Soil temperature increased by ca. 1.1°C in a fruit orchard due to climate change - results of 60 years records

Background: soils act as a buffer in times of climate change alike oceans. The vast majority of biological climate change studies concentrates on arable crops viz plants, while changes brought about on soils are somewhat neglected. Soils are the substrate for the majority of agricultural crops and thereby aid to provide food (and energy). Changes in the soil such as soil temperature are slow and hence long-term records required. In the apparent absence of such orchard data, the objective was to investigate the changes in soil temperature in a fruit orchard based on long-term records, with a monthly breakdown and interpret the results with respect to tree physiology. Methods: Soil temperature was measured at Campus Klein-Altendorf (latitude 50 degrees N) in the Eifel foothills near Bonn, Germany at 20 cm soil depth for 60 years. This experimental station of the University of Bonn is within the Meckenheim fruit growing region in Germany. Soil temperature in the first three decades came from daily manual readings, recorded on filing cards and those of the last three decades from an automated weather station. Results: Soil temperatures were digitized and checked for plausibility before further processing. Two distinct phases were considered in line with the German met office, the first one without and the latter with climate change to achieve a balanced weighted comparison over ca. 60 years. Based on this un-bias approach, the summer soil temperature increased by 0.9 degrees C compared with a larger increase of 1.2 degrees C in the winter. A monthly breakdown showed the strongest increase in soil temperature in January, followed by February to April. Conclusions: The results are discussed with respect to biological activity such as earlier bud break and flowering, but also soil respiration and degradation of organic matter.