Fertilization-induced greenhouse gas emissions partially offset carbon sequestration during afforestation

Newly-planted forests require careful management to ensure the successful establishment of young trees; this can include herbicide application, irrigation, fertilization, or a combination of these treatments. The global rise in nitrogen (N) fertilizer application in managed forest plantations is driven by policies aiming at rapid tree growth and carbon sequestration as a strategy to tackle climate change. However, the impact of N-fertilizer on production and consumption of greenhouse gases (GHG), such as carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4) is poorly understood, particularly when combined with irrigation. As a result, assessing forest GHG balance is key to defining effective climate mitigation strategies through afforestation projects. This study assessed the response of GHG fluxes to irrigation and fertilization on recently afforested lowland arable land in central England, across loamy and sandy loam soils. The application of 180 kg ha-1 of N via an irrigation system, aimed at enhancing wood production and C sequestration, resulted in an increase of CO2 and N2O emissions for both soil types. Particularly, the N2O emission factors (EF; kg N2O/kg N applied) for loamy and sandy loamy soils were 3.9% and 2.1%, respectively, higher than the IPCC default estimate of 1% for agricultural and forest land. Furthermore, both sandy loam and loamy soils showed a distinct transition from being CH4 sinks to sources. Thus, the combined application of irrigation and N-fertilizer had a significant impact on the total Global Warming Potential (GWP), which increased by 34% and 32% for sandy loam and loamy soil, respectively, when compared to their controls. Despite a significant increase in tree growth under fertilized conditions, the offset potential was only partial, highlighting the net contribution to GHG emissions. The outcomes of this study emphasise the potential for significant “carbon-equivalent-debt” from afforestation supported in its early years by irrigation and fertilization.