Agriculture influences ammonium and soluble reactive phosphorus retention in South American headwater streams

Abstract

Agricultural activities can affect the delivery of nutrients to streams, riparian canopy cover, and the capacity of aquatic systems to process nutrients and sediments. There are few measures of nutrient uptake and metabolism from tropical or subtropical streams in general, and even fewer from tropical regions of South America. We examined ammonium (NH4+) and soluble reactive phosphorus (SRP) retention in streams in Brazil and Argentina. We selected 12 streams with relatively little or extensive agricultural activity and conducted whole‐stream nutrient additions and measurements of gross primary production and ecosystem respiration. We used multiple linear regression to determine potential drivers of nutrient uptake metrics across the streams. Nutrient concentrations and retention differed significantly between land use categories. Both NH4+ and SRP concentrations were higher in the agricultural sites (means of 161 and 495 μg l–1, respectively), whereas metabolic rates were slower and transient storage was smaller. Our analysis indicated that agriculture increased ambient uptake lengths and decreased uptake velocities. The regression models revealed that ambient SRP had a positive effect on NH4+ uptake and vice versa, suggesting strong stoichiometric controls. Drivers for nutrient uptake in streams with low‐intensity agriculture also included canopy cover, temperature, and ecosystem respiration rates. Nutrient assimilation in agricultural sites was influenced by a higher number of variables (gross primary production for SRP, discharge, and transient storage for both nutrients). Our results indicate agricultural activity changes both the magnitude of in‐stream nutrient uptake and the mechanisms that control its variation, with important implications for South American streams under agricultural intensification.

Assessing Restoration of Ecosystem Functioning in Brazilian Subtropical and Tropical Streams

“…As an illustration, pictures of the sites with 0%, 60%, and 93% of shading are also shown (from left to right), representing impacted, restored and pristine conditions, respectively. The full description of the sites and methods are provided by Saltarelli (2021) and Cunha et al (2020).…”

Limnology and Oceanography Bulletin Volume 31 Number 1 November 2021 1–29

“…As an illustration, pictures of the sites with 0%, 60%, and 93% of shading are also shown (from left to right), representing impacted, restored and pristine conditions, respectively. The full description of the sites and methods are provided by Saltarelli (2021) and Cunha et al (2020).…”

Land Use Change Influences Ecosystem Function in Headwater Streams of the Lowland Amazon Basin

“…Significant effects of land use on stream nutrient uptake have been widely observed [5,120,121], but studies of nutrient uptake in the tropics remain relatively rare compared to other biomes [110][111][112]117,122,123]. Of those studies that have evaluated land use effects on nutrient uptake dynamics in the Amazon-Cerrado region, most have shown increases in both N and P uptake with changes in land use in the Amazon [97,110,124] but have found mixed results in other regions of South America [122,124]. This emphasizes the importance of further study work to understand how nutrient retention will respond to land use change in expanding tropical agricultural regions.…”