Dynamics of 14C-labeled glucose and ammonium in saline arable soils

Dinâmica de glicose marcada com 14C e amônio em solos salinos

SEÇÃO II - QUÍMICA DO SOLO

Alma Vuelvas-Solórzano^I; Rosalina Hernández-Matehuala^I; Eloy Conde-Barajas^II; Marco L. Luna-Guido^III; Luc Dendooven^III; Marcela Cárdenas-Manríquez^II

^ILaboratorio de Bioingeniería, Departamento de Ing. Bioquímica, Instituto Tecnológico de Celaya. Av. Tecnológico y García Cubas s/n, CP 38010, Celaya Gto., México. E-mail: [e-mail]

^IILaboratorio de Bioingeniería, Departamento de Ing. Ambiental, Instituto Tecnológico de Celaya. Av. Tecnológico y García Cubas s/n, CP. 38010, Celaya Gto., México. E-mail: [e-mail]

^IIILaboratorio de Ecología de Suelos, Departamento de Biotecnología y Bioingeniería, Cinvestav, Av. Instituto Politécnico Nacional 2508, CP 07360, México D.F. México. E-mail: [e-mail]

SUMMARY

Organic matter dynamics and nutrient availability in saline agricultural soils of the State of Guanajuato might provide information for remediation strategies. ¹⁴C labeled glucose with or without 200 mg kg^-1 of NH₄⁺-N soil was added to two clayey agricultural soils with different electrolytic conductivity (EC), i.e. 0.94 dS m^-1 (low EC; LEC) and 6.72 dS m^-1 (high EC; HEC), to investigate the effect of N availability and salt content on organic material decomposition. Inorganic N dynamics and production of CO₂ and ¹⁴CO₂ were monitored. Approximately 60 % of the glucose-¹⁴C added to LEC soil evolved as ¹⁴CO₂, but only 20 % in HEC soil after the incubation period of 21 days. After one day, < 200 mg ¹⁴C was extractable from LEC soil, but > 500 mg ¹⁴C from HEC soil. No N mineralization occurred in the LEC and HEC soils and glucose addition reduced the concentrations of inorganic N in unamended soil and soil amended with NH₄⁺-N. The NO₂^- and NO₃^- concentrations were on average higher in LEC than in HEC soil, with exception of NO₂^- in HEC amended with NH₄⁺-N. It was concluded that increases in soil EC reduced mineralization of the easily decomposable C substrate and resulted in N-depleted soil.

Index terms: dynamics of inorganic N, emission of ¹⁴CO₂ and ¹²CO₂, saline soils.

RESUMO

Os conhecimentos sobre a dinâmica da matéria orgânica e disponibilidade de nutrientes em solos salinos podem ser úteis para nortear a adoção de estratégias de recuperação e manejo. O objetivo deste trabalho foi avaliar o efeito da condutividade elétrica (EC) e presença de N inorgânico na decomposição da matéria orgânica a dois solos salinos localizados em Guanajuato (México). Nesse sentido, glicose marcada com ¹⁴C com ou sem amônio na dose de 200 mg kg^-1 de N foi adicionada em dois tipos de solo com valores de condutividade elétrica de 0,94 dS m^-1 (baixo EC; LEC) e 6,72 dS m^-1 (alta EC; HEC). Amostras de solos foram incubadas durante 21 dias e, ao longo desse tempo, foram avaliadas as concentrações de N inorgânico e a produção de ¹²CO₂ e ¹⁴CO₂. Após o período de incubação, aproximadamente 60 % da glicose marcada com ¹⁴C adicionado ao solo LEC evoluiu como ¹⁴CO₂, mas somente 20 % no solo HEC a evolução de ¹⁴CO₂ foi de apenas 20 %. Decorrido um dia de incubação, menos de 200 mg de ¹⁴C foram extraídos do solo LEC, ao passo que no solo HEC foram extraídos mais de 500 mg de ¹⁴C. Não ocorreu mineralização de N nos dois solos e a adição de glicose reduziu as concentrações de N inorgânico no solo controle e no solo que recebeu N-NH₄⁺. As concentrações de NO₂^- e NO₃^- foram, em média, maiores no solo LEC do que no solo HEC, com exceção do teor de NO₂^-, que foi maior no HEC tratado com NH₄⁺. O aumento da condutividade elétrica promoveu a redução da mineralização de substrato facilmente decomponível e a diminuição do teor de N no solo.

