Prescribed burning and mechanical thinning effects on belowground conditions and soil respiration in a mixed-conifer forest, California

Soil respiration (RS) is a major carbon pathway from terrestrial ecosystems to the atmosphere and is sensitive to environmental changes. Although commonly used mechanical thinning and prescribed burning can significantly alter the soil environment, the effect of these practices on RS and on the interactions between RS and belowground characteristics in managed forests is not sufficiently understood. We: (1) examined the effects of burning and thinning treatments on soil conditions, (2) identified any changes in the effects of soil chemical and physical properties on RS under burning and thinning treatments, and (3) indirectly estimated the changes in the autotrophic soil respiration (RA) and heterotrophic soil respiration (RH) contribution to RS under burning and thinning treatments. We conducted our study in the Teakettle Experimental Forest where a full factorial design was implemented with three levels of thinning, none (N), understory thinning (U), and overstory thinning (O; September to October 2000 for thin burn combination and June and July 2001 for thin only treatments) and two levels of burning, none (U) and prescribed burning (B; fall of 2001). RS, soil temperature, soil moisture, litter depth, soil total nitrogen and carbon content, soil pH, root biomass, and root nitrogen (N) concentration were measured between June 15 and July 15, 2002 at each plot. During this period, soil respiration was measured three times at each point and averaged by point. When we assumed the uniform and even contribution of RA and RH to RS in the studied ecosystem without disturbances and a linear relationship of root N content and RA, we calculated the contributions of RA to RS as 22, 45, 53, 48, and 45% in UU, UO, BN, BU, and BO, respectively. The results suggested that after thinning, RS was controlled more by RH while after burning RS was more influenced by RA. The least amount of RS variation was explained by studied factors under the most severe treatment (BO treatment). Overall, root biomass, root N concentration, and root N content were significantly (p < 0.01) correlated with soil respiration with correlation coefficients of 0.37, −0.28, and 0.29, respectively. This study contributes to our understanding of how common forestry management practices might affect soil carbon sequestration, as soil respiration is a major component of ecosystem respiration.