Abstract
ABSTRACT The aim of the present study was to evaluate the effect of eucalyptus plantations on total organic C (TOC) and total N (TN) stocks as well as the C and N in the fulvic acid (FA), humic acid (HA), humin (HU), light organic matter (LOM) and microbial biomass (MB) fractions in soils with different textures in the eastern part of the state of Rio Grande do Sul, Brazil. Soil samples were collected from the 0-10-cm, 10-20-cm, 20-40-cm, 40-60-cm and 60-100-cm soil layers in a completely randomized experimental design with subdivided plots. Under the eucalyptus plantation, clay loam soil presented lower HA, HU and MB C stocks in the 0-100-cm soil layer compared to reference vegetation, whereas sandy soil presented higher and C and N in HA and LOM as well as C in MB. The observed increase in TOC in eucalyptus plantations was more pronounced in the surface soil layer (0-10 cm), i.e., approximately 150% higher than under native vegetation, which was probably due to the high contribution of the eucalyptus litter. Differences in C and N stocks in soil organic matter (SOM) fractions between eucalyptus plantations and areas with reference vegetation were more pronounced in sandy soil, showing the capacity of the clay fraction to protect SOM.
Highlights
Soil organic matter (SOM) is an important indicator of ecosystem sustainability because it is closely related to soil characteristics and processes that directly and indirectly affect the growth of forest species and is an important global C reservoir
Soil total organic C (TOC) and total nitrogen (TN) were significantly higher under eucalyptus plantations than under native vegetation to a depth of 60 cm for TOC and 20 cm for TN (p
Clay loam soil presented significantly lower C in the humic acid (HA) (C-HA) and HU (C-HU) fractions in the 0-100cm soil layer under the eucalyptus plantation than under the native vegetation (Figure 2), but there were no significant differences in N-HA and N-HU (Figure 3)
Summary
Soil organic matter (SOM) is an important indicator of ecosystem sustainability because it is closely related to soil characteristics and processes that directly and indirectly affect the growth of forest species and is an important global C reservoir. Soil organic matter stocks increase due to shoot and root litter input, the protection offered by the soil, climate conditions, the action of decomposing microorganisms and management practices, and soils with higher clay content usually have a higher C stabilization capacity (Six et al, 2002). Native forest litter is typically composed of residues whose biochemical properties inhibit decomposition (Khanna et al, 2001), and in eucalyptus plantations in southern Bahia, litter accumulation has been attributed to lower contents of N and other nutrients in residues as well as higher litter C/N, lignin/N and (lignin + cellulose)/N ratios (Gama-Rodrigues and Barros, 2002). The high C/N ratio of these residues (between approximately 30 and 100) may result in slower decomposition (Skorupa, 2001), so in eucalyptus plantations, there is a high input of plant residues during the forest rotation and at the time of harvest
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