Isotopic dilution methods to determine the gross transformation rates of nitrogen, phosphorus, and sulfur in soil: a review of the theory, methodologies, and limitations

The rates at which nutrients are released to, and removed from, the mineral nutrient pool are important in regulating the nutrient supply to plants. T hese nutrient transformation rates need to be taken into account when devel oping nutrient management strategies for economical and sustainable product ion. A method that is gaining popularity for determining the gross transfor mation rates of nutrients in the soil is the isotopic dilution technique. T he technique involves labelling a soil mineral nutrient pool, e.g. NH4+, NO 3-, PO43-, or SO42-, and monitoring the changes with time of the size of th e labelled nutrient pool and the excess tracer abundance (atom%, if stable isotope tracer is used) or specific activity (if radioisotope is used) in t he nutrient pool. Because of the complexity of the concepts and procedures involved, the method has sometimes been used incorrectly, and results misin terpreted. This paper discusses the isotopic dilution technique, including the theoretical background, the methodologies to determine the gross flux r ates of nitrogen, phosphorus, and sulfur, and the limitations of the techni que. The assumptions, conceptual models, experimental procedures, and compo unding factors are discussed. Possible effects on the results by factors su ch as the uniformity of tracer distribution in the soil, changes in soil mo isture content, substrate concentration, and aeration status, and duration of the experiment are also discussed. The influx and out-flux transformatio n rates derived from this technique are often contributed by several proces ses simultaneously, and thus cannot always be attributed to a particular nu trient transformation process. Despite the various constraints or possible compounding factors, the technique is a valuable tool that can provide impo rtant quantitative information on nutrient dynamics in the soil-plant syste m.