Moss regulates soil evaporation leading to decoupling of soil and near-surface air temperatures

Abstract

PURPOSE: Moss covers a vast area from the polar to the tropics on a global scale and has important regulatory effects on the biogeochemical processes in the soil. Previous studies had shown that moss on soil could reduce soil temperature in the warm period and plays an important role in the stability of the soil carbon pool in the context of global warming. The cooling effect of soil temperature by moss may be due to its ability to accelerate soil water evaporation. Most vascular plants achieve limited homeothermy by transpiration, but moss has no organs which can regulate transpiration. Can moss also regulate soil water evaporation as the transpiration of vascular plants? The objective of this study was to determine the differences in temperature and evaporation between moss-covered and bare soil. MATERIALS AND METHODS: A year-round mesocosm experiment was carried out to investigate soil temperature and evaporation with and without moss. We used a high time-resolution instrument which could simultaneously monitor soil temperature beneath moss layers, photosynthetically active radiation, and near-surface air temperature (20 cm above the soil) in a 10-min interval. In the meantime, we used a chamber method to monitor soil evaporation and heat flux inside the mesocosms simultaneously. RESULTS AND DISCUSSION: We found that the evaporation of the moss increased drastically when near-surface air temperature exceeded 30 °C, which kept maximum soil temperature around 30 °C. This finding was different from many previous studies which reported that the evaporation of moss was always greater than bare soil or similar to open water surface. The phenomenon we found may be important for moss, which allowed moss to minimize the loss of water at high temperature and maintain a relatively constant temperature. It is known that moss is a C₃ plant, and the high-temperature threshold for the photosynthesis of C₃ vascular plants is also around 30 °C. Thus, the temperature-regulating behavior could improve the net carbon gain for moss and benefit for its survival. To our best knowledge, there is no research reporting this phenomenon. In the context of global warming, the temperature-regulating behavior of moss is very important for its controls on soil carbon dynamics and its other ecological functions, especially at lower latitudes with higher soil temperature. CONCLUSIONS: Our result showed that moss can also achieve limited homeothermy through soil water evaporation similarly as the vascular plant through transpiration, and the temperature threshold was around 30 °C. This homeothermy led to the decoupling of soil temperature and near-surface air temperature. How mosses elevate water evaporation in response to high temperature remains to be a challenge for future research.