sustainability Review Nitrogen Losses and Potential Mitigation Strategies for a Sustainable Agroecosystem Kishan Mahmud 1 , Dinesh Panday 2 , Anaas Mergoum 3 and Ali Missaoui 1,4,5,* 1 Center for Applied Genetic Technologies, University of Georgia, Athens, GA 30602, USA; [email protected] 2 Department of Biosystems Engineering & Soil Science, The University of Tennessee, Knoxville, TN 37996, USA; [email protected] 3 Department of Internal Medicine, School of Medicine and Health Sciences, University of North Dakota, Grand Forks, ND 58102, USA; [email protected] 4 Department of Crop and Soil Sciences, University of Georgia, Athens, GA 30602, USA 5 Institute of Plant Breeding, Genetics and Genomics, University of Georgia, Athens, GA 30602, USA * Correspondence: [email protected]; Tel.: +1-706-542-8847 Abstract: Nitrogen (N) in the agricultural production system influences many aspects of agroe- cosystems and several critical ecosystem services widely depend on the N availability in the soil. Cumulative changes in regional ecosystem services may lead to global environmental changes. Thus, the soil N status in agriculture is of critical importance to strategize its most efficient use. Nitro- gen is also one of the most susceptible macronutrients to environmental loss, such as ammonia volatilization (NH3), nitrous oxide (N2O) emissions, nitrate leaching (NO3), etc. Any form of N losses from agricultural systems can be major limitations for crop production, soil sustainability, and environmental safeguard. There is a need to focus on mitigation strategies to minimize global Citation: Mahmud, K.; Panday, D.; N pollution and implement agricultural management practices that encourage regenerative and Mergoum, A.; Missaoui, A. Nitrogen sustainable agriculture. In this review, we identified the avenues of N loss into the environment Losses and Potential Mitigation caused by current agronomic practices and discussed the potential practices that can be adapted to Strategies for a Sustainable prevent this N loss in production agriculture. This review also explored the N status in agriculture Agroecosystem. Sustainability 2021, during the COVID-19 pandemic and the existing knowledge gaps and questions that need to be addressed. 13, 2400. https://doi.org/ 10.3390/su13042400 Keywords: nitrogen; nitrate leaching; nitrous oxide; soil resilience; soil microbiome; regenerative Academic Editor: Christopher agriculture; ecological ditch Robin Bryant Received: 20 January 2021 Accepted: 18 February 2021 1. Introduction Published: 23 February 2021 The current global population of 7.8 billion is projected to reach over 9 billion by 2050 [1]. This projected boom in population would mandate an approximately 70% increase Publisher’s Note: MDPI stays neutral in global agricultural production to ensure food security in the developed and nearly 100% with regard to jurisdictional claims in in the developing countries [2]. To keep up with this demand, global agriculture will published maps and institutional affil- continue to consume more amendments in both inorganic and organic forms that can iations. support agricultural production and simultaneously battle the food waste crisis where one-third of the annual produced food goes to waste (1.3 billion tons) [3]. Nitrogen (N) is a critical element for all living organisms and assimilation of N by both terrestrial and aquatic plants is limited by its forms in the ecosystem [4]. Copyright: © 2021 by the authors. Over the last five decades, the global N cycle has changed significantly due to the Licensee MDPI, Basel, Switzerland. incessant input of nitrogen fertilizers. Nitrogen cycling involves five major steps; biological This article is an open access article N fixation, ammonification, nitrification, N assimilation into microbial biomass pool, distributed under the terms and and finally denitrification [5]. However, both chemical N fertilizers and organic manure conditions of the Creative Commons are often applied to soil in exceeding amounts for crop growth requirements lead to Attribution (CC BY) license (https:// N loss. Of the applied N for crop growth, only 45–50% is being incorporated into the creativecommons.org/licenses/by/ agricultural products [6] and the remainder is subjected to substantial loss [7]. A significant 4.0/). Sustainability 2021, 13, 2400. https://doi.org/10.3390/su13042400 https://www.mdpi.com/journal/sustainability Sustainability 2021, 13, x FOR PEER REVIEW 2 of 23 Sustainability 2021, 13, 2400 2 of 23 loss. Of the applied N for crop growth, only 45–50% is being incorporated into the agri- cultural products [6] and the remainder is subjected to substantial loss [7]. A significant amount of N in soil and may be lost to the environment as NO , NH , or N O[8,9]. amount of N in soil