Termos de indexação: dinâmica de nitrogênio inorgânico, emissão de ¹²CO₂ e ¹⁴CO₂, solos salinos.

INTRODUCTION

Nowadays, there is an environmental and economic need to understand the role and destiny of N in different ecosystems. Nitrogen cycling is mostly controlled by biological activity and at the same time biological processes are affected by climate and physicochemical soil characteristics. In some extreme environments, such as saline soils, the high electrolytic conductivity (EC) inhibits microbial activity and organic matter decomposition and thus affects N cycling (Johnston & Guenzi, 1963; McCormick & Wolf, 1980; Bandyopadhyay & Bandyopadhyay, 1983; Zahran, 1997; Pathak & Rao, 1998). For instance, mineralization of maize and glucose were inhibited in alkaline saline soils with EC > 10 dS m^-1 and large amounts of NH₄⁺ and NO₃^- were immobilized within short periods of time, reducing N availability (Conde et al., 2005). Additionally, high concentrations of nitrite (NO₂^-) were accumulated in these soils when an easily decomposable substrate plus NO₃^- were added (Vega-Jarquin et al., 2003). Pathak & Rao (1998) reported that ammonification and nitrification were inhibited by high salt concentrations and that, particularly the latter, was very sensitive to the presence of salts.

High concentrations of soluble salts in soil have a negative effect on plant growth in different aspects (Aguirre, 1993; Zahran, 1997). In the first place, high concentrations of specific ions such as Na⁺ are toxic to plants and cause physiologic disorders. Secondly, the presence of salts decreases water root uptake (Bohn et al., 1979; Aguirre, 1993; Pankhurts et al., 2001; Atlas & Bartha, 2002; Bernstein & Kafkafi, 1996; Haynes & Rietz, 2003). And thirdly, certain ions have a negative effect on the solubility of other ions and on microbial biomass activity, e.g., protein synthesis and respiration (Giambiagi & Lodeiro, 1989; Aguirre, 1993; Atlas & Bartha, 2002; Sardhina et al., 2003).

Plant growth and crop yields have decreased in some parts of the State of Guanajuato, Mexico, due to the excessive amounts of salts in the soil, and some parts have become uncultivable. The salt in soil increases since crops are irrigated with saline effluents and by inadequate soil practices. The Agriculture Department of the State of Guanajuato has started a project to investigate how increased salinity affects nutrient availability in the soil and how the addition of organic material might restore soil fertility. As part of this project, the effect of increased salinity on dynamics of inorganic N (NH₄⁺, NO₂^-, NO₃^-) and organic material were investigated. Two agricultural soils with different electrolytic conductivity (EC), i.e. 0.94 dS m^-1 (low EC; LEC) and 6.72 dS m^-1 (high EC; HEC) were amended with or without ¹⁴C-labeled glucose and with or without (NH₄)₂SO₄. The ¹⁴C-labeled glucose is routinely used to determine effects of soil characteristics on mineralization of organic material (Saggar et al., 1999). The objective of this study was to investigate the effect of EC and inorganic N on the decomposition of organic matter in two soils of Guanajuato State, Mexico.

