Temperature and Moisture Interact to Constrain Soil Respiration From Ultisol Soils.

Climate warming has the potential to release large amounts of carbon dioxide from soils; the largest terrestrial carbon store. Previous experiments in temperate and boreal forest ecosystems have shown increased efflux of carbon dioxide in response to soil warming. In contrast to these warming studies, our warming experiment in the highly weathered Ultisols of the Georgia piedmont has not triggered an increase in carbon dioxide efflux. To investigate the potential mechanisms driving this lack of response, we tested the role of temperature, moisture and their interaction in regulating microbial activity. In a fully factorial mesocosm experiment, we incubated soils at 5 temperature levels (5, 15, 25, 30, or 35°C) and 4 moisture levels (5, 10, 20, or 30 %) for several months. These incubations were conducted from soils collected in the winter, spring and summer of 2013. Initially, temperature was the main driver of soil respiration, with moisture further modulating this response. Over the course of the incubation, organic matter substrates became depleted resulting in overall declines in respiration and a statistical interaction between temperature and moisture. Over three seasons, the thermal optimum for soil respiration tracked seasonal changes in temperature. Our results suggest that soil microorganisms are responsive to changes in temperature and moisture and respond physiologically to seasonal changes. Furthermore, substrate limitation in these highly weathered soils may constrain an expected temperature response to soil respiration. These findings help inform predictions of how ultisol soils will respond to climatic warming and shed light on the lack of soil warming response in situ.