Global trends and uncertainties in terrestrial denitrification and N
2
O emissions
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.