Impacts of anthropic pressures on soil phosphorus availability, concentration, and phosphorus forms in sediments in a Southern Brazilian watershed

Abstract

Purpose The transfer of soil sediments and phosphorus from terrestrial to aquatic systems is a common process in agricultural lands. The aims of this paper are to quantify the soil phosphorus availability and to characterize phosphorus forms in soil sediments as contaminant agents of waters as a function of anthropic pressures. Materials and methods On three subwatersheds with different anthropic pressure, water and sediment samples were collected automatically in upstream and downstream discharge points in six rainfall events during the tobacco growing season. Phosphorus desorption capacity from soil sediments was estimated by successive extractions with anion exchange resins. First-order kinetic models were adjusted to desorption curves for estimating potentially bioavailable particulate phosphorus, desorption rate constant, and bioavailable particulate phosphorus. Results and discussion The amount of bioavailable particulate phosphorus was directly correlated with the iron oxide content. The value of desorption rate constant was directly related with the total organic carbon and inversely with the iron oxide contents. Phosphate ions were released to solution, on average, twice as rapidly from sediments collected in subwatersheds with low anthropic activity than from those ones of highly anthropic subwatersheds. Anthropic pressure on watershed can engender high sediment discharge, but these solid particles seem to present low phosphorus-releasing capacity to water during transport due to the evidenced high affinity between phosphorus and iron oxide from sediments. Conclusions Anthropic pressure was related with sediment concentration and phosphorus release to aquatic systems. While natural vegetation along streams plays a role on soil and water depuration, it is unable to eliminate the phosphorus inputs intrinsic to the agricultural-intensive systems. Recommendations and perspectives The contamination of water in watershed by phosphates is facilitated by the erosion process and the traditional tobacco cropping system. Urgent measures for erosion control must be adopted, in accordance to conformations of landscape and the local inhabitants' needs. Among them, those worth pointing out are the adoption of a conservation crop system, regeneration of riparian zone, and reduction of the phosphate doses added to soil for tobacco cultivation.

Degree of phosphorus saturation threshold for minimizing P losses by runoff in cropland soils of Southern Brazil

“…A long-term study in two important river basins in the state of Rio Grande do Sul showed that soluble P levels in freshwater are two-fold higher than the eutrophication limit, and that these high P concentrations are due to nonpoint sources of pollution from agriculture (Diel et al, 2007;Marchezan et al, 2009;Pellegrini et al, 2010). The fact that farmers in Southern Brazil are gradually using broadcast surface P application, in order to reduce time spent on planting, increases the potential for runoff losses.…”

Qualidade de águas subterrâneas captadas em fontes em função da presença de proteção física e de sua posição na paisagem

“…O P, por estar associado à fração sólida, as fontes não protegidas têm maior probabilidade de receberem material externo e apresentarem valores altos de P na água. Contudo, as fontes que compõem o subgrupo C apresentou PT elevado, o que sugere que grande parte do P presente nessas amostras se encontre em estado solúvel, como observaram PELLEGRINI et al (2010 (1) *Significância entre proteção (1 x 2) pelo teste t, a 5%, comparando as médias M1 dos subgrupos (A1vsA2; B1vsB2 e C1vsC2), em que n = 8; (2) * Significância entre locais (A x B x C) pelo teste t, a 5%, comparando as médias M2 dos grupos (AvsB; AvsC e BvsC), em que n = 16; ns é não significativo; A cor é expressa pela Unidade de Hazen (uH) e definida pela Portaria 518/2004(BRASIL, 2004.…”

Phosphorus Forms in Sediments as Indicators of Anthropic Pressures in an Agricultural Catchment in Southern Brazil

“…Moreover, as there was no correlation between SSC and the proportions of cropland and/or forest, it was evident that other factors play an important role in sediment yield, such as distribution of croplands and forests in the landscape. Indeed, the shape of the landscape strongly influences sediment concentration in the catchment, and preserved riparian vegetation can serve as a barrier, decreasing the amount of sediments transferred to the water bodies (Pellegrini et al, 2010). Even when almost all crop fields of the catchment were plowed to transplant tobacco, the values of SSC in all six sampling sites in the three sub-basins and in the catchment outlet were lower than those observed during the storm event that occurred in the fallow period (Table 2).…”

“…These fields are in steeply sloped areas, resulting in a large amount of sediment (Pellegrini et al, 2010;Bonumá et al, 2012;Bonumá et al, 2014;Tiecher et al, 2014).…”