Greenhouse gas emissions in response to nitrogen fertilization in managed forest ecosystems

Nitrogen (N) fertilizer use in managed forest ecosystems is increasing in the United States and worldwide to enhance social, economical and environmental services. However, the effects of N-fertilization on production and consumption of greenhouse gases (GHGs), especially carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) in managed forest ecosystems are poorly understood, unlike in agriculture where effects are well documented. Therefore, a review of the available literature was conducted to comprehend the effects of N-fertilization on CO2, CH4 and N2O emissions in managed forest ecosystems to summarize sources, sinks, and controlling factors, as well as potential mitigation strategies and research gaps to reduce GHG emissions. This review clearly identifies the importance of N-fertilizer management practices on CO2, CH4 and N2O emissions. Potential N management practices to mitigate GHG emissions in managed forest ecosystems include improving N uptake efficiency, identifying and managing spatial variation in soil fertility, using the right fertilizer source at the right time, adopting appropriate methods of N-fertilizer application, and introducing nitrification/denitrification inhibitors. Nitrogen-fertilizer response is affected by soil physical (e.g., moisture, drainage, bulk density, and texture), chemical (e.g., nutrient availability, labile carbon, soil pH, and C/N ratio) and local climatic factors (e.g., temperature, relative humidity, and rainfall). Therefore, the interactions of these factors on GHG emissions need to be considered while evaluating N-fertilizer management practices. Existing studies are often limited, focusing primarily on temperate forest ecosystems, lacking estimation of net emissions considering all three predominant soil-derived GHGs, and were often conducted on a small scale, making upscaling challenging. Therefore, large-scale studies conducted in diverse climates, evaluating cumulative net emissions, are needed to better understand N-fertilization effects on GHG emissions and develop mitigation strategies. Mitigation strategies and research gaps have also been identified, which require the collaborative efforts of forest owners, managers, and scientists to increase adoption of N-fertilization best management practices and understand the importance of N-fertilizer management strategies in reducing emissions and enhancing the net GHG sink potential for managed forest ecosystems.