Analysis of long-term trends in air and soil temperature in a semi-arid region in Iran

Long-term trends in soil temperature are important, but rarely reported, as a sensitive climate indicator and its impact on biological, chemical, and physical processes that occur in the soil. In this study, the simple linear regression and nonparametric Mann–Kendall (MK) trend test were employed to identify trends in monthly air and soil temperature anomalies (ATA and STA, respectively) time series at 5–100 cm depths of a soil profile, based on long-term (1987–2016) observation data of meteorological station of Bajgah Agricultural Experimental station, located in a semi-arid region in Iran. The results indicated a significant warming trend in ATA for February–October, with the highest and lowest rate of warming in March (0.103 °C year−1) and December (− 0.001 °C year−1), respectively. This study further revealed a general and strong increase in monthly means of STA over the studied period. The rates of soil temperature warming trend were 2–3 times greater than those of air temperature. The greatest soil warming increases with the rates of 0.20 and 0.19 °C year−1 were, respectively, observed at 5 and 10 cm soil depths in July. The applied linear trend indicated that the STA at 10 and 30 cm depths have warmed during May–October. In addition, the MK test revealed a significant trend in STA at 10 cm depth in March. Such significant warming trends in subsurface soil depths including 50 and 100 cm were observed for June–December during the whole period.