Soil CO2 evolution and root respiration in 11 year-old loblolly pine (Pinus taeda) plantations as affected by moisture and nutrient availability

We measured soil CO2 evolution rates with (S-ff) and without (S-ms) the forest floor litter and root respiration monthly in 11-year-old loblolly pine (Pinus taeda L.) plantations during the fourth year of fertilization and irrigation treatments. Values of S-ff ranged from less than 1 mu mol.m(-2).s(-1) during the winter to greater than 5 mu mol.m(-2).s(-1) in late spring. Average S-ff was significantly greater in unfertilized relative to the fertilized stands; however, there was no difference in average S-ms among treatments. Soil temperature and the mass of the forest floor (litter) explained most of the difference in S-ff among treatments. Soil temperature and volumetric water content accounted for 70% of the seasonal variation in S-ff. Annual carbon efflux from the soil averaged 14.1 Mg.ha(-1) per year for all treatments. Most of the evolved carbon was derived from root respiration (50-73%). Net ecosystem productivity was -1.1 and 6.9 Mg C.ha(-1) per year for the unfertilized and fertilized stands, respectively. At age 11, the unfertilized stands were functioning as a net carbon source, while fertilized stands were a strong carbon sink. It was concluded that fertilization could decrease the time for a young pine plantation to change from a carbon source to a carbon sink.