Due to concern over the increasing concentration of carbon dioxide in the atmosphere, forest researchers and managers are currently studying the effects of varying silvicultural and harvesting practices on the carbon dynamics of intensely managed forest ecosystems. Soil carbon dioxide efflux resulting from soil microbial activity and root respiration is one of the major components of the total carbon flux in forested ecosystems.

In an effort to examine the response of soil carbon dioxide efflux to changes in soil factors, nutrient availability, temperature, and moisture, soil respiration rates were measured monthly over an entire year in a two-year-old loblolly pine (Pinus taeda L.) plantation subjected to fertilization and mulching treatments. A dynamic, closed-chamber infrared gas analysis system was used to measure efflux rates from plots treated with one of four treatment combinations including: nitrogen (115 kg/ha) and phosphorus (11.5 kg/ha) fertilization with black landscape cloth (mulch), fertilization without mulch, mulch without fertilization, and no treatment (control). For each treatment combination, plots were established at the seedling base and 1.22 m away from the seedling base to examine the effect of seedling roots on soil carbon dioxide efflux rates. Soil temperature and moisture were measured at each chamber position monthly and soil coarse fragments, soil nutrient levels, percent carbon, root biomass and coarse woody debris were measured beneath 64 chambers at the end of the study.

Fertilization had no significant effect on efflux rates during any of our monthly sampling sessions despite the fact that fertilized seedlings experienced significant increases in both above and belowground biomass. Conversely, regression analysis of growing season soil carbon dioxide efflux rates revealed a slightly negative correlation with both total seedling nutrient uptake and biomass. Rates in plots with mulching were significantly higher than rates from non-mulched plots during five monthly measurement sessions, and higher rates in mulched plots during winter months was attributable to warmer soil temperatures. Rates at the seedling base were always significantly higher than rates in plots away from the seedling. Although rates were always higher at the seedling base, the variability observed was only weakly correlated with the amount of pine roots present beneath respiration chambers. Utilizing soil temperature and moisture, soil carbon, and cuvette fine root biomass in a regression model explained 54% of the variance observed in efflux rates across the yearlong study period. Soil temperature alone explained 42.2% of the variance, followed by soil carbon and soil moisture at 5.2% and 2.7% respectively. The amount of pine fine roots under measurement chambers accounted for only 2.4% of the variance. An additional 1.5% was explained by other factors such as soil phosphorus, coarse woody debris, non-pine root biomass, and soil calcium. An examination of the factors affecting the spatial patterns of soil carbon dioxide efflux revealed that total soil carbon and the amount of fine pine root biomass beneath cuvette base rings explain 38% and 11% respectively, of the observed variability in mean annual soil carbon dioxide efflux from differing plots.

The most influential factor affecting soil carbon dioxide efflux during the yearlong study period was soil temperature and modeling of seasonal soil carbon dioxide efflux rates from managed forests using both soil temperature and moisture should be achievable with the establishment of data sets and statistical models covering a range of sites differing in productivity, stand age, and management intensity. The establishment of data sets and statistical models across a variety of forest sites should account for the changing influence of soil carbon levels, aboveground biomass, microbial activity, organic matter inputs, and root biomass on soil carbon dioxide efflux.