Uso de isótopos estables para la cuantificación de la incorporación de carbono orgánico al suelo en plantaciones de segunda rotación de Eucalyptus dunnii

In recent decades, forestry production in Uruguay has grown steadily, establishing itself as an important pillar of the national economy. This development has led to a significant expansion of the area occupied by plantations of exotic species, mainly of the genus Eucalyptus, to the detriment of natural grassland cover. Not only has the forested area increased, but it has also been consolidated over time through second-rotation plantations. This change in land use can affect the physicochemical properties of the soil, particularly soil organic carbon (SOC) stock, a fundamental component of fertility, environmental quality, and ecosystem resilience.

This study was conducted in commercial plantations of second-rotation Eucalyptus dunnii (E. dunnii) belonging to the company UPM Forestal Oriental S.A., located in the department of Paysandú. The overall objective was to quantify the SOC contribution generated by these plantations using isotopic fractionation techniques. Also, SOC contents were compared with those of adjacent areas covered by natural grasslands in order to identify carbon gain or loss processes linked to land use change.

The methodology employed focused on measuring the ratio of stable carbon isotopes (¹³C/¹²C), which allows the origin of carbon to be identified based on the type of photosynthetic metabolism predominant in the vegetation (C₃ or C₄). This isotopic technique revealed a significant contribution of forest cover to SOC.

The results showed that forest cover made a significant contribution to SOC. However, SOC concentrations in the top 10 cm of soil were higher in natural grasslands than in forested areas, suggesting carbon losses following the conversion of grasslands to forest systems. Isotopic analysis showed more negative δ¹³C values under forest cover, indicating a higher proportion of carbon derived from C3 species (Eucalyptus spp.). These findings confirm the new origin of carbon in the soil and suggest that, although COS levels do not yet reach those of native grasslands, they could stabilize at similar ranges over time.