and may be lost to the environment as NO3, NH3, or N2O [8,9]3 . NO3 3 2 NO may also continue to undergo recycling in the soil–water–air system and convert to may also continue3 to undergo recycling in the soil–water–air system and convert to N2O N O and N through the denitrification process and released back to the atmosphere [10]. and N2 through2 the denitrification2 process and released back to the atmosphere [10]. Par- Particularly, N O emission is substantial from production agriculture at the beginning ticularly, N2O emission is substantial2 from production agriculture at the beginning stage of N fertilizerstage application of N fertilizer during application the cropping during season the cropping [11]. Furthermore, season [11]. the Furthermore, potential the potential warming effect of N2O and the short-term cooling effect of NH3 and NOx both has major warming effect of N2O and the short-term cooling effect of NH3 and NOx both has major consequences on global human and environmental health [12,13]. Additional complications consequences on global human and environmental health [12,13]. Additional complica- in estimating N2O arises from the nonlinear nature of N2O emission in response to N tions in estimating N2O arises from the nonlinear nature of N2O emission in response to fertilizer application along with other soil and environmental controlling factors [14]. N fertilizer application along with other soil and environmental controlling factors [14]. Intergovernmental Panel on Climate Change (IPCC) linear model, one of the approaches to Intergovernmental Panel on Climate Change (IPCC) linear model, one of the approaches estimating N loss in terms of N2O, which may overestimate N loss at lower N application to estimating N loss in terms of N2O, which may overestimate N loss at lower N applica- rates while underestimating higher rates [15–17] (Figure1). tion rates while underestimating higher rates [15–17] (Figure 1). Figure 1. The nitrogen cycle Figurein the soil. 1. The nitrogen cycle in the soil. Agricultural contributionAgricultural to contribution global N pollution to global requires N pollution constant requires monitoring constant and monitoring and adoption of necessaryadoption efforts of necessary at the effortsfarm, regional, at the farm, and regional,national andlevels national to mitigate levels and to mitigate and regulate environmentalregulate environmental degradation. Nitrogen degradation. footprint, Nitrogen under footprint, agricultural under context, agricultural is the context, is amount of N thereleased amount from of Nresource released use from in agricultural resource use production in agricultural at every production step of the at every step of production linethe both production upstream line or both downstream upstream [18] or downstream. Nitrogen footprint [18]. Nitrogen can serve footprint as an can serve as indicator of thean usage indicator and of losses theusage of N in and the losses production of N in and the consumption production and of food consumption and of food energy, thereby,and an energy, ecosystem thereby, service. an ecosystem The objective service. of this The review objective was of to this explore review the was ag- to explore the ricultural statusagricultural of N in the status current of N agronomic in the current production agronomic system production and possible system mitigation and possible mitigation strategies to addressstrategies and to minimize address and N loss minimize for sustainable N loss for agriculture. sustainable agriculture. 2. Nitrogen Footprint2. Nitrogen in the Footprint World in the World Nitrogen playsNitrogen such a cardinal plays such role a in cardinal ecosystem role inproductions ecosystem and productions the Earth’s and energy the Earth’s energy balance that anybalance changes that in any the changes N cycle in will the bring N cycle about will profound bring about impacts profound on the impacts global on the global ecosystem andecosystem human health and human[19–21]. healthIt is necessary [19–21]. to It track is necessary the gains toand track losses the in gains the N and losses in cycle and therethe are N several cycle and tools there to do are just several that. For tools instance, to do justa quantifying that. For tool instance, that serves a quantifying tool as an indicatorthat of N serves losses as to an the indicator environment of N lossesfrom different to the environment stages of the from N cycle different ranging stages of the N from productioncycle to ranging consumption from production levels is called to consumption N footprint levels [22]. isGlobally, called N agriculture footprint [ 22]. Globally, dominates N agriculturefootprints [23] dominates. Furthermore, N footprints global [trades23]. Furthermore, connecting