The role of pebbles in the water dynamics of a stony soil cultivated with young poplars

Background and aims Stony soils are widespread and often support plant production; nevertheless, their physical properties remain poorly understood, particularly regarding the role of rock fragments in plant water uptake. Understanding the hydric interactions between rock fragments and fine earth in stony soils remains central to water management in the contect of climate change. Methods A soil water retention curve of stony soil was developed based on a water retention curve for both rock fragments and fine earth. Water transfer between fine earth, rock fragment and plant roots was monitored during experiments under controlled evaporation conditions. Results During desiccation, the water retention curves for fine earth and rock fragments exhibited water movement from the rock fragment to the fine earth. During our evaporation experiment, fine earth water content decreased as soon as desiccation started, whereas the rock fragments released water after several days. We demonstrated that rock fragments are a water reservoir for plants; plants can uptake water directly from pebbles or after it is transferred into surrounding fine earth. However, this transfer could not be measured due to the non-equilibrium state between the two phases. Furthermore, we exhibited how the water potential dynamics in fine earth containers cultivated with poplar was not influenced by the rock fragments content. Conclusions Water in rock fragments can be directly released to plants or released through fine earth, thereby reducing plant water stress during moderate drought periods. Therefore, the assumption that rock fragments constitute an inert phase inhibiting ecological soil functions must be completely reconsidered.