Concentraciones de nitrógeno en la precipitación y escorrentía de parcelas con manejo orgánico y convencional

David Cristóbal-Acevedo*; Elizabeth Hernández-Acosta; María E. Álvarez-Sánchez; Ranferi Maldonado-Torres

Departamento de Suelos, Universidad Autónoma Chapingo. km 38.5 Carretera México-Texcoco. C. P. 56230. Chapingo, Texcoco, Edo. de México. Correo-e: cristobalacevdo@yahoo.com.mx Tel.: 01 595 95 21540 (*Autor para correspondencia).

Received: May 06, 2014. ]]> Accepted: December 11, 2014.

ABSTRACT

The aim of this study was to determine the concentration and amount of nitrates (NO₃^-), ammonium (NH₄⁺) and mineral nitrogen (Nmin = NO₃^- + NH₄⁺) in precipitation and runoff in six plots in Texcoco, State of Mexico, during the rainy season of 2001 and 2010. For this purpose, six runoff plots with corn cultivation were prepared, three with conventional management (CM) and three with organic management (OM). From 2001 to 2010, average concentrations of NO₃^-, NH₄⁺ and Nmin in precipitation increased to 1.307, 0.833 and 2.140 mg·liter^-1, respectively. Minimum values of NO₃^-, NH₄⁺ and Nmin also increased to 0.550, 0.970 and 1.640 mg·liter^-1, respectively. In runoff, average concentrations of NO₃^- in 2001 and 2010 were higher with CM, while concentrations of NH₄⁺ were smaller. Because Texcoco's population increased by 31,049 from 2001 to 2010, we concluded that population growth and development of the area had an effect on the amounts of nitrogen in precipitation, and that OM was an alternative to reduce nitrogen outputs by runoff.

Keywords: Nitrates, ammonium, mineral nitrogen, crop residues.

RESUMEN

El objetivo de este trabajo fue determinar la concentración y la cantidad de nitratos (NO₃^-), amonio (NH₄⁺) y nitrógeno mineral (Nmin = NO₃^- + NH₄⁺) en el agua de precipitación y escorrentía de seis parcelas en Texcoco, Estado de México, durante la temporada de lluvias de los años 2001 y 2010. Para ello, se prepararon seis parcelas de escurrimiento con cultivo de maíz; tres con manejo convencional (MC) y tres con manejo orgánico (MO). Del año 2001 a 2010, las concentraciones promedio de NO₃^-, NH₄⁺ y Nmin en la precipitación incrementaron a 1.307, 0.833 y 2.140 mg·litro^-1, respectivamente. Los valores mínimos de NO₃^-, NH₄⁺ y Nmin también aumentaron a 0.550, 0.970 y 1.640 mg·litro^-1, respectivamente. En la escorrentía, las concentraciones medias de NO₃^- en los años 2001 y 2010 fueron mayores con el MC, mientras que las concentraciones de NH₄⁺ fueron más pequeñas. Debido a que en Texcoco, el incremento poblacional del año 2000 al 2010 fue de 31,049 habitantes, se concluyó que el crecimiento poblacional y el desarrollo de la zona tuvieron efecto sobre las cantidades de nitrógeno en la precipitación, y que el MO fue una alternativa para reducir las salidas de nitrógeno por escorrentía.

Palabras clave: Nitratos, amonio, nitrógeno mineral, residuos de cosecha.

Today, nitrogen and its derivative compounds are studied due to their importance as an essential element in crop production and as a causative agent of environmental impacts, both in the soil and in the atmosphere (Ehhalt et al., 2001; Elliott et al., 2009). In the atmosphere, nitrogen is found in the form of nitrogen monoxide (NO), nitrogen dioxide (NO₂), hydrazoic acid (HN₃), nitrous oxide (N₂O) and ammonia (NH₃), as a result of pollutant emissions due to various anthropocentric activities (Delon et al., 2007). Galloway et al. (2008) and Sutton et al. (2007) argue that high amounts of atmospheric nitrogen are a result of increasing population and that increasing concentrations of the element denotes more polluting impact. Increased nitrogen in the atmosphere can be determined indirectly by measuring nitrate (NO₃^-), ammonium (NH₄⁺) and mineral nitrogen (Nmin) in rainwater. Concentrations in precipitation are variable and depend on the degree of environmental alteration due to human activities. Havlin (1999) indicates that the amount of nitrogen in precipitation is variable and can take values from 1.12 to 56 kg·ha^-1·year^-1. Depending on the location, the changes may occur due to industrial activities and the increase in human and animal populations.

