Direct measurement of dissolved dinitrogen to refine reactive modelling of denitrification in agricultural soils

doi:10.1016/j.scitotenv.2018.07.428

Science of The Total Environment

Volume 647, 10 January 2019, Pages 134-140

https://doi.org/10.1016/j.scitotenv.2018.07.428 Get rights and content

Highlights

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Soils' denitrification rate increased when supplied with acetate.
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Soils showed high N₂ degassing, responsible for the low dissolved N₂ concentrations.
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This is the first model calibrated versus dissolved N₂ and all other redox species.

Abstract

Nitrogen fertilizers used in agriculture often cause nitrate leaching towards shallow groundwater, especially in lowland areas where soil permeability, ploughing, clay content, and the flat topography minimizes surface runoff. The introduction of good agricultural practices to reduce the nitrate amount entering the groundwater system is crucial to ameliorate the kinetic control on nitrate denitrification capacity. With this aim, a series of anaerobic mesocosms, consisting of loamy and clay soils and nitrate rich water, were modelled using acetate and natural organic matter as electron donors. Acetate was chosen because it is the main intermediate in many biodegradation pathways of organic compounds, and hence it is a suitable carbon source for denitrification. To account for the spatial variability of soil parameters, the experiments were performed in triplicates. The geochemical code PHREEQC(3) was used to simulate kinetic denitrification, and equilibrium reactions of gas and mineral phases. The reactive modelling results highlighted a rapid acetate and nitrate degradation rate, a rapid production of dissolved inorganic carbon and dinitrogen, and a steady concentration of dissolved iron and sulphate, suggesting that the main pathway of nitrate attenuation is through denitrification; concomitantly excluding the occurrence of other processes leading to nitrate consumption. In the absence of acetate, the loamy soil, poor of natural organic matter, did not allow to complete the denitrification process.

This modelling study investigates in detail the relationship between the denitrification process in natural soils, with excess and in limitation of organic substrates, and the occurrence and fate of dissolved dinitrogen analysed with a high precision membrane inlet mass spectrometer. Results demonstrate that modelling nitrate degradation processes as a whole, using geochemical datasets and codes, will improve the estimates of agricultural landscapes denitrification and support better nitrogen management, especially in lowland environments.

Graphical abstract

Introduction

Nitrate (NO₃⁻) is considered as the most widespread inorganic contaminant in both surface waters and groundwater due to agricultural fertilization practices (Puckett et al., 2011) and other sources like industrial and sewer systems (Wakida and Lerner, 2005) or wet and dry deposition (Bauer et al., 2007). It has been shown that the agricultural fertilization practices have risen the reactive nitrogen (N) inputs into the terrestrial biosphere up to a factor two respect to the 1860 levels (Galloway et al., 2004). It is also renowned that the ecological and toxicological effects generated by inorganic N pollution in aquatic ecosystems are detrimental (Camargo and Alonso, 2006). Recently, it has been shown that the vadose zone can be considered as a large NO₃⁻ reservoir, actually unaccounted in the global N cycle (Ascott et al., 2017). Thus, to understand the processes that occur in both the vadose and saturated zones, accurate data sets are needed to unravel NO₃⁻ fate from the application to the eventual leaching or transformation. Reactive N attenuation from surface to groundwater systems may take place via bacterial heterotrophic denitrification, using NO₃⁻ as electron acceptor and a carbon (C) source as electron donor, with the production of N gases (Rivett et al., 2008). This process has been extensively studied in superficial ecosystems, showing that the increasing trend of anthropogenic reactive N inputs can be overwhelming for the freshwater ecosystems (Seitzinger, 2008). Besides, other reactive gasses like N₂O and NO are often produced in well drained and tilled agricultural fields via incomplete denitrification (Mosier et al., 1998; Loick et al., 2016; Charles et al., 2017). Although, N₂O and NO production became less important in anaerobic conditions, while N₂ is the dominant denitrification product (Firestone and Davidson, 1989). To prevent such a disaster, best management practices of agricultural lands are more and more required and adopted by single nations or by international agencies (Tilman et al., 2002). One of the possible way out is to try to augment the labile organic content of soils, which in turn can diminish NO₃⁻ leaching via denitrification or by establishing wetlands or biofilters (Dinnes et al., 2002). Acetate is a good candidate as electron donor, since it is the main intermediate species in many biodegradation pathways of organic compounds, thus it is an appropriate C source to trigger denitrification processes (Castaldelli et al., 2013a). Beside denitrification, other processes can occur in the subsurface altering the reactive N fate, like the dissimilatory NO₃⁻ reduction to ammonium (NH₄⁺) (Rütting et al., 2011), anammox and co-denitrification by fungi (Long et al., 2013) or autotrophic NO₃⁻ reduction (Chen et al., 2018). Then, new studies must take into account the role of different pathways that can affect NO₃⁻ in soils. In the recent past, to understand the chemical and biological processes responsible for the natural reduction of NO₃⁻ in soils and groundwater, a number of laboratory experiments and reactive-transport modelling studies have been performed (Mastrocicco et al., 2011; Yan et al., 2016). Numerical models are now also configured to incorporate isotopic fractionation processes controlled by kinetic and equilibrium conditions (Rodríguez-Escales et al., 2014; Vavilin and Rytov, 2015), but actually there is still a lack of accurate measurements and modelling of gaseous end-product of denitrification, like dinitrogen (N₂).

