**AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** SEASONAL SOIL FREEZE/THAW (FT) IMPACTS ABOUT HALF OF THE NORTHERN HEMISPHERE. IT IS A DOMINANT CONTROL ON NUTRIENT AND CARBON CYCLES IN TEMPERATE ECOSYSTEMS AND AFFECTS PERMAFROST, AGROECOSYSTEMS, URBAN ECOSYSTEMS, AND THE INTERFACE BETWEEN THE HUMAN AND BUILT ENVIRONMENT. A FUTURE COMBINATION OF RAPIDLY WARMING WINTERS PUNCTUATED BY EXTREME COLD SNAPS SUGGESTS THAT FT EVENTS ARE LIKELY TO INCREASE IN MID- TO HIGH-LATITUDE REGIONS. YET A LACK OF IN SITU SOIL FROST OBSERVATIONS PRESENTS A KEY KNOWLEDGE GAP IN ASSESSING THE TIMING AND EXTENT OF FROZEN SOIL AND ITS IMPACTS. THE CONVERGENT EFFORT OF THE FROSEN PROJECT BRINGS TOGETHER AN INTERDISCIPLINARY TEAM OF ECOLOGISTS, HYDROLOGISTS, ENGINEERS, AND CLIMATE SCIENTISTS TO DEVELOP AND TEST NOVEL, COMPLEMENTARY SYSTEMS FOR UNDERSTANDING THE SPATIAL AND TEMPORAL VARIATION OF SOIL FROST ACROSS THE LANDSCAPE. THE MANUAL FROST TUBE, REIMAGINED AS A LOW-COST, LOW-POWER, MINIATURIZED IN SITU WIRELESS SENSOR, WILL BE COMBINED WITH GROUND PENETRATING RADAR (GPR) TECHNOLOGY DEPLOYED ON A NEAR-SURFACE, AIRBORNE PLATFORM TO CHARACTERIZE THE SPATIOTEMPORAL HETEROGENEITY OF SOIL FROST. BOTH EFFORTS WILL BE INTEGRATED INTO A STATEWIDE SOIL FROST MANIPULATION EXPERIMENT THAT PROBES THE CONSEQUENCES OF SOIL FT FOR ECOSYSTEM CARBON AND NUTRIENT RETENTION.