Soil nitrogen (N) budgets are used in a global, distributed flow-path model with 0.5° × 0.5° resolution, representing denitrification and N 2 O emissions from soils, groundwater and riparian zones for the period 1900–2000 and scenarios for the period 2000–2050 based on the Millennium Ecosystem Assessment. Total agricultural and natural N inputs from N fertilizers, animal manure, biological N 2 fixation and atmospheric N deposition increased from 155 to 345 Tg N yr −1 (Tg = teragram; 1 Tg = 10 12 g) between 1900 and 2000. Depending on the scenario, inputs are estimated to further increase to 408–510 Tg N yr −1 by 2050. In the period 1900–2000, the soil N budget surplus (inputs minus withdrawal by plants) increased from 118 to 202 Tg yr −1 , and this may remain stable or further increase to 275 Tg yr −1 by 2050, depending on the scenario. N 2 production from denitrification increased from 52 to 96 Tg yr −1 between 1900 and 2000, and N 2 O–N emissions from 10 to 12 Tg N yr −1 . The scenarios foresee a further increase to 142 Tg N 2 –N and 16 Tg N 2 O–N yr −1 by 2050. Our results indicate that riparian buffer zones are an important source of N 2 O contributing an estimated 0.9 Tg N 2 O–N yr −1 in 2000. Soils are key sites for denitrification and are much more important than groundwater and riparian zones in controlling the N flow to rivers and the oceans.

Abstract. In river basins, soils, groundwater, riparian zones and floodplains, streams, rivers, lakes and reservoirs act as successive filters in which the hydrology, ecology and biogeochemical processing are strongly coupled and together act to retain a significant fraction of the nutrients transported. This paper compares existing river ecology concepts with current approaches to describe river biogeochemistry, and assesses the value of these concepts and approaches for understanding the impacts of interacting global change disturbances on river biogeochemistry. Through merging perspectives, concepts, and modeling techniques, we propose integrated model approaches that encompass both aquatic and terrestrial components in heterogeneous landscapes. In this model framework, existing ecological and biogeochemical concepts are extended with a balanced approach for assessing nutrient and sediment delivery, on the one hand, and nutrient in-stream retention on the other hand.

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