MATERIALS AND METHODS

Sampling site

The sampling sites are located in Cuerámaro (HEC soil) and San Francisco del Rincón (LEC soil), State of Guanajuato, Mexico, at 1,726 and 1,804 m asl, with a mean annual temperature of 20.3 and 19.4 °C; and precipitation of 981 and 967 mm, respectively (Terrones-Rincón et al., 2000). Soils in the area originating from aluvio-coluvial deposits are clayey-loamy to clayey-sandy. The soil structure is granular in the top soil and organic matter contents range from 14 to 52 g kg^-1 dry soil; EC in saturation extracts ranges from 0.9 to 6.7 dS m^-1. The cation exchange capacity (CEC) in soils ranges from 34.8 to 40.5 cmol_c kg^-1 dry soil while exchangeable Ca²⁺ from 66 to 90 mg kg^-1, Mg²⁺ from 17 to 26 mg kg^-1, Na⁺ from 82 to 163 mg kg^-1, K⁺ from 15.6 to 19.6 mg kg^-1, and extractable P from 23 to 32 mg kg^-1. CO₃^2- in the exchangeable saturation extract ranges from 15 to 24 mg kg^-1, Cl^- from 82 to 93 mg kg^-1 and SO₄^2- from 230 to 289 mg kg^-1 (Castellanos-Ramos et al., 2000). Approximately 17 % of the study area is irrigated. The natural vegetation consists of species that can be used for cattle raising, such as Bouteloua chasei, Muhlenbergia spp., Prosopis spp., Camedrioteucrium chamaedrys, Bothriochloa spp., Buchloe dactyloides, Aristida divaricata, Liendilla lanuda, Bouteloua curtipendula, Acacia spp., Opuntia spp., and Uncaria tomentosa.

Two agricultural soils with different EC were sampled (Table 1). In each one, three plots of approximately 400 m² were defined from which 30 soil samples were collected by augering the top 0-15 cm layer. The 30 soil samples from each plot were pooled separately and characterized for a total of six soil samples, i.e., soil from two fields and three plots.

Aerobic incubation

The soil samples were passed through a 5 mm sieve and placed in drums containing a beaker with 100 mL distilled H₂O to avoid desiccation. One contained 100 mL 1 mol L^-1 NaOH solution to trap any CO₂ evolved for six days.

Sixty soil sub-samples (30 g) from each field and plot were filled in 120 mL glass flasks. Fifteen sub-samples were amended with a solution containing ¹⁴C-labeled glucose (approximately 1.48 MBq kg^-1), 15 with ¹⁴C-labeled glucose plus (NH₄)₂SO₄, 15 with (NH₄)₂SO₄ and the others were treated with an equal amount of distilled H₂O. The amount of water added resulted in a soil moisture content of approximately 50 % WHC, and the amounts of C and N added as glucose and NH₄⁺ were approximately 1,000 mg kg^-1 C and 200 mg kg^-1 N. Three flasks were chosen at random from each treatment. Soil was extracted (30 g) for inorganic N and ¹⁴C with 120 mL 0.5 mol L^-1 K₂SO₄ solution to provide zero time samples. The samples were shaken for 30 min, filtered through Whatman No 42 paper® and stored until analysis at -20 °C (Jenkinson & Powlson, 1976).

The glass flasks were placed in 945 mL glass jars with 10 mL distilled H₂O. One contained a vessel with 20 mL 1 mol L^-1 NaOH solution to trap CO₂ evolved and another a vessel with 20 mL 2 % H₃BO₃ solution to trap volatilized NH₃. The jars were sealed airtight and stored in the dark for 21 days at 22 ± 1 °C. After 1, 3, 7 and 21 days, three jars selected at random from each treatment were opened, and the vessels containing NaOH and H₃BO₃ were removed. An aliquot of 0.2 mL 1 mol L^-1 NaOH solution was taken and analyzed for ¹⁴C activity. The remaining NaOH and H₃BO₃ solution was titrated with appropriate concentrations of H₂SO₄ to determine CO₂ and NH₃ trapped, respectively. The soil was removed from the three flasks and 30 g was extracted with 120 mL 0.5 mol L^-1 K₂SO₄ solution. The samples were shaken for 30 min and filtered through Whatman N° 42 paper® and the inorganic and organic N and ¹⁴C were measured as described for zero time samples (Conde et al., 2005).

