Nitrous oxide emissions from grassland ecosystems: Magnitude, spatio-temporal pattern and attribution
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Manure/fertilizer nitrogen (N) application and N deposited by livestock play an important role in nitrous oxide (N2O) emission from agricultural systems, but relatively little attention has been given to understanding the response of N2O emissions to grassland management at the global scale, including those from manure. In this study, we parameterized and calibrated the global land ecosystem model (The Dynamic Land Ecosystem model; DLEM) to simulate the spatio-temporal variations in N2O emissions from both intensively managed and extensively managed grasslands during 1961-2014. In addition, we attributed total N2O emissions from grassland ecosystems to different sources including manure/ mineral fertilizer N application, manure N deposition and climate change. The DLEM simulations with multiple environmental changes and nitrogen input in the form of manure deposition and manure/fertilizer application show that N2O emissions from grassland ecosystems increased significantly from 1.79 Tg N2O-N/yr in 1961 to 2.68 Tg N2O-N/yr in 2014 (1Tg = 1012 gram) at the rate of 0.017 Tg N2O-N/yr2 (p < 0.05). Regionally, Europe contributed to 31% of the total emissions, followed by Asia (25%), Africa (23%) and North America (15%) during 1961-2014. Interestingly, DLEM simulations indicate that the contribution of developing regions (Asia and Africa) to N2O emissions has been increasing significantly largely due to an increase in manure deposition from livestock. Among various sources, manure deposition contributed to 56% of the total emissions, followed by manure application (21%) and fertilizer application (7%) during 1961-2014. Likewise, simulation results show that although managed grasslands (pasturelands) occupy 24% of the global grassland area, they contribute up to 77% of the total emissions, primarily due to high level of N inputs in the form of urine, feces and mineral N. It is likely that future increases in livestock production could stimulate N2O emissions, unless fertilizer and manure N management practices that promote nitrogen use efficiencies of grassland ecosystems are implemented.