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Critical Zone Research and Observatories: Current Status and Future Perspectives

The Critical Zone is defined as the layer of the land surface that extends from the bottom of the weathering zone (above fresh bedrock) to the top of the vegetation canopy. This layer that ranges from a few centimeters in some locations to tens of meters at other locations provides the support for agriculture, forestry, pasture, and other ecosystems, and hence sustains life. Over the past decade with funding from the National Science Foundation (and other sources) numerous research groups have set up Critical Zone Observatories (CZOs) in natural or managed ecosystems. This paper summarizes the progress made over the past 5 years on research in the Critical Zone and advocates for a global network of CZOs (some already set up in the U.S., Europe, China, and Australia) for study of Critical Zone processes in various climate, soil, vegetation, and landscapes. The Critical Zone embraces atmosphere, biology, ecology, soil science, geophysics, geochemistry, geomorphology, and hydrology research. However the Critical Zone is under tremendous stress due to growing human population, increased agriculture, deforestation, and human built structures all increase soil erosion and loss. The spatial and temporal scale of processes in the Critical Zone ranges from the pore scale to the continental scale and from seconds to geological time scales. Over the last 5 years there have been over 200 peer-reviewed articles  published on various topics related to the Critical Zone.

Diagram of critical zone

The authors categorize Critical Zone research in three aspects of deep science – deep time, deep depth, and deep coupling. The deep science emphasizes the coupling of the different processes as listed in the previous paragraph. In deep time, the time scales of the different processes fall between the diurnal scales (fluctuation of soil temperature, vegetation growth) to the thousand-year scale (weathering and formation of soil). All other processes occur in between these two ends of the spectrum. Another important aspect of studies at CZO sites is the link between past history of processes and their use for prediction of the future. These include soil formation rate of the past to predict the future and the prediction of Critical Zone services from land use management. In deep depth, the studies integrate the root zone to the deep weathered bedrock. These two layers are linked via the infiltration or preferential flow of water and play a role in bedrock disaggregation and physical and chemical weathering due to groundwater flow. As larger vegetation has roots that go deeper than the traditional surface layer (say 1-2 m) the withdrawal of water from the deeper layers by the roots thereby affects these layers. The coupling between biogeochemistry and hydropedology helps to understand the flow and transport of elements and bedrock weathering.

Understanding Critical Zone services is of great value for management. This includes land use management, growth of agricultural crops, forest management, and other water resources management. There exists a global network of CZOs as mentioned earlier but work has to be carried out to coordinate research efforts and relate results between different climate, soil, biomes, and ecosystem regions. Also a library of models for the Critical Zone needs to be established so that they are easily accessible along with exchange of data sets to various researchers across the globe.

Read the full open access article in Vadose Zone Journal