Nitrogen Transformations in Flowpaths Leading from Soils to Streams in Amazon Forest and Pasture

Abstract

The modification of large areas of tropical forest to agricultural uses has consequences for the movement of inorganic nitrogen (N) from land to water. Various biogeochemical pathways in soils and riparian zones can influence the movement and retention of N within watersheds and affect the quantity exported in streams. We used the concentrations of NO3 − and NH4 + in different hydrological flowpaths leading from upland soils to streams to investigate inorganic N transformations in adjacent watersheds containing tropical forest and established cattle pasture in the southwestern Brazilian Amazon Basin. High NO3 − concentrations in forest soil solution relative to groundwater indicated a large removal of N mostly as NO3 − in flowpaths leading from soil to groundwater. Forest groundwater NO3 − concentrations were lower than in other Amazon sites where riparian zones have been implicated as important N sinks. Based on water budgets for these watersheds, we estimated that 7.3–10.3 kg N ha−1 y−1 was removed from flowpaths between 20 and 100 cm, and 7.1–10.2 kg N ha−1 y−1 was removed below 100 cm and the top of the groundwater. N removal from vertical flowpaths in forest exceeded previously measured N2O emissions of 3.0 kg N ha−1 y−1 and estimated emissions of NO of 1.4 kg N ha−1 y−1. Potential fates for this large amount of nitrate removal in forest soils include plant uptake, denitrification, and abiotic N retention. Conversion to pasture shifted the system from dominance by processes producing and consuming NO3 − to one dominated by NH4 +, presumably the product of lower rates of net N mineralization and net nitrification in pasture compared with forest. In pasture, no hydrological flowpaths contained substantial amounts of NO3 − and estimated N removal from soil vertical flowpaths was 0.2 kg N ha−1 y−1 below the depth of 100 cm. This contrasts with the extent to which agricultural sources dominate N inputs to groundwater and stream water in many temperate regions. This could change, however, if pasture agriculture in the tropics shifts toward intensive crop cultivation.