Long-term trends and variability of soil temperature in contrasting agro-climatic zones of Tamil Nadu, India

Soil temperature is a key regulator of soil physical, chemical and biological processes, influencing nutrient cycling, root growth and overall ecosystem functioning. Long-term evaluation of soil thermal regimes is essential for understanding climate-driven changes in agricultural systems. This study analysed daily morning and afternoon soil temperature at 5, 10 and 20 cm depths (2009–2019) at Kovilpatti (semi-arid) and Ooty (high-altitude), Tamil Nadu, India. Trend detection was performed using the Mann-Kendall or modified Mann-Kendall test based on data independence and Sen’s slope estimator quantified trend magnitude (°C yr-1). Soil temperature exhibited strong seasonal, depth-wise and diurnal variability at both locations, with Kovilpatti consistently recording higher temperatures and greater diurnal amplitudes than Ooty. Results revealed significant cooling trends in Ooty, particularly during winter at shallow depths (e.g., -0.49 °C yr-1 at 5 cm in January), indicating increasingly colder soil conditions during winter, which could stress soil biota. In contrast, Kovilpatti showed widespread warming in morning soil temperatures across depths and seasons (up to 0.41 °C yr-1 at 10 cm in February), while afternoon temperatures exhibited mixed patterns, including notable cooling at 10 cm during monsoon and post-monsoon periods. Depth-wise analysis confirmed increasing morning temperature with depth and decreasing afternoon temperature with depth at both sites. These contrasting trends highlight the influence of regional climate on soil thermal behaviour. Rising soil temperatures in semi-arid regions may intensify thermal stress and necessitate modified irrigation and soil conservation practices. Thus, understanding such site-specific soil temperature responses is critical for developing climate-resilient agricultural management strategies.