Dissolved nutrient exports from natural and human‐impacted Neotropical catchments

Abstract

Aim Neotropical biomes are highly threatened by land‐use changes, but the catchment‐wide biogeochemical effects are poorly understood. Here, we aim to compare exports of dissolved nitrogen (N) and phosphorus (P) from natural and human‐impacted catchments in the Neotropics. Location Neotropics. Methods We measured streamwater nutrient concentrations and exports in 20 south‐eastern Brazilian catchments with different land uses (natural Cerrado/semi‐deciduous forest, pasture, intensive agriculture and urban areas) and conducted a meta‐analysis on nutrient exports from Neotropical catchments, both natural and human‐impacted. Results Organic forms dominated dissolved nutrient exports in central/south‐east Brazil in both natural and human‐dominated catchments. Our meta‐analysis suggests that there is wide geographic variability in the natural dominance of organic versus inorganic nutrient exports across the Neotropics, and for N a tendency for inorganic and organic forms to vary inversely across sites. We found strong land‐use effects, especially in urban areas. In areas naturally dominated by organic N exports, land use did not overturn that dominance, but rather increased the concentration of both inorganic and organic N. In catchments dominated by inorganic N exports or showing equivalent organic and inorganic exports, land use also caused organic exports to be dominant. Land‐use effects on P were unclear, probably owing to a paucity of available data, but our results from south‐east Brazil suggested that land use also increases dissolved P exports. Main conclusions We show that dominance of inorganic versus organic nutrient exports is highly variable across natural Neotropical catchments, a pattern contrasting with the general tendency for dominance of organic N in natural temperate catchments. Further, we found a general dominance of organic N in most human‐impacted catchments, which is in contrast to strong dominance of inorganic N in most human‐impacted temperate catchments. Finally, we find that urbanization exerts the strongest impacts on nutrient exports, thus underscoring wastewater treatment as a critical management priority under future land‐use change.

Nutrient export and elemental stoichiometry in an urban tropical river

“…In central Brazil, small watersheds with significant urban cover had a median NO 3 − ‐N concentration of only 0.22 mg/L (Gücker et al. ), considerably lower than those in the Río Piedras.…”

“…Gücker et al. () showed a clear pattern of increasing SRP in response to urbanization in Brazilian Cerrado watersheds, with concentrations doubling (17–35 μg/L) and N:P ratio increasing dramatically relative to their undeveloped watersheds. These urban sites in Brazil were half the average value that we obtained in the highly urbanized Río Piedras watershed (74 μg/L), likely due to its very high human population density.…”

“…Work by Gücker et al. (, b) shows that, for tropical inland streams in Brazil, pastures and urbanization increase concentrations of DOC, DON, and inorganic nutrients, and alter the stoichiometry of dissolved organic matter. The role of small tropical rivers in coastal nutrient dynamics is not as well understood, despite the disproportionately important role that those draining mountainous volcanic islands play in the global carbon cycle (Lloret et al.…”

Contrasting nitrogen fluxes in African tropical forests of the Congo Basin

“…, Gücker et al. ), and net soil N transformations (Vitousek and Matson ). Although, most of these proxies are considered integrative, they only offer limited insight in the mechanisms behind contrasting N cycle patterns between ecosystems.…”

Living in a browning environment: Effects on Daphnia’s growth and fatty acid pattern

“…Worldwide urbanization and agricultural land use increases and alters the export of terrestrial carbon to aquatic ecosystems (Graeber et al ), with DOM being by far the largest fraction (Gücker et al ) and having pronounced effects on the FA distribution, e.g., in large tropical rivers (Boëchat et al ). These current worldwide impact of land use and climate change, not only alters HS dynamics—brownification—in temperate and boreal lakes (Williamson et al ; Scharnweber et al ; Creed et al ) but also in the tropics (Gücker et al ). Therefore, it is extremely important to understand how HS input and metabolism might affect food web structure directly and indirectly.…”