Could Eucalyptus harvest residues be a major source of soil carbon?

Background/Question/Methods

Eucalyptus forests are considered a key low-carbon land use, efficiently sinking atmospheric CO₂. Non-or-less-commercial components (Harvest residues – HR) account for 40 % of the aboveground biomass at harvesting. Keeping HR in the field could enhance CO₂ sequestration, especially if they could be transformed into stable forms of soil organic carbon (SOC). We set up an experiment in a new frontier of Eucalyptus forest expansion in Southern Brazil. Micro-plots of PVC inserted 10 cm into the soil were used to assess the effect of simulated management practices [removal of all residues, only bark removal and maintenance of all residues, all of them combined with external nitrogen addition (+N) or not (-N)] on decomposition rate, C transfer and C stabilization. After 1 yr., whole micro-plots were sampled, 0 – 1 cm soil layer was separated and used for analysis. We assess HR management effects on soil organic matter (SOM) fractions [Particulate organic matter (POM) and Mineral-associated organic matter (MAOM)] by changes in C content and changes in the natural abundance of ¹³C, as well as changes in soil microbial community by ¹³C-PLFA.  

Results/Conclusions

Until 1 yr., around 50 % of HR have been decomposed. Bark presence slowed down decomposition (p<0.05). HR removal decreased C content in both SOM fractions (p<0.05). Losses were higher in MAOM fraction. HR combined with +N increased C content in MAOM fraction by 2.73 and 7.34 g kg^-1 in the absence and presence of bark, respectively. C losses in MAOM were found with bark removal -N treatments. Losses in POM were also expressive in absence of bark in –N treatments. ¹³C depletion was only observed in POM fraction and was greater with bark presence. HR presence positively altered PLFA concentrations. Gram positive and Actinobacteria concentration was higher in absence of bark, and Fungi was higher with bark presence. Greater HR recovery occurred in Actinobacteria, Gram negative and Fungi groups. In +N treatments, HR recovery in Fungi was reduced, while in Actinobacteria and Gram negative more C derived from HR was found, in absence and presence of bark, respectively. Results had shown an important role of HR on SOM properties. Combined analysis of changes in SOM fractions indicates that HR, especially when bark is kept, enhance C transformation into more stable forms. POM seems to be the first step of the transformation pathway of HR into SOC.