The expansion and intensification of soya bean agriculture in southeastern Amazonia can alter watershed hydrology and biogeochemistry by changing the land cover, water balance and nutrient inputs. Several new insights on the responses of watershed hydrology and biogeochemistry to deforestation in Mato Grosso have emerged from recent intensive field campaigns in this region. Because of reduced evapotranspiration, total water export increases threefold to fourfold in soya bean watersheds compared with forest. However, the deep and highly permeable soils on the broad plateaus on which much of the soya bean cultivation has expanded buffer small soya bean watersheds against increased stormflows. Concentrations of nitrate and phosphate do not differ between forest or soya bean watersheds because fixation of phosphorus fertilizer by iron and aluminium oxides and anion exchange of nitrate in deep soils restrict nutrient movement. Despite resistance to biogeochemical change, streams in soya bean watersheds have higher temperatures caused by impoundments and reduction of bordering riparian forest. In larger rivers, increased water flow, current velocities and sediment flux following deforestation can reshape stream morphology, suggesting that cumulative impacts of deforestation in small watersheds will occur at larger scales.

20 21Large-scale soy agriculture in the southern Brazilian Amazon now rivals 22 deforestation for pasture as the region's predominant form of land use change. Such 23 landscape level change can have substantial consequences for local and regional 24 hydrology, which remain relatively unstudied. We examined how the conversion to soy 25 agriculture influences water balances and stormflows using stream discharge (water 26 yields) and the timing of discharge (stream hydrographs) in small (2.5 to 13.5 km 2 ) 27 forested and soy headwater watersheds in the Upper Xingu Watershed in the state of 28Mato Grosso, Brazil. We monitored water yield for one year in three forested and four 29 soy watersheds. Mean daily water yields were approximately four times higher in soy 30 than forested watersheds, and soy watersheds showed greater seasonal variability in 31 discharge. The contribution of stormflows to annual streamflow in all streams was low (< 32 13% of annual streamflow), and the contribution of stormflow to streamflow did not 33 differ between land uses. If the increases in water yield observed in this study are typical, 34 landscape-scale conversion to soy substantially alters water-balance, potentially altering 35 the regional hydrology over large areas of the southern Amazon. 36 37 3 Introduction 38

20 21Large-scale soy agriculture in the southern Brazilian Amazon now rivals 22 deforestation for pasture as the region's predominant form of land use change. Such 23 landscape level change can have substantial consequences for local and regional 24 hydrology, which remain relatively unstudied. We examined how the conversion to soy 25 agriculture influences water balances and stormflows using stream discharge (water 26 yields) and the timing of discharge (stream hydrographs) in small (2.5 to 13.5 km 2 ) 27 forested and soy headwater watersheds in the Upper Xingu Watershed in the state of 28Mato Grosso, Brazil. We monitored water yield for one year in three forested and four 29 soy watersheds. Mean daily water yields were approximately four times higher in soy 30 than forested watersheds, and soy watersheds showed greater seasonal variability in 31 discharge. The contribution of stormflows to annual streamflow in all streams was low (< 32 13% of annual streamflow), and the contribution of stormflow to streamflow did not 33 differ between land uses. If the increases in water yield observed in this study are typical, 34 landscape-scale conversion to soy substantially alters water-balance, potentially altering 35 the regional hydrology over large areas of the southern Amazon. 36 37 3 Introduction 38

Agricultural intensification offers potential to grow more food while reducing the conversion of native ecosystems to croplands. However, intensification also risks environmental degradation through emissions of the greenhouse gas nitrous oxide (N2O) and nitrate leaching to ground and surface waters. Intensively-managed croplands and nitrogen (N) fertilizer use are expanding rapidly in tropical regions. We quantified fertilizer responses of maize yield, N2O emissions, and N leaching in an Amazon soybean-maize double-cropping system on deep, highly-weathered soils in Mato Grosso, Brazil. Application of N fertilizer above 80 kg N ha−1 yr−1 increased maize yield and N2O emissions only slightly. Unlike experiences in temperate regions, leached nitrate accumulated in deep soils with increased fertilizer and conversion to cropping at N fertilization rates >80 kg N ha−1, which exceeded maize demand. This raises new questions about the capacity of tropical agricultural soils to store nitrogen, which may determine when and how much nitrogen impacts surface waters.

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.