Spatial heterogeneity of soil CO2 efflux at ADVEX site Norunda in Sweden

Abstract: This study investigates the spatial variation in soil carbon dioxide (CO2) efflux in a boreal mixed forest mainly dominated by Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies L.) in the ADVEX expermental site in Norunda, in central Sweden. Soil CO2 efflux measurements were performed in 72 collars around the main tower by using a portable LI-6250 infrared gas analyzer connected to a home-made dark chamber. The average soil CO2 efflux normalized to temperature of 10 oC (R10) from the forest floor in Norunda as calculated from 72 collars was 2.09±1.36 [μmol m-2 s-1]. This was the sum of the total soil respiration mainly originating from root respiration (autotrophic), microbial respiration (heterotrophic) in addition to the respiration from forest floor vegetation which was not removed from the upper soil layer during the measuring of soil CO2 efflux. Measurements were performed between the 7th and 12th of June 2007 during the daytime between 11 am and 4 pm. The spatial heterogeneity of soil respiration as expressed by coefficient of variation (CV) was 65%, which reflects inconsistency in soil CO2 efflux among the 72 collars in the study area. In order to investigate the reasons for spatial variation in soil CO2 efflux, a number of variables were measured in each collar directly after CO2 efflux was measured. First, depth of the soil layer above boulders showed a significant relationship with R10 (R2=0.48). Second, humus thickness also showed a significant relationship with R10 (R2=0.21). Third, there was a significant difference in R10 with micro-topography (between normal and elevated points). Fourth, a weak negative correlation was found between R10 and weight of the dry mosses with R2=0.06. Fifth, a non- significant relationship between R10 and distance, diameter and species of the nearest three trees to the collar was found. The variables that measured in the field account about 50% the reasons behind spatial variation in soil CO2 efflux, therefore others variables are suggested to investigate for future studies.