Abstract
Crops can uptake only a fraction of nitrogen from nitrogenous fertilizer, while losing the remainder through volatilization, leaching, immobilization and emissions from soils. The emissions of nitrogen in the form of nitrous oxide (N2O) have a strong potency for global warming and depletion of stratospheric ozone. N2O gets released due to nitrification and denitrification processes, which are aided by different environmental, management and soil variables. In recent years, researchers have focused on understanding and simulating the N2O emission processes from agricultural farms and/or watersheds by using process-based models like Daily CENTURY (DAYCENT), Denitrification-Decomposition (DNDC) and Soil and Water Assessment Tool (SWAT). While the former two have been predominantly used in understanding the science of N2O emission and its execution within the model structure, as visible from a multitude of research articles summarizing their strengths and limitations, the later one is relatively unexplored. The SWAT is a promising candidate for modeling N2O emission, as it includes variables and processes that are widely reported in the literature as controlling N2O fluxes from soil, including nitrification and denitrification. SWAT also includes three-dimensional lateral movement of water within the soil, like in real-world conditions, unlike the two-dimensional biogeochemical models mentioned above. This article aims to summarize the N2O emission processes, variables affecting N2O emission and recent advances in N2O emission modeling techniques in SWAT, while discussing their applications, strengths, limitations and further recommendations.
Highlights
Introduction to Soil and Water Assessment Tool (SWAT)SWAT is a process-based, continuous hydrological model which was initially developed to assist management of water-resources system and non-point sources of pollution in large river basins [106].Over the years, it has been extensively tested in multiple river basins across the globe and has been reported to replicate their quantitative and qualitative aspects satisfactorily [107,108]
Clay soil has been reported to have higher nitrous oxide (N2 O) fluxes compared to sandy clay loam and loess for the same moisture content in several studies, which is due to the higher soil organic carbon and mineral levels in clay soils [53,70]
Among different biogeochemical cycles included in SWAT, the nitrogen cycle (N-cycle) module is the one responsible for N2 O emissions
Summary
The protective layer of ozone in the Earth’s atmosphere keeps it warm and habitable by trapping high-energy ultraviolet rays. Anthropogenic activities have led to the emission and corresponding accumulation of other gases, such as carbon dioxide (CO2 ), methane (CH4 ) and nitrous oxide (N2 O) These gases have more complex structures than ozone and can absorb a significant amount of heat and restrict its dissipation. A multi-model simulation suggested that, with each increase in N2 O in stratosphere, ozone gets depleted 5–10 times, leaving the earth vulnerable to harmful radiative forcing [4]. This highlights an imminent need of understanding N2 O emission processes
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