Land use change, soil degradation and water erosion: converting grasslands to croplands in the Rolling Pampa (Argentina).

VANGELI, SEBASTIÁN; CASTIGLIONI, MARIO G; BEHRENDS KRAEMER, FILIPE; CHAGAS, CELIO

Rio de Janeiro

Congreso; 21 World Congress of Soil Science; 2018

International Union of Soil Sciences

One third of Argentina is affected by water erosion and, to a lesser extent, by wind erosion.Recently the area and rate of these processes grew significantly. The main reasons are overgrazing,deforestation, land use change and inadequate cropping practices. We analyzed the effect ofgrasslands conversion into croplands, particularly its impact on some fragile hydrohalomorphic lowlands belonging to an 865 km2 basin, characteristic of the Rolling Pampa. Firstly, we used GIS,remote sensing and USLE to measure the increment of the surface devoted to annual cropping between 1987 and 2015 for the whole basin and to estimate its effect on water erosion and sediment yield. Then, we selected and studied the soil properties of a pilot site located on the fragile lowlands belonging to the alluvial plain of this basin, which shifted its land use from livestock to grain production recently. We assessed changes in physical, chemical and biological soil properties compared to a control plot. With rainfall simulations, we studied runoff and its quality,emphasizing on physical (sediments) and chemical (phosphorus, glyphosate and AMPA) yield. Then, we selected some soil properties that were sensitive to land use change in this lowland: bulk density (BD), aggregate stability (AS) and soil organic carbon (SOC) and used them as indicators of the state of degradation for other similar situations corresponding to the basin´s alluvial plain. For that purpose, we compared paired crop and grasslands plots in the rest of this alluvial plain. Duringthe period 1987-2015, annual crops area expanded over 26% of the whole basin, and currently occupies 80% of its surface. Water erosion estimates increased from 3.6 to 4.4 t ha-1yr-1, while sediment yield increased by almost 22,000 t yr-1(31%). Regarding the fragile alluvial plain, SOC and AS decreased significantly because of the agricultural expansion (SOC: 13.2 g kg-1in agriculture vs. 19.2 g kg-1in livestock, AS: 1.95 mm in agriculture vs. 3.06 mm in livestock). BD increased from1.17 g cm-3in livestock to 1.24 g cm-3in agriculture. Microbial biodiversity also decreased significantly. Rainfall simulation experiments showed that contribution of sediments and glyphosate to the stream water from these lowlands increased significantly. However, we did not detect AMPA in runoff water and the phosphorus levels did not show significant differences between crops and grassland plots.