Soil and microbial respiration in a loblolly pine plantation in response to seven years of irrigation and fertilization

Because Soil CO2 efflux or soil respiration (R-S) is the major component of forest carbon fluxes, the effects of forest management on R-S and microbial biomass carbon (C), microbial respiration (R-H), microbial activity and fine root biomass were studied over two years in a loblolly pine (Pinus taeda L.) plantation located near Aiken, SC. Stands were six-years-old at the beginning of the study and were subjected to irrigation (no irrigation versus irrigation) and fertilization (no fertilization versus fertilization) treatments since planting. Soil respiration ranged from 2 to 6 mu mol m(-2) s(-1) and was strongly and linearly related to soil temperature. Soil moisture and C inputs to the soil (coarse woody debris and litter mass) which may influence R-H were significantly but only weakly related to R-S. No interaction effects between irrigation and fertilization were observed for R-S and microbial variables. Irrigation increased R-S, fine root mass and microbial biomass C. In contrast, fertilization increased R-H, microbial biomass C and microbial activity but reduced fine root biomass and had no influence on R-S. Predicted annual soil C efflux ranged from 8.8 to 10.7 Mg C ha(-1) year(-1) and was lower than net primary productivity (NPP) in all stands except the non-fertilized treatment. The influence of forest management on R-S was small or insignificant relative to biomass accumulation suggesting that NPP controls the transition between a carbon source and sink in rapidly growing pine systems. (C) 2009 Elsevier B.V. All rights reserved.