Nitrogen can reach ecosystems through rainwater, but there is also an outlet at the time the event occurs through runoff. Pierson et al. (2001) reported annual losses of 1.5 to 13.5 kg·ha^-1 of N in runoff in southern Georgia, USA. These authors indicate that nitrogen losses depend on the volume of runoff and concentrations of the element present. On the other hand, one of the practices that helps maintain balance in agricultural ecosystems is organic farming, which uses only crop residues to provide nitrogen to the soil compared to conventional agriculture which adds synthetic chemical fertilizers. In this regard, Berry et al. (2006) argue that organic systems have the potential to meet supply and demand with sufficient amounts of nitrogen available from crop residues; however, it should be borne in mind that organically-produced waste tends to have low nitrogen levels and very low mineralization rates, so its availability in the form of nitrates and ammonia is very slow. In the case of conventional agriculture, nitrogen availability is higher because large amounts of the element are provided in the form of mineral fertilizers, so the output from crop fields is greater (Duilio-Torres, Florentino, & López, 2005). However, global use of synthetic nitrogen fertilizers is causing regional and global problems by overloading this element and its compounds. In the last decade there has been great concern about the loss of nutrients from agricultural systems and the consequences for the health and sustainability of aquatic ecosystems (Carpenter et al., 1998; Sharpley et al., 1994).

In the present study two nitrogen dynamic processes in ecosystems were quantified: contributions (inputs) by precipitation and losses (outputs) by runoff in two periods with different urban environments. The Universidad Autónoma Chapingo Agricultural Experiment Station and its environment in 2001 did not have the demographic pressure (pressure exerted on natural resources due to overpopulation) that it had in 2010. The increase in this pressure resulted from widespread growth in the municipality of Texcoco, which had 204,102 inhabitants in 2000 and 235,151 in 2010 (Instituto Nacional de Estadística y Geografía [INEGI], 2000, 2010), as well as the growth of Mexico City's suburbs. These factors have an impact on nitrogen emissions due to the increase in factories, greater automobile use and human activities. Therefore, the objectives of this study were to determine the behavior of the concentrations and amounts of NO₃^-, NH₄⁺ and Nmin in rainwater and runoff from corn plots with organic and conventional management in two periods with an interval of nine years. This was done in order to: a) establish the behavior of NO₃^-, NH₄⁺ and Nmin concentrations as a function of time and quantify their magnitudes in rainwater and runoff; b) determine whether urban development increases nitrogen concentrations and amounts in rainwater and c) establish whether organic management with input from nitrogen-containing crop residues causes a decrease in the concentrations and quantities of the element in runoff, compared to conventional management using synthetic chemical fertilizers.

MATERIALS AND METHODS

The study was carried out at the Universidad Autónoma Chapingo Agricultural Experiment Station (CAEUACH), State of Mexico, in the Efraím Hernández Xolocotzi organic farm (19° 29' 00'' NL - 98° 53' 00'' WL; 2,250 masl). According to García (1988), the climate is classified as C(w₀)(w)b(i')g which corresponds to temperate subhumid, with mean annual rainfall of 645 mm, mean annual temperature of 15 °C and early frosts in late September and late frosts in April. The soil is sandy loam inceptisol with bulk density of 1.4 g-cm^-3, depth of 1.2 m and slope of 2 %.

