Evolution of the impact of land use changes and agricultural practices on sediment delivery in the Uruguayan Pampa

South America has experienced significant landscape transformations over the last century, with the expansion of agriculture (pasture, cropland, plantations) at the expense of natural ecosystems (forest, grassland).

More specifically, the Rio de la Plata Grasslands composing the Pampa biome, a temperate grassland ecosystem, mainly located in Uruguay and north Argentina, is among the regions with the highest global rates of land-use change, thereby threatening its biodiversity, land and water resources. 

 

The consequences of agricultural development in this region have been poorly documented since its beginning. 
Retrospective analysis using sediment coring can provide valuable insights into these impacts over extended periods. 
Such a retrospective was successfully conducted by Foucher et al (2023) \cite{foucher_inexorable_2023}. Nevertheless, their sediment core did not reach the reservoir's bottom, limiting the reconstruction of these processes to the post-1990 period.

 

In this study, we are analysing a sediment core collected in the Rincon del Bonete dam, draining a 39,500 km² catchment, and dated back to 1948. Various analyses were performed along this sedimentary archive in order to date and characterise the sediment properties (gamma spectrometry, high-resolution geochemical content analysis (XRF), pesticides) and their changes with time. 
Statistical analyses of the sediment fluxes enabled the differentiation of distinct phases in the sediment delivery process.

 

The Rincon del Bonete catchment in Uruguay has undergone substantial changes of land-use and farming practices, reflecting the broader challenges of environmental degradation in the Pampa region.
Available data over the region show that forest plantations expanded from less than 1\% of the area in 1985 to over 10\% in 2022. Concurrently, agricultural and pastoral land use increased by over 250\% between 1985 and 2022, while natural grasslands declined from covering 80\% of the basin to just 60\%. 
Results show that these changes have led to four distinct phases in sedimentation recorded in the lake archive: an initial period (1948-1964) of reservoir filling and early basin degradation in the northern Brazilian part of the catchment, characterised by extensive DDT insecticide use; a second period (1964-1985) of conventional tillage agriculture with a mix of agriculture-pasture and the beginning of intensive pesticides use in Uruguay (1970-1980). The third phase (1985-2007) was then characterised by a shift to no-tillage agriculture, afforestation, with a notable expansion of this practice occurring between 1999-2005, and the observation of an associated decrease of sediment delivery. During the final phase (from 2007 onwards), rapid and large agricultural expansion under continuous no-tillage practices and wood harvesting led to a large usage of pesticide and to an increase of sediment delivery despite a second notable phase of afforestation in 2007-2014.

 

This study highlighted the influence of land use changes and agricultural practices on sediment delivery since WWII, revealing the occurrence of high sedimentation rates during early conventional tillage and the onset of pesticide use, followed by a reduction of these rates during the transition to no-tillage and afforestation, and a marked increase with large-scale agricultural expansion and wood harvesting.