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
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.
Section snippets
Soil samples collection and preparation
The two soils used in this study were collected during the field trials of the LIFE + “Improved flux Prototypes for N2O emission reduction from Agriculture” (IPNOA) project, carried out at two different sites in the Tuscany region of Italy: the “Enrico Avanzi” Centre for Agro-Environmental Research of the University of Pisa, located in San Piero a Grado (Pisa, Italy) (43°40′48″N 10°20′55″E) and the “Terre Regionali Toscane” Center for Innovation Testing, located in Cesa (Arezzo, Italy)
Cumulative N2O emissions
Cumulative N2O emissions were significantly affected by soil type, N fertilizer formulation and WFPS level, and significant interactions were recorded between: (i) soil type and N fertilizer formulation, and; (ii) N fertilizer formulation and WFPS level (p < 0.001).
N2O emissions from CS, when averaged among treatments, stood at 359 ± 101 μg N2O-N kg− 1 dry soil, significantly lower than the 1121 ± 374 μg N2O-N kg− 1 dry soil recorded in CLS.
In terms of the effect of N fertilizer formulation, N2O
Discussion
This study highlighted that the interaction between the soil characteristics and the nitrogen fertilizer formulation is a key factor regulating the magnitude of N2O emissions. Meanwhile, the effect of the WFPS level on N2O emissions turned out to be independent of the soil type and the N fertilizer formulation. Another key finding of this study is that only nitrification inhibitors showed any potential to mitigate N2O emissions, although this mitigation potential was strongly dependent upon the
Conclusions
This work investigated how fertilizer management could be improved to reduce soil N2O emissions by optimizing the fertilizer formulation. In particular, the manuscript reported results concerning the effect on N2O emissions of the interaction among factors, that have been usually evaluated separately in previous studies in recent literature.
The key finding of this study is the significant effect of the fertilizer formulation on N2O emissions and its strong interaction with the soil type.
Acknowledgements
This research was carried out with the support of the European Commission's LIFE financial instrument, within the framework of the project LIFE + “Improved flux Prototypes for N2O emission reduction from Agriculture” IPNOA (LIFE/11 ENV/IT/302, www.ipnoa.eu). We would like to acknowledge Dr. Marc Hervé and Dr. Andreas Pacholski from EurochemAgro for providing us with fertilizers and for their valuable contributions to this manuscript.
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