From 2001 until 2010, CAEUACH lots B6 and B16 were cultivated with corn. The first lot was managed conventionally (CM) with input of mineral fertilizers (200 kg·ha^-1 of N); the second lot had organic management (OM) using corn crop residues (leaves and stems) from the cycle previous to the year assessed. The periods analyzed were from June 22 to September 27 (97 days), 2001, and from June 23 to September 29 (98 days), 2010. In each lot, three 16 m² runoff plots were established with tanks to collect, quantify and take runoff samples, which were taken every 24 h on rainy days. Precipitation was measured with a rain gauge (authors' own design). Because the lots with OM and CM are 6 m apart, no replications were taken in precipitation measurement; in the case of runoff, three replications were performed. Thus, for each precipitation and runoff event in 24 h, one water sample for measuring precipitation and three for runoff were taken for each plot with CM and OM. Water samples were taken in 100-mL polypropylene bottles and placed in a refrigerator at 4 °C, for subsequent determination of NO₃^- and NH₄⁺ by steam distillation (Bremner, 1965). The amounts of NO₃^- and NH₄+ (g·ha^-1) for each event were obtained with the data from the NO₃^- and NH₄⁺ (mg·liter^-1) concentrations in precipitation and runoff, and also using the volumes of water that fell as rain and flowed as runoff in 1 ha. The behavior of the NO₃^- and NH₄⁺ concentrations in precipitation was analyzed as a function of time with the aid of graphs. The variables analyzed were: nitrates in precipitation (NO₃^--Pr), ammonium in precipitation (NH₄⁺-Pr), mineral nitrogen in precipitation (Nmin-Pr), nitrates in runoff from organic plots (NO₃^--Run-O), nitrates in runoff from conventional plots (NO₃^--Run-C), ammonium in runoff from organic plots (NH₄⁺-Run-O), ammonium in runoff from conventional plots (NH₄⁺-Run-C), mineral nitrogen in runoff from organic plots (Nmin-Run-O) and mineral nitrogen in runoff from conventional plots (Nmin-Run-C). The parameters of central tendency and dispersion were obtained for each variable.

RESULTS AND DISCUSSION

In 2001 and 2010, 47 and 61 rainfall events occurred, respectively. Figures 1 and 2 show that NO₃^- and NH₄⁺ concentrations in rainwater were higher in 2010 than in 2001. The beginning of the rainy season had the highest concentrations, tending to decrease in 2001 and remain constant or increase slightly in 2010. Table 1 shows that average nitrogen concentrations in precipitation in the form of NO₃^-, NH₄⁺ and Nmin increased by 106.7, 74.6 and 68.0 %, respectively, in 2010 compared to 2001. Accordingly, it can be inferred that there was a higher nitrogen concentration in the atmosphere in 2010; the increases were less than 120 %, an amount reported by Delon et al. (2007). According to Magnani et al. (1998), the higher concentrations may be due to increased nitrogen in the atmosphere resulting from population and vehicle growth. Figures 1 and 2 also show that in the two years evaluated, variability is greater at the beginning and then tends to decrease, so that an effect of atmospheric washing out and stabilization of concentrations can be inferred, since an increase in rainfall events decreases the ion concentration in the atmosphere. The above effect coincides with the findings of Pérez-Suárez, Cetina-Alcalá, Aldrete, Fenn, and Landois-Palencia (2006). According to Austin et al. (2004), this behavior is also because during the dry season there is an accumulation of nitrogen in the atmosphere and in the soil, with pulses or emissions of NO and NO₂ occurring with the onset of the rainy season. The number of precipitation events was higher in 2010, so there should have been a greater dilution effect on NO₃^- and NH₄⁺ concentrations. However, this effect did not appear; on the contrary, concentrations increased slightly over time, indicating increased emission of NO and NO₂.

In 2001 and 2010, the highest concentrations corresponded to NH₄⁺ (Figures 1 and 2), coinciding with Pérez-Suárez et al. (2006), who found the same behavior in two sites in the State of Mexico. Table 1 shows that the mean concentrations of NH₄⁺ in precipitation are higher than those of NO₃^-, which is linked to greater emission of nitrogen by activities related to urbanization (Galloway, Likens, & Hawley, 1984). According to Oyarzún, Godoy and Leyva (2002), NH₄⁺ concentrations are a reflection of farming activities in the area. On the other hand, an increase in NO₃^- concentrations in rainwater is a manifestation of increased pollution due to activities that emit NO and NO₂ (Galloway et al., 1984).

The maximum concentration values of NO₃^- and Nmin are similar in 2001 and 2010, while the NH₄⁺ concentration in 2010 increased by 33.1 % (Table 1). On the other hand, in 2010, the minimum concentration values of NO₃^-, NH₄⁺ and Nmin rose by 366.6, 646.6 and 273.3 %, respectively, compared to 2001.

