Anthropogenic nitrogen emissions during the Holocene and their possible effects on remote ecosystems

Abstract

Reactive nitrogen {(Nr} = {NH3-N} + {NOx-N)} is an important atmosphericpollutant, contributing to acidification, eutrophication and biodiversitychanges in ecosystems. This study estimates Nr emissions from anthropogenicsources on a global scale since the advent of agriculture ∼8000{B.C.}, using a simple model based on the development of human populationand per capita factors of Nr emissions originating from livestockproduction, biomass burning (biofuel use and forest and savannahburning), and other anthropogenic sources (humans and pets, N-fertilizeruse, and fossil fuel combustion). The estimated global cumulativeanthropogenic emissions of Nr to the atmosphere are ∼17.4 Pg N(8.6 Pg {NH3-N} and 8.8 Pg {NOx-N)} for 8000 {B.C.} through the year2000 {A.D.}, with 28% of this amount emitted during 1850--2000 {A.D.},42% during 1--1850 {A.D.}, and 30% during the previous 8000 years.Forest and savannah burning represent the major cumulative flux ofboth {NH3-N} and {NOx-N} (3.5 and 5.8 Pg, respectively). Livestockproduction and biofuel burning are responsible for emissions of 3.3and 1.2 Pg {NH3-N}, respectively, while the application of syntheticfertilizers contributes 0.26 Pg {NH3-N.} The different duration ofbiofuel and fossil fuel use (10,000 versus ∼150 years) causes thehigher cumulative {NOx-N} emissions from biofuel than from fossilfuel use (1.9 versus 1.1 Pg). The cumulative Nr emissions on a landarea basis are 1.3 and 3.0 Mg N ha−1 globally and in Europe, respectively.Since an estimated 60% of Nr emitted in Europe is also depositedthere, the average cumulative anthropogenic Nr deposition would be∼1.8 Mg N ha−1, representing ∼30% of the current N pools inforest and alpine meadow soils of European glaciated areas (i.e.,soils of similar age as the emissions). Despite large uncertaintiesin the model (13.7--30.5 Pg N over the last 10,000 years), the relativetemporal distributions of total cumulative Nr emissions vary withinrelatively narrow ranges for different assumptions, with 70%--84%of the emissions occurring prior to 1850 {A.D.} We conclude thatthe majority of the total cumulative Nr flux from anthropogenic sourcesover the last 10,000 years occurred in the preindustrial period andcould have increased soil N pools of some remote ecosystems muchearlier than is currently assumed.