Evaluation of the CENTURY model with laboratory measured soil respiration
The CENTURY model is widely used in assessing the effect of management on soil C dynamics. However, recent testing of the model revealed that it performed unsatisfactorily in simulating soil C changes in southwestern Saskatchewan, suggesting that the model may need further testings and modifications. Evaluations of the model were made with measurements conducted in an laboratory experiment which included different straw placements (incorporated in the soil and applied on the soil surface) and soil water regimes (continuously moist and moist-dry). Results from model testing revealed some weaknesses of the model and modifications were made to improve model performance. The temperature function was modified to slightly increase the relative decomposition rate when temperatures were below the reference temperature. The moisture function was modified to reduce the relative decomposition rate when the soil moisture was very low. The modified model also assumed that soil mineral N is readily available for the use of decomposition of soil C pools, but only about 6.6 mg m-2 d-1 of soil N is available for C pools on the soil surface. When N availability is less than that required for maximum decomposition rates of soil or surface pools, decomposition rates of these pools were reduced until supply met demand. The modified model improved simulations of daily C fluxes, cumulative CO2 emissions and soil mineral N. To use this modified model for estimating soil respiration in the field, further studies on the N availability for soil surface C pools and the dryness of surface-placed residue are needed. The concentration of CO2 in the atmosphere has increased by about 25% since the beginning of the industrial revolution. There are concerns that continuing increases in levels of CO2 and other greenhouse gases will contribute to global warming. Soils contains about three times as much C as the atmosphere, and they have the potential to store additional C (Campbell and Zentner, 1993). Agricultural soils on the Canadian prairies contain about 3 Pg soil organic carbon (SOC) in the top 30 cm layer, which is about 20 times the amount of CO2-C emitted annually by fossil fuel combustion in Canada. Research shows that if properly managed agricultural lands could be an important sink for C. Management options to enhance C storage in Canadian prairie soils include: decreasing summer fallow frequency, reducing tillage, including legumes in crop rotations, proper fertilization, and growing forage and trees on marginal lands (Campbell and Zentner, 1993; Campbell et al., 1995). For example, reduction in tillage intensity, especially no-tillage (NT) cropping, has been shown to increase SOC at various locations (Janzen, et al. 1998). Although changes in SOC occur when soil management practices are altered (Mann, 1986), it is common for these changes to remain undetectable for 10 or 20 years. The reason is that because of the inherent spatial variability of SOC in the field, too many samples are required to be taken to ensure that small differences can be statistically separated (Campbell et al., 1976; Campbell et al., 2000). Thus, we often use a process-based simulation model that describes soil organic matter turnover and nitrogen cycling dynamics in soils to estimate management induced SOC changes. The CENTURY model (Parton et al., 1987) is one of such models that is the most widely used and has been extensively evaluated in various ecosystems (Scholes et al., 1997). However, the recent testing of the CENTURY model revealed that it performed unsatisfactorily in simulating soil C changes in a 30-yr crop rotation experiment in southwestern Saskatchewan (Campbell et al., 1999), suggesting that the model may need further testings and modifications for use on the Canadian prairies. Because of the problems associated with SOC measurement and the variability of environmental conditions in the field, it is difficult to rigorously test the mechanism of a process-based soil organic matter model. Alternatively, the model can be readily tested against measurements of CO2 emissions from a controlled laboratory experiment. The objectives of this study were thus: (1) to test the validity of the CENTURY model with the soil respiration measured from laboratory experiments and (2) to address the weaknesses revealed during the model testings by modifying the model.
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