Investigating the Root Zone Critical Interface in Intensively Managed Critical Zones

The Critical Zone (CZ) in the Midwestern United States has transformed from predominantly prairie landscapes to highly productive row-crop agriculture that requires intensive management such as tillage, tile drains and fertilizer inputs. The Critical Interfaces CZ Network (CINet) project focused on three critical interfaces that are important regulators of material storage, transport and transformation in the CZ: the near-land surface, the active root zone and the river corridor. To investigate the root zone critical interface, we established instrument clusters called MIRZ (Management Induced Reactive Zone) in Illinois and Nebraska on both agriculture and restored prairie land management. The study sites differ in climate and geology: Illinois has wetter conditions (100 cm MAP) with loess over glacial till and extensive tile drainage, while Nebraska is drier (78 cm MAP), formed in loess, and lacks artificial drainage. At each site, precipitation, soil porewater (sampled at 20, 60, 110, and 180 cm depths), surface waters, tile drains, groundwater and soil gases were collected biweekly. In addition, co-located sensors were installed to monitor soil moisture, temperature, electrical conductivity, oxygen, carbon dioxide, and meteorological conditions at hourly intervals. Bulk soil measurements included geochemistry, carbon/nitrogen concentrations, mineralogy, density and particle size. Key findings from the MIRZ root zone measurements suggest that land use strongly controls how quickly water moves through soils and how much geochemical alteration occurs before water reaches streams. Longer water residence times and greater water–mineral interaction occur in agricultural soils, whereas stronger soil structure and deeper root systems in restored prairies promote rapid infiltration and more limited geochemical alteration. The geochemical similarity between agricultural porewaters and stream or tile-drain waters highlights strong hydrologic connectivity and implies that agricultural land use fundamentally alters root-zone structure, water flow paths, and ultimately stream geochemistry at the watershed scale. The diverse and deeply rooted prairie vegetation also influences soil gases, with higher carbon dioxide production rates and enhanced seasonal variability in prairie soils compared to agricultural soils. The widespread conversion of Midwestern USA prairies to intensive agriculture has therefore altered solute, carbon, and gas fluxes throughout the root zone critical interface, including the depth and intensity of the reactive zone where weathering, nutrient cycling, and carbon storage occur.