Soil sustainability challenges in eucalypt afforestation of floodplain grasslands in Paraguay’s Humid Chaco: Carbon and phosphorus dynamics

Abstract

The floodplains of the Humid Chaco (HC) ecoregion in Southern Paraguay comprise an area of great environmental value, which contributes to maintaining regional climate stability while supporting a rich biodiversity of flora and fauna. Throughout the region, soils play a fundamental role in C sequestration and water regulation but are strongly influenced by the water table. In recent years, different afforestation plans have led to thousands of hectares of land being covered with eucalyptus plantations, involving intensive drainage and tillage that strongly affect the water table. The intensive afforestation could potentially result in the loss of large amounts of soil organic matter (SOM), influencing nutrient cycling and ecosystem viability in the long term. The present study aimed to evaluate the impact of Eucalyptus camaldulensis plantations on natural grasslands in the HC, focusing on SOM quantity and quality and P dynamics. A representative area of the HC, including E. camaldulensis plantations of various ages (ranging from 2 to 8 years) and contiguous native grasslands were selected for study. Soil samples (0–5, 5–10, 10–20, 20–30 cm) were obtained to determine total C and N concentrations, and C:N ratio, and then SOM quality (by DSC-TGA, and solid-state 13C CP-MAS NMR spectroscopy) and the different P forms present (by 31P NMR spectroscopy) were analysed in the upper soil layer (0–5 cm). The intensive site preparation prior to plantation establishment initially caused a notable reduction (50 %) in soil organic carbon (SOC) and large increases in the carbon-to-nitrogen (C:N) ratio at all depths studied but especially in upper soil layers. These parameters gradually recovered as the trees grew, although the values did not return to the original levels measured in native grasslands. Thermal analysis and 13C NMR spectroscopy revealed initial loss of labile and recalcitrant SOM compounds, with more noticeable loss of the most labile fractions. More than 40 % of the P in the natural grasslands was present as organic P forms (monoesters), indicating the high potential for P immobilization and preservation. Afforestation led to a substantial loss of soil extractable P, which affected both organic (monoesters and diester) and inorganic (orthophosphate) forms. This could have a significant impact on the short and medium-term P reserves, inducing P limitation and threatening ecosystem sustainability. Considering the short and medium-term impacts observed after the transformation of natural grasslands into E. camaldulensis plantations, strategic planning, establishing buffer zones with natural vegetation and applying proper soil management are urgently required to preserve SOM and nutrients in the floodplains of the Humid Chaco.