies. The study "Land Use Effects on Soil Organic Matter in Manali Watershed of Thrissur District, Kerala", investigates the ramifications of various land-use systems on the composition, quality, and quantity of SOM. This research highlights pathways to sustainable land stewardship by exploring the relationship between land use systems and soil health. Soil samples were collected from six major land-use systems in the Manali watershed: natural forest, rubber, coconut, rice, banana, and nutmeg with four samples from each system, and analyzed for their physicochemical and biological properties. Various soil carbon fractions, including total organic carbon, labile carbon, particulate organic carbon, microbial biomass carbon, and mineralisable carbon, were measured. Forest soils had the highest carbon fractions and microbial activity, indicating superior organic matter content and biological health. In contrast, nutmeg and banana soils had the lowest. Forest soils recorded the highest Carbon Pool Index (1.92), Carbon Lability Index (0.93), and Carbon Management Index (174.65), reflecting enhanced carbon stability, while nutmeg and banana soils showed the lowest values. The study revealed strong interconnections between soil physicochemical, biological, and carbon fraction properties. Soil organic carbon showed positive correlations with total nitrogen (0.821***) and soil aggregation (0.686***), underlining its importance in nutrient retention and structural stability. Biological properties, such as enzyme activities and microbial populations, were closely linked with carbon fractions, including total organic carbon, microbial biomass carbon, particulate organic carbon, and labile carbon, emphasizing the central role of organic matter in supporting microbial activity and nutrient cycling. Total nitrogen also correlated positively with microbial populations and enzyme activities, highlighting its role in soil metabolism. Additionally, microbial activity was associated with soil aggregation, suggesting its contribution to maintaining soil structure. The strong interrelationships among carbon fractions indicate their collective influence on organic matter stability and nutrient availability. Humic acid and humin fractions were extracted and purified from soil samples to evaluate the impact of different land-use practices on soil organic matter stability. The findings revealed that forest soils yielded the highest levels of humic acid at 23.96 g kg⁻ ¹ and humin at 16.48 g kg⁻ ¹, highlighting their superior organic matter stability. Conversely, soils from banana and nutmeg plantations showed the lowest yields, highlighting the significant influence of land-use practices on soil quality. Additionally, forest soils had the highest proportion of humic acid, while rubber plantation soils exhibited the lowest. Humin content varied across land uses, with coconut soils containing the highest percentage and rubber plantations the lowest. The chemical characterisation of soil humic acid across various land use systems revealed notable differences in acidity and elemental composition. Rice systems recorded the highest levels of total acidity, reaching 11.13 meq/g, along with carboxylic groups at 4.57 meq/g and phenolic groups measuring 6.56 meq/g, while rubber soils exhibited the lowest values, indicating significant variability in humic acid properties. Elemental analysis also showed variation, with the forest system containing the highest concentrations of carbon, hydrogen, nitrogen, and sulphur, suggesting a richer organic matter content. In contrast, banana soils had the lowest elemental levels, whereas rice and rubber soils maintained moderate compositions. The carbon to nitrogen (C/N) ratio peaked in banana soils at 22.99, followed closely by coconut soils at 22.36. In contrast, forest soils had the lowest ratio of 17.26, reflecting differences in organic matter quality and potential decomposition dynamics among the land use systems. The structural characterization of soil humic acid was conducted using UV-VIS, VIS-IR, FTIR, fluorescence, and Raman spectroscopy. The UV-Vis spectra showed a distinct peak between 221 and 226 nm, followed by a steady exponential decline from 230 to 700 nm. Absorption coefficient ratios varied, with the lowest E2/E4 (5.05) and E2/E6 (25.73) ratios in forest soils, and the highest in rice (6.99) and rubber (32.80) systems. The E4/E6 ratio ranged from 4.59 in rice to 5.12 in rubber. VIS-IR spectra exhibited a general decrease in absorbance with wavelength. Forest soils had the highest absorbance at lower wavelengths, indicating greater concentrations of organic chromophores in forest soil humic acid and increased activity in the visible and near-infrared regions. FTIR spectra showed broad peaks in the high-wavenumber region (4000–3000 cm⁻ ¹) and sharp peaks in the fingerprint region (1500–400 cm⁻ ¹), indicating functional group variations such as C=O, COOH, –OH, and C=C across systems. Fluorescence spectra showed a broad emission peak near 450 nm, with rice soils displaying the highest intensity, suggesting higher humification. Raman spectra revealed diverse vibrational modes, with coconut and rubber soils showing the most complex patterns. These findings highlight significant structural differences in humic acid across land-use systems, emphasizing the impact of land management on soil organic matter characteristics. The electrochemical properties of soil humic acids varied notably across land use systems, particularly in surface charge and particle size. All systems showed negative zeta potentials, with rubber plantations showing the highest negative potential of –66.25 mV, followed by forest soils at –61.50 mV, and banana plantations at –60.20 mV, indicating strong colloidal stability. Average particle sizes ranged from smaller in rice soils (354.50 nm) to larger in forest soils (1739.05 nm), reflecting differences in aggregation and structure. This study underscores the profound influence of land use systems on the composition, quality, and stability of soil organic matter. Forest soils exhibited the highest carbon fractions, microbial activity, and enzymatic functions, indicating superior soil health, while banana and nutmeg soils recorded the lowest. Humic acids varied in composition, acidity, and elemental content, with forest soils demonstrating the highest organic matter stability. Spectral and electrochemical analyses revealed differences in structural characteristics, surface charge, and particle size, reflecting variations in soil aggregation. These findings highlight the impact of land use practices on soil organic matter dynamics and emphasize the need for sustainable management to enhance soil quality and fertility.