Spotting ethylene in forest soils—What influences the occurrence of the phytohormone?

Due to the rapid microbial degradation, ethylene (ethene; C 2 H 4 ) detection in most soils is difficult. The knowledge about factors favoring the occurrence of C 2 H 4 is mainly based on laboratory experiments which are not necessarily representative for real field conditions in forest soils. We report results from a total of 24 measuring plots in southwest Germany and more than 50,000 gas samples, including long-term forest monitoring sites and other study sites involving liming, artificial soil compaction, and temporary waterlogging. Many of the patterns and influencing factors identified in the laboratory were confirmed. In well-aerated forest soils, C 2 H 4 was detected in 1.3% of the gas samples. C 2 H 4 detection was higher at oxygen (O 2 ) concentrations below 10% in the soil. The effect was positively correlated with bulk density in compacted soils and CO 2 , CH 4 , and N 2 O concentrations—factors that indicate reduced microbial activity and thus also facilitate the detection of C 2 H 4 . It could be detected 3.5 times more often in spruce than in beech stands, indicating that plant species plays a role and was not found to follow any distinct spatial or temporal pattern with the exception of seemingly random spatio-temporal clusters of root-born C 2 H 4 near large roots of spruce trees. The long observation period and large dataset of this field study allowed valuable insights into C 2 H 4 occurrence in forest soils under natural conditions supporting the theory that the plant hormone C 2 H 4 is produced as a warning for anaerobic soil areas that are limiting root growth.