Abstract
Climate change mitigation strategies have increased the demand for wood products, resulting in an urgent need to increase wood production. One approach is to fertilize forest land, but this can influence greenhouse gas (GHG) fluxes within the ecosystem. The aim of this study was to examine the effects of forest N fertilization on soil CH4 and N2O fluxes in young Norway spruce (Picea abies (L.) Karst.) stands in southern Sweden. The gas fluxes were measured using flow-through non-steady-state dark chambers. In the first, long-term, experiment, half of the stand was fertilized twice (once in 2014 and once in 2016) with 150 kg ha−1 of N, and gas flux measurements were taken throughout 2014–2017. In the second, dose, experiment, 0, 150, 300, or 450 kg ha−1 of N was added to the stand in April 2016, and gas flux measurements were taken during April-December 2016. The dose experiment showed that the sink strength of CH4 decreased with increasing amounts of N; the long-term experiment indicated that repeated fertilization decreased the CH4 sink strength over time. Additionally, the long-term experiment indicated that, while significantly higher N2O emissions were recorded in the fertilization years, this was not detected in subsequent years, suggesting the effect to be short-lived. In the dose experiment, fertilization tended to increase the N2O emissions relative to the amount of fertilizer. However, despite the significant effects of fertilization on these GHGs, the summed fluxes were a fraction of the net uptake of C at the sites, as recorded in another study. These findings suggest that fertilizing forest land with commercial NP or NPK fertilizers corresponding to 150 kg ha−1 of N, the level used in operational forestry in Sweden today, can be conducted without changing CH4 and N2O fluxes to any great extent.
Highlights
Climate change caused by rising levels of carbon dioxide (CO2) and other greenhouse gases (GHGs), such as methane (CH4) and nitrous oxide (N2O), in the atmosphere, is one of the greatest challenges of our time, and the risks of rising temperatures caused by GHG emissions are critical to life on Earth (IPCC, 2013)
Our study adds to the scant literature on fluxes of these potent GHGs from mineral soils, as previous work has primarily been carried out on organogenic soils (Maljanen et al, 2010; Matson et al, 2009; Ojanen et al, 2019; Shrestha et al, 2015; Siljanen et al, 2020), and provides new information about how CH4 and N2O fluxes are affected by forest fertilization of mineral soils
This study indicates that fertilizing forest land in Sweden with ≤ 150 kg ha− 1 of N, to increase wood production and meet the future demand for forest products, does not significantly decrease the uptake of CH4 nor the increase in N2O emissions
Summary
Climate change caused by rising levels of carbon dioxide (CO2) and other greenhouse gases (GHGs), such as methane (CH4) and nitrous oxide (N2O), in the atmosphere, is one of the greatest challenges of our time, and the risks of rising temperatures caused by GHG emissions are critical to life on Earth (IPCC, 2013). Of whether a forest is used to store carbon (C), or its wood utilized to replace fossil fuel or C-intensive materials, the growth rate of the forest is important to maximize its potential to contribute to climate change mitigation (Lundmark et al, 2014). In Sweden, most fertilization of forest land comprises one or more applications of 150 kg ha− 1 of N during a rotation period (Hedwall et al, 2014) in middle-aged and older boreal forests This method of fertilization can enhance stem growth by 13–20 m3 ha− 1 during a 10year period (Nohrstedt, 2001), while more intensive fertilization re gimes can result in much larger increases in wood production and further increase the potential to mitigate climate change. Biannual fertilization with 125–150 kg ha− 1 of N in young forest stands can result in incremental increases of 7 m3 ha− 1 yr− 1 over a six-year period (Bergh et al, 2008)
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