Global trends and uncertainties in terrestrial denitrification and N_2O emissions

摘要

Soil nitrogen (N) budgets are used in a global, distributed flow-path model with 0.5° × 0.5° resolution, representing denitrification and N_2O 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 1/4 teragram; 1 Tg 1/4 1012 g) between 1900 and 2000. Depending on the scenario, inputs are estimated to further increase to 408-510 Tg N yr21 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 yr21 between 1900 and 2000, and N_2O-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_2O-N yr~(-1) by 2050. Our results indicate that riparian buffer zones are an important source of N_2O contributing an estimated 0.9 Tg N_2O-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.