Nitrate fate in a Mexican Andosol: Is it affected by preferential flow?

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Abstract

Andosols are the dominant soils in the Valle de Bravo basin, the origin of a significant amount of Mexico City's drinking water. The main land use is agriculture and most of the existing surface water bodies are eutrophic. Nitrogen fertilizer is used extensively. There have been very few studies on nitrate (NO3) fate in this type of soil and region. Comprehensive laboratory studies were conducted to determine the fate of NO3 in an Andosol profile from Valle de Bravo, in order to assess the risk of water resources contamination. Nitrate retention was analysed statically (using batch experiments) and dynamically (using intact and packed soil columns) at different soil depths and its competition with Cl was evaluated. Complementary laboratory experiments were conducted to study water transport through the columns and nitrogen transformations in the soil. In batch and columns, NO3 adsorption was linear in the range of concentrations studied and higher in the deepest soil layer. Preferential flow pathways were found in the unaltered deeper soil layers, while tillage activity in the top layer destroyed the pore continuity. In spite of the deeper soil layer's greater capacity for NO3 retention, the presence of preferential flow pathways coupled with high rainfall intensities, makes the NO3 mobile below the root zone at 1 m depth and increases the risk of groundwater contamination. The results illustrate the complexity of nitrate fate in Andosols and can be used to improve agricultural practices in the central Mexico region.

Highlights

► Nitrate fate through Andosol is affected by physical, chemical and biological processes. ► Agriculture modifies Andosol properties decreasing its capacity to retain NO3. ► Preferential flow increases the nitrate mobility towards groundwater resources. ► Soil surface ploughing destroys natural flow pathways reducing the rapid transport.

Introduction

Extensive research on preferential flow has been conducted on temperate soils. A complete review of the principles and controls on preferential flow and transport in soil has been published by Jarvis (2007). Preferential flow and transport phenomena can occur at all scales from the pore scale of 10−3 m to large regions of 106 m (Clothier et al., 2008). There is a great emphasis on how these flows heterogeneously transport a range of substances, including nitrate (Seo and Lee, 2005), organic compounds (Wehrer and Totsche, 2008), biota, pesticides (Kahl et al., 2008), and colloids (Burkhardt et al., 2008). However studies on preferential flow in soils from volcanic origin are scarce and do not find consistently preferential flow in this soil type. Preferential flow has been observed in Andosols using fluorescent tracers (McLeod et al., 1998, Duwig et al., 2008) and by infiltrometry on undisturbed cores (Clothier et al., 2000). Maruyama et al. (2003), Eguchi and Hasegawa (2008) and Prado et al. (2009) found that the flow bypassed a part of the soil matrix, and that preferential flow occurred in tubular macropores. On the contrary, neither Magesan et al. (2003) nor Sansoulet et al. (2007) found preferential flow in their study on intact cores. Preferential flow in soils is associated with the earlier appearance of agrochemicals in groundwater than in soils with homogeneous wetting. It poses a particular problem in agricultural areas used as catchments for water supply. This is the case with the Cutzamala system which provides 21% (19 m3 s−1) of Mexico City's water supply.

The Cutzamala system is located on the west side of Mexico City in the Trans Mexican Volcanic Belt and the study site is inside Valle de Bravo watershed, the most important sub catchment of the Cutzamala river basin. Field observations (Secretaría de Ecología, 1999) showed that crop productivity in Valle de Bravo watershed is low, encouraging farmers to apply agrochemicals. Farmers often are not trained and usually do not conduct any soil fertility measurements. Olvera-Viascán et al. (1998) observed that most of the surface water bodies in the Valle de Bravo watershed were eutrophic with the highest phosphorus and nitrogen loadings entering the Valle de Bravo reservoir through the Amanalco river system. Merino-Ibarra et al. (2008) studied the physical and chemical limnology of Valle de Bravo reservoir and found high-chlorophyll-a values and cyanobacterial blooms taking place during the stratification period (February to October). They concluded that water quality in Valle de Bravo reservoir was mainly affected by eutrophication. Studies on agrochemical fate in the soil of the catchment are thus an important first step in understanding the origin of the contamination sources and reducing the water eutrophication.