The main purpose of this research was to quantify the main biogeochemical reactions governing N₂ exsolution and transport through the vadose/saturated zone interface, using laboratory mesocosms simulated via the reactive geochemical code PHREEQC(3) (Parkhurst and Appelo, 2013).

As far as the authors are aware, this is the first biogeochemical modelling study in agricultural soils constrained by major redox species, combined with a high precision membrane inlet mass spectrometer (MIMS) employed to track N₂ production.

Section snippets

Study sites

In Italy, the Po River valley is the largest and most intensively farmed alluvial plain, and is heavily impacted by NO₃⁻ contamination in groundwater (Soana et al., 2017) and surface water (Panepinto et al., 2016). In the coastal area of the Po River valley, four sites (representative of the most common soil types) have been extensively studied in the last decade within various national and international research projects. The four sites have different soils characteristics: sandy, peaty,

Reactive modelling

NH₄⁺ concentrations were always below detection limits (<0.5 μmol/L), CH₄ concentrations never exceeded 0.5 μmol/L, with most values <0.2 μmol/L, without any temporal increase or difference among treatments (data not shown).

Fig. 2 illustrates that the model results fit well with the observed reagents and products of denitrification during the experiments. The primary electron donor (acetate for CCR-Ace and SAP-Ace mesocosms and SOM for CCR and SAP mesocosms) and the terminal electron acceptor

Discussion

Although, the MIMS technique does not distinguish between denitrification and anammox, which are both N₂ producing processes, anammox was excluded as active processes due to the absence of NH₄⁺; for the same reason also DNRA was not considered. Since CH₄ was always close to detection limit, methanogenesis was considered as an insignificant pathway in organic matter degradation in the two soils studied. The monitoring of the redox sensitive geochemical species (Fig. 3), allowed excluding other

Conclusions

The reactive modelling fully described the complete NO₃⁻ degradation to N₂ in both acetate amended and unamended mesocosms, in conjunction with the concomitant DIC increase. These evidences, corroborated by the good model fit, indicate that the main pathway of NO₃⁻ attenuation in these soils is denitrification in presence of acetate as electron donor, while calcite and dolomite acted as a buffer for pH, preserving optimal denitrification conditions. On the contrary, when labile C sources were

Acknowledgments

This work was financially supported by the Emilia-Romagna Region within the Rural Development Programme (PSR) 2014-2020 and within the POR FESR 2007-2013 Programme for the development of the regional High Technology Network. A special thank goes to Dr. Fabio Vincenzi for his technical support and to Dr. Elisa Soana for the calculations of the MIMS data. We would like to thank two anonymous reviewers for their positive feedbacks and remarks.

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