Solute and sediment export from Amazon forest and soybean headwater streams

Abstract

Intensive cropland agriculture commonly increases streamwater solute concentrations and export from small watersheds. In recent decades, the lowland tropics have become the world's largest and most important region of cropland expansion. Although the effects of intensive cropland agriculture on streamwater chemistry and watershed export have been widely studied in temperate regions, their effects in tropical regions are poorly understood. We sampled seven headwater streams draining watersheds in forest (n = 3) or soybeans (n = 4) to examine the effects of soybean cropping on stream solute concentrations and watershed export in a region of rapid soybean expansion in the Brazilian state of Mato Grosso. We measured stream flows and concentrations of NO , PO , SO , Cl , NH , Ca , Mg , Na , K , Al , Fe , and dissolved organic carbon (DOC) biweekly to monthly to determine solute export. We also measured stormflows and stormflow solute concentrations in a subset of watersheds (two forest, two soybean) during two/three storms, and solutes and δ O in groundwater, rainwater, and throughfall to characterize watershed flowpaths. Concentrations of all solutes except K varied seasonally in streamwater, but only Fe concentrations differed between land uses. The highest streamwater and rainwater solute concentrations occurred during the peak season of wildfires in Mato Grosso, suggesting that regional changes in atmospheric composition and deposition influence seasonal stream solute concentrations. Despite no concentration differences between forest and soybean land uses, annual export of NH , PO , Ca , Fe , Na , SO , DOC, and TSS were significantly higher from soybean than forest watersheds (5.6-fold mean increase). This increase largely reflected a 4.3-fold increase in water export from soybean watersheds. Despite this increase, total solute export per unit watershed area (i.e., yield) remained low for all watersheds (<1 kg NO N·ha ·yr , <2.1 kg NH -N·ha ·yr , <0.2 kg PO -P·ha ·yr , <1.5 kg Ca ·ha ·yr ). Responses of both streamflows and solute concentrations to crop agriculture appear to be controlled by high soil hydraulic conductivity, groundwater-dominated hydrologic flowpaths on deep soils, and the absence of nitrogen fertilization. To date, these factors have buffered streams from the large increases in solute concentrations that often accompany intensive croplands in other locations.

Identifying the Molecular Signatures of Agricultural Expansion in Amazonian Headwater Streams

“…At this site in the Brazilian Amazon, traditional geochemical approaches to assess impacts on stream geochemistry (nitrogen, phosphorus, and sediment loading) appeared to suggest limited impacts of agriculture on downstream aquatic ecosystems because of the high inorganic nutrient fixation and retention capacity of the Oxisols in the region (Neill et al, ; Jankowski et al, ; Riskin et al, , ). However, our results show that crop agriculture causes divergent DOC concentrations and a distinct DOM molecular composition compared with forested streams.…”

“…Aquatic autochthonous sources and microbial degradative processes have recently been shown to result in a diversity of N‐ and S‐containing formula (D'Andrilli et al, ; Riedel et al, ). Small impoundments of water that exist in cropland catchments at Tanguro Ranch and throughout the region (Macedo et al, ; Riskin et al, ) may provide hotspots for DOM production, or elevated microbial decomposition due to warmer soils may be prevalent in cropland watersheds leading to this molecular signature. On a seasonal basis, N‐containing formula were lowest during the late wet season samples and were highest in the early wet season samples for both cropland and forest streams (Table ).…”

“…Despite declining OM inputs in croplands, soil OC stocks between cropland and forest sites below the upper 10 cm were not different in this region (Nagy et al, ). Coupled with the fact that streams are typically dominated by groundwater inputs (Elsenbeer, ; Riskin et al, ), this suggests that the soils in this region are acting passively to the changing OM inputs from conversion of forest to croplands. Therefore, the changing signature derived from shifting OM sources in surface soils and litter layers from forest to cropland is then directly translocated to streams making them sentinels for detecting change.…”

“…Double‐cropping with corn began in 2005 and to date has occurred mainly on the northern part of the farm. The mean annual temperature in the region is 27°C, and mean annual precipitation is 1,800 mm/year with almost all precipitation falling between September and April (Riskin et al, ).…”

“…Interestingly, despite these large changes across the Amazon cropland region, the inorganic solute concentrations in small Amazon lowland headwater streams remain nearly identical in watersheds draining either forest or croplands (Riskin et al, ). This limited effect of forest conversion to croplands on stream inorganic solute concentrations in this region of the Amazon contrasts with large increases of solute concentrations in streams draining cropland watersheds in other regions (Howden et al, ; Johnes & Heathwaite, ; Mattsson et al, ; Mayer et al, ).…”