Andosols show high water retention, good permeability, and generally high organic matter content. They are highly productive agricultural soils (Shoji et al., 1993, Dahlgren et al., 2004). The existence of amorphous short range compounds combined with high organic matter content confers a variable charge to Andosols (Shoji et al., 1993). The presence of these short range compounds and the high saturated hydraulic conductivity (around or above 10−5 ms−1, Armas-Espinel et al., 2003) make the study of ion dynamics in Andosols complex. Nitrate (NO3) movement through soils depends on water movement and sorption processes. Sorption of NO3 in Andosols has been well documented (Kinjo and Pratt, 1971a, Sansoulet et al., 2007). Transport studies have been conducted on repacked columns under transient flow in horizontal columns (Katou et al., 1996) and vertical lysimeters (Feder and Findeling, 2007), under saturated conditions in the laboratory (Qafoku et al., 2004) and field studies under unsaturated flow conditions (Payet et al., 2009). Using classical “static” laboratory methods, various authors have linked NO3 sorption to the positive charges of allophanes. Nitrate sorption capacity varies depending on the amount and type of allophanes (Al or Si rich allophanes) (Ryan et al., 2001), the soil solution pH (Schalscha et al., 1973), the solution ionic strength (Sposito, 1984), and competition with other anions such as Cl (Kinjo and Pratt, 1971a, Katou et al., 1996, Feder and Findeling, 2007). However, little work has been done on solute movement in Andosols using intact soil columns under unsaturated flow conditions (Magesan et al., 2003, Eguchi and Hasegawa, 2008, Prado et al., 2009). This is fundamental to understand what happens to NO3 in Andosols under field conditions.

In the present study, nitrate transport was studied in an Andosol profile to a depth of 0.9 m from the Valle de Bravo basin. The goal was threefold: (i) to assess the importance of each process affecting nitrate fate i.e. preferential flow, sorption and transformation, (ii) to compare the results obtained by different laboratory methods, under static or dynamic conditions, and (iii) to evaluate the competition between NO3 and Cl. For this purpose, a range of independent laboratory experiments were combined, making this study a more complete work than previous studies where NO3 and Cl leaching were examined on Andosols. Soil NO3 retention was studied both dynamically (displacement in intact and repacked soil columns) and statically (batch experiments) at three different depths. Competition between NO3 and chloride ions was studied by analysing Cl behaviour in the same experiments. Soil mineralogy, chemistry, and exchange capacities affecting NO3 transfer were evaluated. Bacterial NO3 transformations such as nitrification and denitrification were analysed using incubation experiments.

Section snippets

The study site

The Cutzamala system is located on the west side of Mexico City in the Trans Mexican Volcanic Belt (Prado et al., 2007). Half of the system supply of water comes from Valle de Bravo reservoir (394 × 106 m3, Tortajada and Castelán, 2003). Valle de Bravo basin, part of the Cutzamala system, is located in the Trans-Mexican Volcanic Belt and is characterised by the predominance of volcanic rocks from the Cenozoic Era, with basalt in the lower part of the basin, and andesitic rocks in the upper part.

Physical equilibrium: packed soil columns

Fig. 2a shows the breakthrough curves for NO3 and H218O obtained for the packed soil columns corresponding to 5–30 cm depth (P1). The H218O BTC had a Gaussian shape, with a centre of gravity at one pore volume after the gravity centre of tracer application. The symmetrical Gaussian shape signifies physical equilibrium in the system as H218O is an inert water tracer. The NO3 BTC was also symmetrical, showing the presence of chemical equilibrium; it was retarded in relation to the H218O curve

Physical equilibrium vs. non equilibrium

Solute transport occurred in physical non equilibrium in all but one of the intact soil columns which demonstrates that the natural structure of the studied Andosol contains preferential flow pathways where water flux is more rapid than predicted by the classical convection dispersion equation (Parker and van Genuchten, 1984). Prado et al. (2009) confirmed this fact by determining the morpho-geometrical parameters of the soil porous network by image analysis of thin sections obtained from the

Conclusions

Reactive solute transport through soil is the result of several interacting physical, chemical and biological processes.

Nitrate fate in the studied Andosol profile was affected by preferential flow, this process increased the NO3 mobility reducing the resident solute time and so increasing the risk for groundwater contamination once the NO3 reaches the soil depths below the root zone. Preferential flow was found in all but one intact column, with around 27% of the pore space being by-passed

Acknowledgements

The authors thank Francisco Otero, Rafael Puente and Edith Cienfuegos of the “Laboratorio de Geoquímica Isotópica (LUGIS)”, and Lucy Mora of the “Laboratorio de Edafología” of the “Instituto de Geología”, UNAM. The DGAPA-PAPIIT Project number IN116310 for financial support.

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