Effect of organic and conventional plot management on NO₃^-, NH₄⁺ and Nmin concentrations in runoff

In 2001 and 2010, 21 and 17 runoff events, respectively, were analyzed. By analyzing the effect of plot management on runoff water, Table 1 shows that the mean concentrations of NO₃^- in 2001 and 2010 were higher in plots with CM compared to those with OM. This can be explained by the dominance of nitric forms due to the addition of fertilizers and the greater availability of NO₃^- (Cookson et al., 2006). For both the organic and conventional treatments, mean concentrations of NO₃^- and Nmin in runoff water decreased from 2001 to 2010; by contrast, the NH₄⁺ concentration slightly increased, with the increase being greater in the conventional treatment. The foregoing does not coincide with what is asserted by some authors (Cermak, Gilley, Eghball, & Wienhold, 2004; Flores-López et al., 2009; Porta, López-Acevedo, & Roquero, 1999), who hold that NH₄⁺ concentrations are related to the decomposition of crop residues, so that in the present study the NH₄⁺ concentration in the plots with organic management should have increased from 2001 to 2010.

The mean NO₃^- concentration values found in runoff water with the two management methods (CM and OM) in 2001 and 2010 are lower than those found by Camas-Gómez et al. (2012), which were 9.2, 26.3 and 12.3 mg·liter^-1 as an average of six events with three agroforestry system treatments (corn in conservation tillage, corn in living wall barriers and corn interspersed with fruit trees). In general, in 2001, mean concentrations of NO₃^-, NH₄⁺ and Nmin⁺ were higher in runoff for both OM and CM, while in 2010, concentrations were higher in precipitation.

Table 1 shows that the coefficients of variation (CV) of the NO₃^-, NH₄⁺ and Nmin concentrations in runoff were higher in 2001 compared to 2010. On the other hand, in 2001 and 2010, the CV of NO₃^-, NH₄⁺ and Nmin in precipitation were higher than those in runoff. This indicates that there was no regularity in the two years analyzed to be able to say that the NO₃^-, NH₄⁺ and Nmin concentrations have more or less variability in precipitation or runoff.

Table 2 shows the amounts of NO₃^-, NH₄⁺ and Nmin in precipitation were higher in 2010 compared to 2001 with increases of 24.2, 17.07 and 20.27 %, respectively, although the maximum and minimum values were higher for 2001. It can also be seen in this table that the data were more variable in 2001 compared to 2010. This variability is explained by the maximum and minimum values that provided a greater range in 2001. On the other hand, it can be seen that for both 2001 and 2010 the discharges of NO₃^-, NH₄⁺ and Nmin were higher for CM, which coincides with the behavior of the concentrations in Table 1. In the two years evaluated, the average amounts of NO₃^- and NH₄⁺ were higher in precipitation than in runoff.

Table 3 presents a balance of inputs and outputs of NO₃^-, NH₄⁺ and Nmin. This table shows that the nitrogen in the precipitation was mainly in the form of NH₄⁺. The difference between NH₄⁺ and NO₃^- was about 1 kg·ha^-1, for both 2001 and 2010. In the latter year, the amount of Nmin that entered by precipitation was higher (2 kg·ha^-1) than in 2001. The amounts of NO₃^- and NH₄⁺ contributed by precipitation were higher than those found by Pérez-Suárez et al. (2006) in a study conducted in Desierto de los Leones National Park, Mexico City. The authors reported values of 3.43 kg·ha^-1 NO₃^- and 4.70 kg·ha^-1 NH₄⁺ over a period of three months. The differences are probably due to the fact that Desierto de los Leones is a forest area.

CONCLUSIONS

The methodology used in this study allowed us to analyze the differences in concentration and amounts of nitrogen that entered with rainwater and came out with runoff in plots with organic and conventional treatments, in two periods with an interval of nine years. The results have environmental and productive implications because the amounts of nitrogen in the form of NO₃^-, NH₄⁺ and Nmin that entered the agroecosystem with precipitation increased from 2001 to 2010. This is likely due to the effect of population growth and development of the area on nitrogen concentrations in the atmosphere because of increased emissions. On the other hand, the organic production system was an alternative to reduce nitrogen outputs in the form of NO₃^-, NH₄⁺ and Nmin in runoff, compared to the conventional system. The amounts of nitrogen in the form of NO₃^- and NH₄⁺ that entered through precipitation were considerably higher than those that came out through runoff. Nitrogen that entered the conventional and organic systems by precipitation was mainly in the form of NH₄⁺, and nitrogen that came out by runoff was mostly in the form of NO₃^-.

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