Net annual change

+ 120

+ 1470

a With permission from Paustian et al. (1990).

a With permission from Paustian et al. (1990).

of the net primary production. Microbial respiration dominates faunal respiration by a ratio of 9:1, with heterotrophic CO2 evolution returning 32% of the primary production to the atmosphere. The annual change in the residues in the alfalfa field is attributable to the buildup of the plant residues in the perennial system.

Studies of substrate degradation in soil require both short intervals between early measurements and extension of the measurements over time intervals long enough to capture the full range of turnover times. Several tools are available for determining the pool sizes and dynamics of SOM fractions. The soluble components, and even some of the cellulose of plant residues, decompose within hours to days. If measurements are delayed, the degradation of the microbial products, rather than that of the original substrate, is measured. Incubations in the laboratory or in the field, extending to hundreds of days or longer, are needed to evaluate the more persistent components. Generally, incubations incorporating respiration measurements and curve-fitting analysis are sufficient for determining the short-term dynamics. Soil organic matter components also occur in intermediate pools with turnover times of 10-100 years. In some cases, the turnover rates of these constituents can be measured by using the stable isotope 13C (Balesdent et al., 1987). This isotope accounts for 1.1% of the CO2-C of the air. Plants with the C3 photo-synthetic pathway discriminate more against this isotope than do plants with the C4 pathway, resulting in differential 13C enrichment of plant materials. Residue inputs and SOM turnover can be measured using mass spectrometry in soils formed under C3 plants (cool season grasses, trees) if C4 plants such as maize or sorghum are then grown or if residues from these plants are added. The reverse sequence of crops will also make measurements possible. The natural abundance of 13C in the plants and soil may provide insufficient differentiation. In this case, artificially labeling plant materials with 13C by growing them in closed chambers with enriched 13CO2 or 14CO2 may provide a better tracer. Studies examining the transformations of N, such as plant uptake or mineralization, can be done using the 15N stable isotope tracer.

The slow decomposition rates of the most resistant fractions, making up 50% of the soil C and persisting for hundreds to thousands of years, are not easily measured with normal tracer techniques. For this, we resort to C dating, which utilizes the much longer half-life of naturally occurring 14C (Paul et al., 1997). Such studies have found that the average age (mean residence time) of organic matter in the surface of temperate agricultural soils ranged between modern and 1100 years, with an average of 560 years. Deeper in the soil profile (50-100 cm), the average age was 2757 years and ranged from 1500 to 6600 years. It is important to note that studies of nutrient and organic matter turnover are best conducted on well-characterized, long-term plots where the yield or primary productivity components, soil type, and long-term climate and management controls are known.

model selection and evaluation

As mechanistic soil-crop-water-atmosphere models become increasingly accepted as tools for analyzing agronomic or environmental issues, users are being faced with an increasing number of models to choose from. Ideally, a simulation model would include all of the processes dictating the dynamics of SOM or nutrient elements at a level of detail that represents the current state of the art in understanding, and model selection would be a moot point. Unfortunately, an ideal model for complex and heterogeneous agroecosystems is impractical. Not all system processes are fully understood and thus each model includes various approximations and simplifications. In addition, the documentation available for each model generally does not refer to the validity, limits, and potential applications of the model, which would provide some guidance in the selection of the most appropriate model.

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