Climate-Change Affected Processes and Reactions in Soils: A Review of Molecular and Microscale Investigations and Upscaling Efforts.

Climate change variables [higher atmospheric CO₂ concentration and air temperature (2-4°C or greater), and intensive rainfall and/or heavy storms] and extremes (heat wave and dry spells) may cause long-lasting changes in soil properties that are currently poorly understood.  The soil responses to climate change would be complex because of the intricate network of simultaneous, coupled and/or sequential soil chemical, biological and hydrological reactions and processes.  In addition, chemical elements, nutrients and contaminants involved in these reactions and processes (which are often time-dependent) are distributed in the soil solid, liquid and gas phases.  Finally, scale-dependent effects related to the solid phase mineralogical, chemical and physical spatial heterogeneities are also important. Source/sink behavior of soil in terms of carbon storage and elemental cycling depends on molecular-scale reactions and processes.  However, state-of-the-art climate-land models utilize km-scale grid size while the molecular-scale processes are studied within nano/micrometer-scales. A spatial scaling gap of many orders-of-magnitude does therefore exist. Incorporation of molecular-scale process knowledge in upscaling models is currently limited by the complexity of the soil system (reaction network), limitations in up-scaling schemes and computational power.  Several methods are being proposed to examine the impact of climate change reactions and processes in soils. Small scale measurements can be used to characterize the physical, chemical, mineralogical and related microbiological properties of the soil; then biogeochemical modeling can be used to investigate the susceptibility of different soil types to changes in properties due to climate change variables, and to make predictions of the impact of these changes on elemental cycling.  The scaling issue could be addressed by using hyperspectral remote sensing data and geostatistical methods. This paper includes a discussion about important and challenging aspects of climate effects on soils and upscaling, reports recent discoveries, identifies key research needs, and highlights research opportunities offered by the changing climate.