tors at work in soil, that is, climate, vegetation, parent material, and time. These state factors interact with human activity to provide quantitative understanding of additional soil responses that can be used to determine the potential long-term impact of soil management decisions (intentional and unintentional) on the soil resource.

Tracers are available from natural and human-made (i.e., from atomic weapons testing) isotopes; however, the number of these tracers is decreasing because of the elapsed time since those tracers were introduced into the atmosphere. The analytical tools exist to use these tracers as reliable measures of the indicators. Some of the reasons that tracers are not more widely used include a lack of understanding in the scientific community of the potential use of tracers to address soil science questions, a perceived expense of isotope measurements, and the need for geochemists familiar with tracer methods to work with soil scientists in defining questions that the use of tracers can answer. Trumbore suggested that a combination of recent methodological advances and framing of critical questions makes this an appropriate time for a more systematic application of a suite of tracers to study problems in soil science.

Trumbore presented three examples of how tracers can be applied to soil science research: (1) use of inert or biologically unreactive tracers to separate physical from biological and chemical processes, (2) the use of time-sensitive tracers to determine the rates of soil processes on several timescales, and (3) the use of isotopic or elemental fingerprints to determine the relative importance of different processes or sources of elements in soil and soil solution. She discussed these in the context of important soil geochemistry research topics.

Tracers can be applied to identify nutrient supply to plants through separation of weathering, recycling, and dust inputs into soil nutrient pools. These applications provide insights into the dynamics of nutrients in different soils. Tracers can also be used to evaluate trace gas emission from soils. Soils serve as sinks and sources of greenhouse gases; however, tracers can serve as indicators of the interacting processes occurring within the soil volume. Quantification of erosion rates, deposition within the landscape, and restoring soil is a complex set of processes. Tracers have been applied to the question of soil restoration, addressing the question of time required for restoration. Tracers have been used as tools to fingerprint sources of soil-derived materials that move from the landscape into nearby water bodies, providing quantification of the source and movement of soil materials for environmental quality assessments.

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