Characteristics, lability and distribution of phosphorus in suspended sediment from a subtropical catchment under diverse anthropic pressure in Southern Brazil

Abstract

Measuring lability of anthropogenically enriched sediment-bound phosphorus (P) to water is a useful tool in ecological monitoring and planning mitigation to tackle diffuse P sources to surface waters. In current study, the sediment-associated total P, P fractions, and P lability accounting for river P legacy was determined. We monitored six representative headwater sub-catchments differing in land-use and management i.e., forest, recently turned low agriculture under conventional-tillage (CT), low agriculture under CT dominated with Ilex paraguariensis plantations, intense agriculture under CT, intense agriculture under no-tillage (NT), and highly anthropic urban receiving untreated sewage; and five sites on the main river in Guaporé river from October 2012 to October 2014. Total P and individual P fractions in 63-μm particle size were determined following the Hedley fractionation and the mean TP concentration lied between 752mgkg−1 in forest to as high as 1738mgkg−1 in highly anthropic sediments. The individual P fraction when pooled for different P lability indicated 10, 35, and 55% of TP as labile, moderately labile and non-labile P, respectively. With a land use shift from forest to highly anthropic urban, an increase in labile P (172%), moderately labile P (705%) and non-labile P (874%) was observed. The consistent higher values of non-labile (Pi+PoHID-0.5) and residual P in urban, high agriculture and main river sediment indicates the high P affinity due to the presence of high Fe/Al oxides. Temporal analysis revealed that regardless of P lability, a higher concentration was found in winter months (July to November) particularly under CT sub-catchment showing high sediment production and increased transport of added P during tobacco cultivation period. The results provided a baseline to revisit the current fertilization and soil management practices to minimize the river P legacy for sustainable ecological management.