Soil properties and organic matter quality in relation to climate and vegetation in southern Indian tropical ecosystems

Abstract

Soil organic matter (SOM) quantity and quality can be directly altered by climate, and SOM has been proposed as both a significant source and sink of carbon dioxide. To understand the factors that influence SOM, the present study focuses on the relationship between precipitation and soil physical and chemical properties along a rainfall–vegetation gradient in a seasonally dry tropical forest in the Western Ghats, southern India. Soil pH decreased with increasing rainfall, but soil organic carbon (SOC) and nitrogen (N) slightly increased. Rainfall had strong positive correlations with standing litter (Pearson’s r = 0.85), clay content (r = 0.69) and recalcitrant organic matter (r = 0.73) (all P < 0.05), and chemical properties SOC (r = 0.85), N (r = 0.71), magnesium (r = 0.65), iron (r = 0.64) and zinc (r = 0.75) (all P < 0.05). The heavy fraction, which was bound to mineral particles, comprised more than 60% of the SOM in all sites. The heavy fraction was positively correlated with rainfall and showed a higher proportion in the high rainfall forest soils. Thermal recalcitrant organic matter, such as carbohydrates – and other aliphatic compounds, such as lignin and polyphenols – were also higher in high rainfall forest soils, suggesting a greater effect of SOM turnover. The SOM content and quality were lower in the dry forest soils, which were mainly limited by annual precipitation, but clay content also contributed significantly to SOC storage.