Retaining eucalyptus harvest residues promotes different pathways for particulate and mineral-associated organic matter

Eucalyptus plantations have replaced other (agro)ecosystems over 5.6 Mha in Brazil. While these plantations rapidly accumulate carbon (C) in their biomass, the C storage in living forest biomass is transient, and thus, longer-term sustainability relies on sustaining soil organic matter (SOM) stocks. A significant amount of harvest residues (HR) is generated every rotation and can yield SOM if retained in the field. Yet, there is little information on how managing eucalyptus HR changes SOM dynamics. We used isotopic and molecular approaches in a 3-yr field decomposition experiment where a native grassland has been replaced by eucalyptus plantations to assess how HR management practices influence content and chemistry of two distinct SOM fractions [particulate (POM) and mineral-associated organic matter (MAOM)] at two soil depths (0-1 and 1-5 cm). The management practices investigated were HR removal (-R), only bark removal (-B), and retention of all HR (including bark, +B), combined with two levels of nitrogen (N) fertilization [0 (-N) and 200 (+N) kg/ha]. N fertilization inhibited HR decomposition (P = 0.0409), while bark retention had little effect (P = 0.1164). Retaining HR, especially with bark, increased POM-C and MAOM-C content (2.1- and 1.2-fold, respectively), decreased POM-delta C-13 (1.2-fold), and increased inorganic N retention (1.7-fold) compared with plots where HR had been removed. Inorganic N applications, however, diminished the positive impacts of bark retention. Although the influence of HR management was most pronounced in POM, retaining HR reduced potential soil C mineralization by up to 20%. POM and MAOM chemistry shifted over time and revealed distinct influence of HR on the formation of these fractions. We demonstrate that HR management alters SOM dynamics and that retaining HR, particularly including bark, enhances SOM retention. With continuing conversion of native grassland ecosystems to eucalyptus, long-term sustainability will require careful HR and fertilizer management to balance total biomass harvest with sustaining belowground SOM concentrations.