Chemical analysis

The pH was measured in 1:2.5 soil/H₂O suspension using a glass electrode (Thomas, 1996). Total C was determined by oxidation with potassium dichromate (K₂Cr₂O₇) and titration of excess dichromate with (NH₄)₂FeSO₄ (Kalembasa & Jenkinson, 1973), and inorganic C by adding 5 mL 1 mol L^-1 HCl solution to 1 g air-dried soil and trapping CO₂ evolved in 20 mL 1 mol L^-1 NaOH. Total N was measured by the Kjeldhal method using concentrated H₂SO₄, K₂SO₄ and HgO to digest the sample (Bremner, 1996), soil particle size distribution by the hydrometer method as described by Gee & Bauder (1986) and CEC with the barium acetate method (Jackson et al., 1986).

The NH₄⁺, NO₂^- and NO₃^- in the 0.5 mol L^-1 K₂SO₄ extracts were measured colorimetrically with an automatic Skalar San plus System (the Netherlands). The ¹⁴C in the extracts was measured with a scintillation counter (Beckman LS6000SC, USA). The CO₂ in the 1 mol L^-1 NaOH was determined by titration with 0.1 mol L^-1 HCl (Jenkinson & Powlson, 1976). The WHC was measured in soil samples water-saturated in a funnel and left to stand overnight and defined by weight differences (Conde et al., 2005).

Statistical analysis

The cumulative production of ¹²CO₂ and ¹⁴CO₂ was regressed on elapsed time using a linear model forced to pass through the origin, but allowing different slopes (production rates) for each treatment. This approach was supported by the theoretical considerations that no ¹²CO₂ and ¹⁴CO₂ was produced at time zero and a control (flask without soil) accounted for atmospheric ¹²CO₂ and ¹⁴CO₂.

The data of soil characteristics, concentration of NH₄⁺, NO₂^- and NO₃^-, and extractable ¹⁴C were subjected to one-way analysis of variance using PROC GLM (SAS, 1989) to test for significant differences between treatments with Tukey's Studentized Range test. Significant differences between treatments for ¹²CO₂ and ¹⁴CO₂ production were determined using PROC MIXED (SAS, 1989).

RESULTS

Extractable ¹⁴C and production of ¹⁴CO₂

The CO₂ production rate was significantly higher in LEC soil amended with NH₄⁺ than in the unamended soil, but lower than in soil amended with ¹⁴C labeled glucose or ¹⁴C labeled glucose plus NH₄⁺ (p < 0.05) (Figure 1a, Table 2). The CO₂ production rate was significantly higher in HEC soil amended with ¹⁴C labeled glucose or NH₄⁺ than in the unamended soil, but lower than in soil amended with ¹⁴C labeled glucose plus NH₄⁺ (p < 0.05) (Figure 1b, Table 2). The CO₂ production rate was 1.3 times and significantly higher in LEC soil than in HEC soil (mean of all treatments) (p < 0.05).

The production of ¹⁴CO₂ was greater in LEC soil amended with glucose than in soil amended with glucose plus NH₄⁺ (Figure 1c, Table 2). The production of ¹⁴CO₂ was lower in HEC soil amended with glucose than in soil amended with glucose plus ammonium soil (Figure 1d, Table 2). The ¹⁴CO₂ production rate was 1.4 times and significantly higher in LEC soil than in HEC soil (mean of all treatments) (p < 0.05).

