The primary mechanism for C uptake onto land is photosynthesis, yet long-term terrestrial reservoirs of C reside in soils. Sequestration generally occurs where net primary production exceeds C losses to decomposition, leaching export, and/or combustion. Soil C can be physically protected in cold or wet soil environments, or chemically protected by recalcitrant organic compounds, C stabilization onto mineral surfaces, and/or microaggregate formation. Furthermore, feedbacks and responses occur between soil, plant, and water systems that retard or accelerate the uptake and release of C. The development of isotopic tracers for atmosphere-geosphere interactions has strengthened our ability to quantify the inputs, turnover, movement, and transformation of soil C. For example, 14C analysis can provide insight into multiple soil processes, while beryllium isotopes (10Be and 9Be) have been useful in investigating soil development, sedimentation, and mineral weathering. Radiocarbon and 10Be both have atmospheric inputs, but their differences in terrestrial biogeochemical cycling can discern landscape (erosion, deposition) and site-scale (plant-atmosphere) processes. Modeling, particularly inversion modeling, can help to constrain rates of soil processes where terms for mathematical models are ‘solved’ by discreet measurements. For soils in high latitude ecosystems, we have constrained C losses to decomposition and combustion, and have investigated interactions among combustion, decomposition, thermal regimes, and hydrology through data-model iterations. In temperate ecosystems, we have constrained rates of loess deposition and hillslope evolution, and addressed the distribution of eolian and alluvial sediment for a small drainage basin. In these examples, sink/source thresholds exist during transitions between disturbance and adjustment. The timing and magnitude of such thresholds are highly sensitive to feedbacks among decomposition, nutrient cycling, and soil climate. Ultimately, an understanding of current soil processes, as well as the legacy and interrelationships of those mechanisms at local, landscape and regional scales, is needed to accurately assess the fate of soil C reserves under a changing climate.