Impacts of logging on soil organic carbon and heterotrophic respiration in tropical forests in Borneo

Selective logging is the practice of extracting selected commercial trees from natural production forests. The intensity of logging correlates with a reduction in biodiversity, wood production and biomass stocks. Less is known about the relationship of logging to soil organic carbon (SOC) and how it changes or recovers over time. Empirical measurements in Borneo provided SOC, soil respiration, aboveground and belowground net primary productivity (NPP) from intact old-growth forest (OGF) as well as from moderately to heavily logged (LOG) forest sites. Soil carbon (C) content and heterotrophic respiration (R_h) was higher in LOG sites than in OGF sites. Moderately logged forest (logged > 10 years ago) contained more SOC than heavily logged forest (logged approx. 7 years ago). NPP was used to estimate the C input to the soil. All these data were used to test the biochemical model ECOSSE (Estimating Carbon in Organic Soils – Sequestration and Emissions) to calculate SOC for the study sites. The model performed well in simulating the soil respiration of OGF and generated acceptable results for LOG sites in the validation process. The results for logged forests showed an increase in R_h over the first 15 years, with some sites showing either a further increase over the next 15 years or stabilizing at a higher level compared to pre-disturbance conditions for other sites. However, for all modelled cases, a break was observed after 30 years, when R_h decreased to a lower level (but not as low as for OGF) before reaching a new equilibrium. At the same time, SOC begins to increase. Spatial modelling showed the results for Borneo under logged conditions and the potential of storing C if logging was reduced. Only 22% of Borneo is under old-growth forest; the results show moderate to high C losses if this region is subjected to logging. Overall, the results show the disturbance of SOC and Rh through logging over periods longer than 30 years.