The amount of extractable ¹⁴C was similar in LEC soil amended with glucose and glucose plus NH₄⁺ over time (Figure 1e). A sharp drop was detected after the first day and a flattening out afterwards. Based on the concentration of ¹⁴C in the soil extracts, approximately 150 mg C of the added glucose could not be detected one hour after its application in LEC soil and 100 mg glucose-C in HEC soil. Less than 10 mg kg^{-1 14}C soil was measured after 21 days. The extractable ¹⁴C decreased more slowly in HEC soil than in LEC soil and the residual concentration of ¹⁴C was > 68 mg kg^{-1 14}C in soil amended with glucose plus NH₄⁺ after 21 days (Figure 1f). The extractable ¹⁴C decreased faster in HEC soil amended with glucose plus NH₄⁺ than in soil amended with glucose and the residual concentration was also higher in the latter, i.e., 245 mg kg^-1. ¹⁴C. The amount of extractable ¹⁴C was significantly higher in HEC than in LEC soil (mean of all treatments) (p < 0.05) (Table 2).

Inorganic N

The emissions of NH₃ from both soils < 4 mg kg^-1 N soil and treatment had no significant effect on the mean NH₃ concentrations volatilized from both the LEC soil and HEC soils (Table 2). The mean amount of NH₃ volatilized was similar for LEC soil and HEC soil (mean of all treatments) (p < 0.05).

The NH₄⁺ concentrations in the unamended LEC soil and HEC soil remained < 4 mg kg^-1 NH₄⁺-N and glucose addition decreased these values < 0.2 mg kg^-1 NH₄⁺-N (Figure 2a,b). The concentration of NH₄⁺ was lower in soil amended with glucose plus NH₄⁺ than in soil amended with NH₄⁺ alone (Table 2). The mean concentrations of NH₄⁺ were similar in both soils (mean of all treatments) (p < 0.05).

The pattern of NO₂^- concentrations in the unamended LEC soil and soil enriched with NH₄⁺ was similar, i.e., a small decrease over time, with concentrations < 2 mg kg^-1 N (Figure 2c). The NO₂^- concentration in the glucose-amended soil also decreased over time, though the values were higher than in the unamended soil and soil enriched with NH₄⁺. In the glucose-amended soil plus NH₄⁺, the NO₂^- concentrations increased sharply to > 7 mg kg^-1 NO₂^- N on day 1 and 3 and then decreased sharply to < 2 mg kg^-1 NO₂^- N. The NO₂^- concentrations in the unamended HEC soil and soil enriched with glucose showed a similar pattern, i.e. a small decrease over time, with concentrations < 2 mg kg^-1 N (Figure 2d). The NO₂^- concentrations in the glucose-amended plus NH₄⁺ increased to a maximum of 5 mg NO₂^- N after three days, and then decreased. In the NH₄⁺-amended soil, the concentrations of NO₂^- showed a similar pattern as in glucose plus NH₄⁺ amended soil, but the maximum was lower, reached after seven days. The mean NO₂^- concentration was similar in LEC and HEC soil (mean of all treatments) (p < 0.05) (Table 2).

The NO₃^- concentration did not change significantly over time in unamended LEC, but decreased in the glucose-amended soil (p < 0.05) (Figure 2e). In the NH₄⁺ plus glucose and NH₄⁺ amended soil, NO₃^- increased over time with the highest increase found in the NH₄⁺ amended soil. The NO₃^- concentration did not change significantly over time in the unamended HEC soil, the glucose and glucose plus NH₄⁺-amended soil, but increased in the NH₄⁺-amended soil (p < 0.05) (Figure 2f). The mean concentration of NO₃^- was lower in HEC soil than in LEC soil (mean of all treatments) (p < 0.05) (Table 2).

