Measurement and modeling of soil CO[subscript]2 flux in a temperate grassland under mowed and burned regimes

Abstract

Soil-surface CO2 flux (R[subscript]s), which is a large component of the carbon (C) budgets in grasslands, usually is measured infrequently using static or dynamic chambers. Therefore, to quantify annual C budgets, estimates of Rs are required during days when no direct measurements of R[subscript]s are available. Other researchers have developed empirical models based on soil temperature, soil volumetric water content ([theta]v), and leaf area index (LAI) that have provided reasonable estimates of Rs during the growing season in ungrazed tallgrass prairie. However, the effects of mowing and grazing, which are common in grasslands, on predictions of Rs from those models are uncertain. Predictions of R[subscript]s during dormancy (post-senescence to spring fire) also are uncertain. Data from a year-long mowing study, which simulated grazing, were used to refit these models. Output from the models then was compared to independent data collected from nearby prairie sites. Results showed that LAI must be included to accurately estimate R[subscript]s in mowed prairie ecosystems. When LAI was not included in the model, predicted daily R[subscript]s following mowing was nearly four times greater than measured R[subscript]s, and cumulative, annual R[subscript]s was overestimated by 95-102%. When LAI was included in the model, predictions of R[subscript]s were comparable to measured R[subscript]s in the mowing study. Annual estimates of cumulative Rs ranged from 3.93 to 4.92 kg CO[subscript]2 m[superscript minus]2. When comparing the model with independent chamber data from nearby sites, cumulative R[subscript]s during those studies was within [plus minus]9% of cumulative estimates calculated from measured R[subscript]s. The model overestimated daily R[subscript]s during a dry period, suggesting a nonlinear response of R[subscript]s to soil water content; matric potential may be more appropriate than [theta]v for modeling R[subscript]s. Data suggest that R[subscript]s, in addition to being dependent on soil temperature and soil water content, is dependent on the photosynthetic capacity of the canopy and the subsequent translocation of C belowground.