Future climate change will be influenced significantly by the organic carbon dynamics in agricultural and forest soils, because the massive amounts present in carbon pools are susceptible to increased net mineralization due to soil disturbance and respiratory loss by soil organisms. To quantify the effects of agricultural management and component respiration on the carbon budget, the daily version of the CENTURY (DayCENT) model and the Carbon- and Nitrogen-coupled Canadian Land Surface Scheme (CN-CLASS) model were employed. Based on substantial field measurements for quantifying the carbon budget in terms of plant carbon allocation, net ecosystem production, component respiration and soil organic carbon (SOC) at daily and half-hourly time-steps, both models were examined for their ability to predict the effects of agricultural management and phenology on carbon dynamics. Our study sites were located at the University of Guelph Elora Research Station and the Borden Forest Research Station, southern Ontario, Canada.

Effects of conventional tillage (CT) and no-till (NT) practices on carbon dynamics in a cropfield was modeled by application of a 5000 year equilibrium simulation to ensure the development of steady-state and representative SOC pools, followed by a 9-year simulation of agricultural management practices. The effects of phenology on multi-year transformation of litterfall and SOC, and the potential for carbon sequestration were quantified. During our initial studies, we found that the plant phenology algorithms used in CN-CLASS were not fully constructed, resulting in a high uncertainty in the simulations. Using the DayCENT agricultural simulation, a new agricultural module for CN-CLASS was designed and tested. Furthermore, to quantify the respiratory losses in deciduous forests using CN-CLASS, we examined the dynamics of component respiration in comparison with vegetation growth and further traced the contribution of soil respiration sources from litterfall, SOC and root respiration.

In summary, phenology played a fundamental role in regulating the carbon inflow/outflow dynamics in both agricultural and forest ecosystems. A flexible crop-specific phenological algorithm is required to mitigate the uncertainty of carbon budgets in land surface models. At the agricultural site, the total carbon sink during the growing season is estimated as 477.5 g C m-2. CT enhanced the SOC decomposition relative to NT by 38.4, 93.7 and 64.2 g C m-2 yr-1 for corn, soybean and winter wheat crops, respectively. The annual variation of the total SOC pool was greater in CT than NT due to tillage effects on carbon transfer from the active surface SOC pool to the active soil SOC pool at a rate of 50–100 g C m−2 yr−1. The adoption of NT accounted for a 10.7 g C m-2 yr-1 increase in the slow SOC pool. At the deciduous forest site, the total ecosystem CO2 emission was estimated at 1366 g C m-2, that mainly due to heterotrophic respiration (57%) and maintenance respiration (37%). Annual soil respiration was estimated as 782 g C m-2, which was mainly from the SOC pools (60%).