DISCUSSION

Concentration of extractable ¹⁴C

After one day, < 200 mg ¹⁴C was extractable from LEC, but > 500 mg ¹⁴C from HEC soil. After one day, 20 % of the added glucose was mineralized to ¹⁴CO₂, with an efficiency of 60 % for C (Payne, 1970), so only 53 % of the glucose was mineralized although 80 % could not be detected in the soil. Part of the ¹⁴C-labeled glucose was taken up by the microorganisms in LEC soil without being metabolized. A similar lag was observed in HEC soil. Approximately 10 % of the added glucose was mineralized to ¹⁴CO₂, at an efficiency of 60 % for C, so only 27 % of the glucose was mineralized although 50 % could not be accounted for in HEC soil. The lag between glucose uptake and utilization for biosynthesis has often been reported (Payne & Wiebe, 1978; Bremer & van Kessel, 1990; Bremer & Kuikman, 1994). It indicates, as stated by Coody et al. (1986), that CO₂ evolution is a poor indicator of glucose uptake rates by soil microbes.

The NH₄⁺ addition reduced the amount of extractable ¹⁴C and thus stimulated decomposition of ¹⁴C-labeled glucose. Saline soils are often N depleted and the addition of NO₃^- or NH₄⁺ to glucose-amended soil increases CO₂ production and a priming effect (as explained below) may also be observed (Conde et al., 2005).

¹²CO₂ and ¹⁴CO₂ production

Approximately 9.4 % of the soil organic matter C was mineralized within 21 days in LEC and 7.7 % in HEC soil. The percentage of organic C mineralized was lowest in the HEC soil. This suggests that salts decreased microbial activity. The decrease in CO₂ production, however, was lower than reported by Pathak & Rao (1998), who found that when EC increased from 1.1 dS m^-1 to 96.7 dS m^-1, the CO₂ respiration decreased from 2.1 to 0.9 g kg^-1 CO₂-C in Sesbania-amended soil after 90 days. However, they added salts to soil (NaCl and CaCl₂) to increase EC whereas the high EC in the HEC soil was the result of a slow salt increase due to irrigation. The microorganisms in the soils used by Pathak & Rao (1998) were not adapted to high salt contents and this might have strongly inhibited their activity. Salt addition to non-saline soils would require the adaptation of microorganisms to osmostic and/or specific ion stress and would therefore inhibit their activity more strongly.

Approximately 60 % of the glucose-¹⁴C added to LEC soil was evolved as ¹⁴CO₂ and 50 % in LEC soil amended with glucose plus NH₄⁺. The percentage of mineralized glucose-C in the LEC soil was similar to values reported in literature. Bremer & Kuikman (1994) found that approximately 60 % glucose was mineralized in a sandy loam soil and silt loam soil after 35 days when > 576 mg kg^-1 of glucose C soil had been added.

The amount of glucose-¹⁴C mineralized was only 37 % in HEC soil. The higher EC in HEC than in LEC soil presumably explained the lower amount of glucose ¹⁴C respired to ¹⁴CO₂, although the effect of other soil characteristics on glucose decomposition can not be ruled out. Clay content has often been found to affect the decomposition of organic material fertilization (Sørensen, 1981; van Veen et al., 1985; Amato & Ladd, 1992) as does the pH (Saggar et al., 1999), but pH and clay were similar in both soils. The specific surface area of the clay, and the nature rather than the amount of the clay mineral has also been found to affect the decomposition of organic material (Saggar et al., 1996; Torn et al., 1997), but this effect is presumably more important in the long term than in short-term laboratory incubation experiments. Considering the above, it appears that EC was the factor that explained most of the reduction in mineralization of glucose added.

Compared to the unamended soil, the application of glucose increased CO₂ production to 784 mg kg^-1 C in LEC soil and to 525 mg kg^-1 C in HEC soil after 21 days, but the amount of ¹⁴CO₂ produced was only 660 and 374 mg kg^-1 C in LEC soil and HEC soil, respectively. The accelerated decomposition of unlabeled soil organic matter following the addition of organic material has often been referred to as a "priming effect", and has been a matter of controversy for many years (Brookes et al., 1990). However, the conditions for an apparent priming effect, such as non-uniformly labeled substrate or a great substrate addition, are absent in this experiment and the bicarbonate in the soil solution could not explain the differences either, as they were similar in both soils, i.e., 0.26 cmol_c kg^-1 HCO₃^-. Brookes et al. (1990) described situations where a true priming effect is mainly caused by an increased turnover of microbial cell C (after glucose addition) or by an increased decomposition of native soil organic matter (after addition of rye-grass). In the first situation, the new microbial biomass partly replaces the native, resulting in a greater production of unlabeled CO₂. In the second, the new microbial biomass is added to the already present native biomass and the increase in CO₂ production results from an increased contact between the native soil organic matter and enzymes produced by a more active microbial population.

