Working In the Heart of the Critical Zone

While the term "Critical Zone" may conjure up science fiction visions, the real-life Critical Zone is an area of immense importance to life on earth.

"The Critical Zone is the area from the base of groundwater to the top of the vegetation canopy," explains Julie Pett-Ridge, Assistant Professor of Geochemistry and Biogeochemistry in the Department of Crop and Soil Science. The term "Critical Zone" was coined about 12 years ago to describe the special place where the hydrosphere, lithosphere, atmosphere, and biosphere intersect. There is a lot going on in this area: soil creation, water movement, and nutrient cycling.

Soil is at the heart of the critical zone, Pett-Ridge points out. Understanding critical zone processes is at the center of her research into biogeochemistry, the study of coupled biological and geochemical elemental cycles on the Earth’s surface.

Pett-Ridge's research focuses on three processes occurring in the critical zone: chemical weathering, soil formation, and the supply of rock-derived nutrients to ecosystems. Chemical weathering, which is the breaking down of minerals in rocks when exposed to water, leads to soil formation and the release of elements such as phosphorus, calcium, and potassium that can be taken up by plant roots.

Pett-Ridge's work has led her to work with the Critical Zone Exploration Network, a group of scientists funded by the NSF who are studying the processes that occur in the Critical Zone. CZEN works in a number of observatory sites across the US. Pett-Ridge has worked at three sites – the Luquillo Mountains of Puerto Rico, the Hawaiian Islands  and a soil chronosequence in Merced, CA, where she has collected samples of soil, stream and rainwater, as well as plant tissue.

"These CZEN sites are valuable to us because they have previously been well studied from both an ecological and a geological perspective," explains Pett-Ridge. "Hydrology data are available too, so we can build off of that."

Working to understand the processes occurring in the Critical Zone has practical implications. For example, Pett-Ridge's work into biogeochemical cycling can lead to a better understanding of how a soil’s nutrient supply capacity changes in response to different land management practices or changing climactic conditions.

Pett-Ridge admits that she is fascinated by the complex interactions among geology, soils, climate, water and plants.

"The Critical Zone is where all these processes intersect, and it sustains all terrestrial life on the planet."