Phosphorus in soils and fluvial sediments from a Cerrado biome watershed under agricultural expansion.

Over the past decades, lands alongside Gurguéia River have witnessed rapid expansion of soybean agriculture which has increased soil degradation and affected nutrient concentration in sediment, especially phosphorus (P). The present study aimed to quantify the P concentration in soils under different land uses (i.e., croplands, grasslands, and cerrado) and fluvial sediments (suspended sediment, channel bank, and riverbed sediments), assessing pollution over the main watercourse in cerrado biome Gurguéia watershed, located in Piauí State, Brazil. In total, 136 composite soil samples at a depth of 0-5 cm, under different land uses, as well as 51 composite fluvial sediment samples were collected over the watershed. The land use change from native cerrado had resulted in an increase of total phosphorus (TP) whose concentration was higher in cropland areas, followed by suspended sediment, channel bank, and riverbed sediments. This high concentration in cropland areas resulted from phosphate fertilizer inputs. The transfer of phosphorus to water bodies was evidenced, since an increase of TP was observed in suspended sediment, channel bank and riverbed  sediments. Mineralogical signatures in sediments were identified by X-ray diffraction analysis which showed the occurrence of kaolinite, illite, smectite, iron oxides, and other minerals in lesser proportions. The presence of 1:1 minerals was higher in riverbed sediments and downstream sampling points, while 2:1 minerals were present in higher proportions in suspended sediment and channel bank sediment, as well as at the upstream and middle sampling points. This finding shows that land use change from cerrado to cropland due to soybean agriculture expansion might increase P discharges from terrestrial to aquatic environments, with sediments being the major carrier of this element.

Soil loss and sediment yield in a perennial catchment in southwest Piauí, Brazil.

Soil and water are vital natural resources. However, due to their indiscriminate use, these resources are being seriously threatened. Therefore, it is essential to manage them in a sustainable way and leave them for future generations. Population and agricultural areas have expanded, deforesting native landscapes for cultivation and pastures. As a result, soil loss from agricultural areas is increasing the amount of sediment transport in water courses. The objective of this study was to quantify soil loss and sediment yield from the Corrente dos Matões sub-basin (CMSB). These measurements are essential to quantify the environmental impact of advancing agricultural frontiers. The Universal Soil Loss Equation (USLE) was applied due to its wide use, compatibility with GIS, and data availability. The suspended sediment transport was calculated by collecting samples with DH-48 sampler. From the application of USLE, the average soil erosion contributed very little to sediment delivery in the watercourse, with a magnitude of only 0.37 t ha-1 year-1. The highest soil loss was associated with greater slope and was observed in areas with agriculture or under the absence of vegetation cover. The low transport of suspended sediments in CMSB is due to the existence of preserved sites. About 99% of the sub-basin had a low degree of erosion. The high degree of soil loss was attributed to the cliffs and the development of agricultural activities leaving the soil uncovered. This work will help identify the most susceptible areas to water erosion for optimizing the allocation of financial resources for the preservation of this natural resource.