Improving the management of mineral fertilizers for nitrous oxide mitigation: The effect of nitrogen fertilizer type, urease and nitrification inhibitors in two different textured soils

Highlights

• Nitrogen fertilizer type is an important regulating factors for soil N₂O.

• NH₄-based fertilizer produced higher N₂O than NO₃-based fertilizers.

• Effect of WFPS level on N₂O soil emissions was independent by soil type.

• Effectiveness of nitrification inhibitors was strongly influenced by soil type.

• Urease inhibitors were not effective in reducing N₂O emissions.

Abstract

The production of nitrous oxide (N₂O) in soil is mainly related to nitrification and denitrification processes, taking place under aerobic and anaerobic conditions, respectively. Oxygen, ammonium (NH₄⁺) and nitrate (NO₃⁻) concentrations in soil are the main factors controlling the N₂O production processes. Many studies confirm that N₂O emissions in agroecosystems are stimulated by increasing the N fertilizer rate. Beside the N rate, the fertilizer formulation may affect soil N₂O emissions, though the effects are still unclear and may be different according to soil type, soil water content and oxygen availability expressed as soil water filled pores space (WFPS). Enhanced efficiency fertilizers, such as fertilizers with nitrification and urease inhibitors (NIs, UIs) may slow down N₂O production processes, although different results were reported regarding their effectiveness.

This study contributed to the evaluation of the effect of mineral N fertilizer formulation on N₂O and CO₂ emissions, through two laboratory experiments carried out on soil cores distributing to two different textured soils: i) nitrate or ammonium-based fertilizer under increasing WFPS levels (Exp.1), and; ii) NIs and UI with various N fertilizers, including a new NI (DMPSA) (Exp.2).

The highest N₂O and CO₂ emissions were recorded after the application of an ammonium-based fertilizer in both soils, suggesting the preponderance of nitrifying organisms. The results confirmed that WFPS is a key driver for N₂O emissions, with highest N₂O emissions occurring at WFPS > 75%, while CO₂ emissions decreased linearly with increasing WFPS across all fertilizer treatments and in both soils. NIs were effective only in the soil with the highest nitrification activity and the lowest clay content, decreasing N₂O emissions by an average of 50%, while the UI did not reduce N₂O emissions in either soil. These results proved that soil characteristics significantly affect NIs capacity to mitigate N₂O emissions from soil.

Introduction

There is high interest within the scientific community in processes that regulate nitrous oxide (N₂O) emissions from agricultural soils, mainly because soil is a major source of this potent and ozone-depleting greenhouse gas (Bouwman et al., 2002, Ravishankara et al., 2009). N₂O emissions from agricultural soils account for about 70% of the total annual N₂O emissions at the European scale (European Environment Agency, 2015). The production of N₂O by soils is primarily attributed to the microbial processes of nitrification and denitrification. Through nitrification, in aerobic conditions, ammonium ion (NH₄⁺) is oxidized to form nitrate ion (NO₃⁻), and the availability of NH₄⁺ and oxygen are the dominant factors controlling the process (Firestone and Davidson, 1989). During denitrification, in anaerobic conditions, NO₃⁻ is reduced to N₂ and N₂O can be emitted as an intermediate product. The availability of carbon (C), NO₃⁻ and oxygen are the major factors controlling microbial denitrification (Tiedje, 1988). However, a complex set of processes are involved in the production of N₂O in soil, as was clearly summarized by Butterbach-Bahl et al. (2013), and the relative proportion of the end products of these processes is influenced by different factors such as environmental conditions, and the biological and physico-chemical characteristics of the soil. Moreover, the processes contributing to the emission of N₂O from agricultural soils are often regulated by management practices, with nitrogen (N) fertilization playing a key role in stimulating N₂O emissions, especially when the N rate exceeds crop demand (Crutzen et al., 2008, Snyder et al., 2014, Sainju, 2016). The total worldwide consumption of N fertilizer is expected to increase in order to feed a growing population, meaning that greenhouse gas (GHG) emissions, especially N₂O, are also expected to rise (Food and Agriculture Organization of the United Nations (FAO), 2009; Tilman et al., 2011). Previous studies have indicated, as regards N fertilizer management, that the N rate, fertilizer formulation and fertilizer splitting are key aspects influencing N₂O emissions (Sainju, 2016, Burger and Venterea, 2011). Laboratory and field studies have studied whether the application of an NO₃-based or NH₄-based fertilizer can influence the intensity of N₂O emissions by affecting the NH₄⁺ and NO₃⁻ soil concentrations. However, limited studies compare different N fertilizer types, often reporting contradictory results. The lack of clear results on the effect of N fertilizer formulation is probably due to the strong relation between N₂O emissions, fertilizer application technique and soil properties (Lebender et al., 2014, Stehfest and Bouwman, 2006, Tenuta and Beauchamp, 2003). Thus, further studies are necessary in order to draw solid conclusions for the individuation of N₂O emission mitigation strategies. Together with NH₄⁺ and NO₃⁻ soil concentration, water filled pore space (WFPS), which depends on the combination of water availability and soil physical and chemical characteristics, is often indicated as the main soil parameter regulating N₂O emissions (Dobbie and Smith, 2003). In fact, WFPS regulates the microbial processes of nitrification and denitrification by affecting the oxygen availability in soil (Del Prado et al., 2006). In field trials, high peaks in N₂O emissions are frequently detected after rainfall events, caused both by a stimulation of mineralization and a change in gas diffusivity (Ball, 2013, Chen et al., 2013, Laville et al., 2011). In particular, it is commonly indicated that N₂O from nitrification occurs with WFPS of around 50–60%, while N₂O derived from denitrification peaks occur at a WFPS higher than 60%, though these figures may change in response to different soil properties (Davidson et al., 1991, Grundmann and Rolston, 1987). Previous studies have reported that when WFPS is above 80%, oxygen is limiting and N₂O may be reduced to N₂, though this threshold may vary depending on the soil texture, soil temperature, nitrogen content and pH level (Bouwman, 1998, Chapuis-Lardy et al., 2007).

Furthermore, in regards to N fertilizer formulation, enhanced efficiency N fertilizers, such as fertilizers with nitrification (NIs) or urease inhibitors (UIs), were developed to improve the efficiency of N use by crops and minimizing the N losses. The NIs act by delaying the bacterial oxidation of NH₄⁺, halting the nitrifying activity in soil, while the UIs slow down the hydrolysis of urea. NIs and UIs can potentially mitigate N₂O soil emissions, given their effect on the presence of NO₃⁻ and NH₄⁺ in soil (Akiyama et al., 2010, Khalil et al., 2009, Ruser and Schulz, 2015). However, it is difficult to draw any specific conclusions about the N₂O mitigation potential of NIs and UIs, because their efficiency depends on site-specific parameters such as climate conditions, soil properties and management practices. This uncertainty is especially relevant for N₂O mitigation by UIs (Barth et al., 2001, McGeough et al., 2016, Francisco et al., 2011). The main objective of this study was to analyze the N₂O emissions from two different textured soils in a laboratory experiment on soil cores as a response to: i) different N fertilizer formulations (nitrate vs ammonium-based fertilizers) at increasing WFPS levels (experiment 1), and; ii) the application of different N fertilizers with or without NI or UI (experiment 2). Finally, to expand our knowledge of the potential effect of the factors being analyzed on soil respiration, carbon dioxide (CO₂) emissions were also measured in both experiments.