Global trends in terrestrial denitrification and N2O emissions for the period 1900-2050

Estimates of global terrestrial denitrification and nitrous oxide (N2O) emission are presented for the period 1900 to 2000 and scenarios for the period 2000-2050 based on the Millennium Ecosystem Assessment. Soil nitrogen (N) budgets are used in a global distributed flow-path model with 0.5 by 0.5 degree resolution, representing denitrification and N2O emissions from soils, groundwater and riparian zones. Total agricultural and natural N inputs from N fertilizers, animal manure, biological N fixation and N deposition increased from ~155 to ~345 Tg of N yr-1 (Tg = teragram; 1 Tg = 1012 g) between 1900 and 2000; depending on the scenario, inputs will further increase to ~408 to ~510 Tg of N yr-1 in 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 per year in 2050, depending on the scenario. Estimates indicate that N2 production from denitrification increased from 52 to 96 Tg yr-1 between 1900 and 2000, and N2O-N emissions from 10 to 12 Tg of N yr-1. The major part (70%) of global N2 and N2O-N (92%) production occurred in soils in 2000. A further increase of denitrification is foreseen to 142 Tg N2-N and 16 Tg of N2O-N yr-1 in 2050. Our results indicate that riparian buffer zones are an important source of N2O. 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. The total (temporary) storage in deep groundwater between 1900 and 2000 amounts to around 376 Tg of N. Despite the removal of N through denitrification, the N flow from diffuse sources on land to rivers increased from 38 to 65 Tg of N yr-1 between 1900 and 2000, with a further increase of up to 84 Tg of N yr-1 in 2050. The major causes of uncertainty in our estimates are the difficulties associated with measurements and models of denitrification. With the projected increase in N flow through the soil and hydrological system, the availability of electron donors may become the limiting factor for denitrification in soils, groundwater and riparian zones in the future.