Contributions of nitrogen deposition and nitrogen fertilizer application to greenhouse gases balance in China

Background/Question/Methods Human activities largely enhanced the amount of reactive nitrogen (N) deposited onto land surface and fertilized to cropland in China. For the past century, atmospheric and anthropogenic N additions have substantially stimulated terrestrial carbon uptake, and thus alleviated global warming. Many studies confirmed that China acted as a significant carbon sink caused by a wide range of environmental factors, among which N amendment is a critical contributor for determining the carbon sequestration capacity. However, little is known on the roles of atmospheric N deposition and N fertilizer application to climate changes if simultaneously considering the exchanges of CO₂, CH₄ and N₂O between land ecosystems and the atmosphere induced by N enrichment. Here we used a process-based ecosystem model, Dynamic Land Ecosystem Model (DLEM), to distinguish and quantify the biogenic fluxes of three greenhouse gases (GHGs) resulted from increased N deposition to land ecosystems and N fertilizer application to agricultural land in China during 1901-2005.

Results/Conclusions Our study finds that N-induced terrestrial CO₂ uptake has been largely offset by the emissions of CH₄ and N₂O caused by N enrichment, especially for the recent decades. However, both atmospheric N deposition and anthropogenic N fertilizer application are found to mitigate climate warming in terms of global warming potential (GWP) from three GHGs budget. Ambient CO₂ reduction induced by reactive N inputs is overwhelming compared to the release of CH₄ and N₂O. It is noticeable that the CO₂ sink stimulated by N fertilizer application leveled off and began to decline in the 1980s while N₂O emission continued to increase with fertilization amount. The net GWP of three GHGs would continually increase and lead to climate warming if N fertilization rate kept rising or overused.