Understanding nitrogen and carbon biogeotransformations and transport dynamics in saturated soil columns
Introduction
The use of treated, partially treated and untreated urban wastewater in agriculture has been a common practice for centuries in developing countries. The use of wastewater in the agricultural sector has enormously substituted both water and nutrient demands in the form of nitrogen. Nitrogen (N) is an important agricultural input which plays a major role in crop production (Tavakkoli and Oweis, 2004). It is important to use an optimum amount of water and nitrogen for best management of crop production in agricultural fields because the application of an excess amount of water can cause nitrogen leaching below the root zone (Chen et al., 2007, Gheysari et al., 2009, Wang et al., 2010), thus causing economic losses for farmers. Due to the lack of regulation and monitoring in this regard, most of the groundwater resources are contaminated which can cause a health threat to humans. Effective management of groundwater quality requires a thorough scientific understanding of complex physical, chemical and biological processes that govern the transport in the subsurface environment.
NO3− transport in flooded soil is governed by a complex interplay between chemical, biological and physical factors such as flow conditions (Fillery et al., 1984, Antonopoulos, 1993), nitrogen sources, climate, soils, microbial ecology, soil oxic condition, plant uptake, geology, groundwater geochemistry, the land surface area contributing to the well, and travel time in the, aquifer (Reddy et al., 1989, Lee et al., 2006, Desimone and Howes, 1998, Rubol et al., 2013, Ma et al., 2016a, Ma et al., 2016b). A good understanding of transport (advection and dispersion) processes along with chemical and biological reactions (adsorption, biotransformation) is essential for developing a model for sustainable irrigation practices and policymaking. Moreover, modeling tools open a window for irrigation scheduling, water and nutrient addition according to crop needs, and deciding remediation technology (Zhao et al., 2009). But, the knowledge about contaminant loading, solute velocity, dispersion, transformation rates and soil properties is essential.
Numerous laboratory scale controlled column experiments were carried out to address the leaching and sorption process (Jellali et al., 2010, and Al-Darby and Nasser, 2006) and biotransformation (Munoz-Leoz et al., 2011, Liang et al., 2012, Mekala and Nambi, 2016). The biodegradation process was mostly considered as first order kinetics for simplification (Zhao et al., 2014). Moreover, the studies that have considered the biodegradation process ignore the effect of substrate inhibition on biodegradation which seems to greatly affect the reaction rates. The evaluation of column experiments with numerical models allows quantification of individual processes and their parameters (Wehrer and Totsche, 2008).
Several numerical reactive transport models have been developed and are used to explain the contaminant transport in saturated (Lee et al., 2006, MacQuarrie and Sudicky, 2001, Chowdary et al., 2004, Kinzelbach et al., 1991) and unsaturated porous media (Tindall et al., 1995, Berlin et al., 2013, Gaonkar et al., 2016). Most of these models address the microbial degradation as lumped parameter ignoring the processes of vital importance such as nitrification and denitrification and also lack experimental validation. Though models such as HYDRUS by Simunek and van Genuchten (2008) and RT3D by Clement et al. (1998), are able to give the spatial and temporal distribution of water flow and NO3− transport, they are more complicated, data intensive and appropriate for field studies. For simplification and scientific understanding of the nitrogen dynamics, there is always a need for a simple one-dimensional model. Some researchers have developed numerical models to simulate the transport and transformation of NO3− through laboratory column experiments (Chun et al., 2009, Peyrard et al., 2011 and Tafteh and Sepaskhah, 2012). But they consider only a single process and also represent microbial degradation in simple kinetics. In addition, modeling the fate and transport of nitrogen species in flooded soil is quite complicated due to a series of nitrogen transformation processes (Yoshinaga et al., 2004, Evans et al., 2006) and coupled physical, chemical and biological processes (Nakasone et al., 2004, Liang et al., 2004).
In this study, the vertical transport and dynamics of nitrogen in an agricultural soil under flooded conditions are simulated. The model considered in this study accounts for all the important N-transformation processes namely, sorption, leaching, nitrification and denitrification with substrate inhibition effect which usually is not taken into consideration. Moreover, the model accounts for efficiency factor to consider the deviation in biokinetic constants between batch and column studies. In addition, the model requires a minimum number of parameters that will effectively quantify N-transformation processes which can assess the risk of groundwater contamination. The ultimate goal is to provide a simple but comprehensive model that can estimate N leaching loads in groundwater governed by the bio and hydrogeochemical pathways which can improve fertilizer management.
Section snippets
Materials and methods
This study was conducted in a saturated soil column of 35 cm depth packed with wastewater irrigated soil. The soil was obtained from wastewater irrigated fields near the wastewater treatment plant of Indian Institute of Technology, Madras Tamil Nadu, India (IITM) at 1 m depth by continuous coring which was dried, sieved and stored for subsequent studies. This method of soil sampling would provide an easy and representative way for nitrifier and denitrifier enrichment from the soil (Elbanna et
Nitrogen species transport modeling with sorption
Solute transport in soil and groundwater is affected by a large number of physical, chemical, microbial processes and media properties. The conceptual understanding of the process was given in Fig. 1.
Nitrogen loss from irrigated cropland, particularly from sandy soils, significantly contributes to the NO3− contamination of surface water and groundwater sources. The major processes involved in the vertical transport are sorption, nitrogen fixation, ammonification, nitrification, immobilization,
Results and discussion
Initially batch studies were carried out to understand the sorption, desorption and biodegradation mechanisms in soil. The kinetic sorption, desorption coefficients, and biokinetic parameters obtained from batch studies were used as input to the model parameters. Further continuous column studies were carried out for validation of the developed model.
Conclusions
The nitrogen dynamics in saturated soil based on advection-dispersion-reaction equations was studied. The primary focus of this reactive transport modeling is to account for the entire biogeochemical phenomenon happening simultaneously in saturated soil affecting the nitrogen and carbon compound transport during wastewater irrigation. The sorption, desorption and biokinetic parameters estimated in the batch studies were used as model inputs to simulate the transport and transformation processes
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