Compared to the soil amended with NH₄⁺, the application of glucose plus NH₄⁺ increased the CO₂ production to 364 mg kg^-1 C in LEC soil and to 369 mg kg^-1 C in HEC soil after 21 days, but the amount of ¹⁴CO₂ produced was 515 and 520 mg kg^-1 C for LEC soil and HEC soil, respectively. As such, the addition of NH₄⁺ to the glucose-amended soil induced a negative priming effect. The addition of different substances to the soils might cause not only an acceleration of mineralization or positive priming effect, but also a reduction or a negative priming effect. A negative priming effect has often been reported for N, e.g. N immobilization and a temporal N unavailability, but less often for C. It is difficult to indicate which mechanisms might contribute to a negative priming effect, but Kuzyakov et al. (2000) stated that when a negative priming effect occurs the microbial biomass switches from metabolizing soil organic matter to easily available C sources, stimulated by NH₄⁺. Another possibility is that the addition of an easily decomposable substrate plus NH₄⁺ induced the growth of an active glucose-decomposing population that inhibits the activity of a passive population decomposing soil organic matter. It might also be that the addition of NH₄⁺ itself inhibited microbial activity by the formation of NH₃. NH₃ is toxic and known to inhibit microbial activity.

Inorganic N

The factors that normally most affect NH₃ volatilization are concentrations of NH₄⁺, pH and water content (Kirchmann & Witter, 1989). Both soils had similar water contents, pH and mean NH₄⁺ concentrations so the mean amounts of NH₃ volatilized were similar (Table 2).

It has often been reported that nitrification and especially oxidation of NO₂^- to NO₃^- is inhibited by high salt concentrations (Darrah et al., 1987; Low et al., 1997; Pathak & Rao, 1998). However, in the experiment reported here the mean NH₄⁺ and NO₂^- concentrations were similar so it appeared that the nitrification process was not inhibited.

The concentration of inorganic N (sum of NH₄⁺, NO₂^- and NO₃^-) decreased over time in HEC soil amended with NH₄⁺, but not in LEC soil (Figure 3). The amount of N lost through NH₃ volatilization was similar in both soils so HEC was N-depleted, but not the LEC soil. Additionally, the concentration of inorganic N in HEC amended with glucose plus NH₄⁺ was lower than in LEC soil, i.e. N immobilization was higher in HEC than in LEC soil.

CONCLUSIONS

1. Organic matter decomposition and N mineralization were most affected in the HEC soil.

2. In HEC soil, the addition of NH₄⁺ led to N immobilization.

3. NH₄⁺ and NO₂^- oxidation was not inhibited in the HEC as it was in the LEC soil.

ACKNOWLEDGEMENTS

We thank J.M. Ceballos, M. Mercado and I. Vargas for technical assistance and L. M. Salgado for assistance with the scintillation counter. The research was funded by the Department of Biotechnology and Bioengineering, Cinvestav, Mexico City, Mexico and Fondo Sectorial de Investigación Ambiental SEMARNAT-CONACyT research Grant FOSEMARNAT-C01-424. A. V-S and R. H-M received grant-aided support from CONACyT and CONCYTEG.

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Recebido para publicação em dezembro de 2007 e aprovado em abril de 2009.

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Dynamics of ¹⁴C-labeled glucose and ammonium in saline arable soils

Dinâmica de glicose marcada com ¹⁴C e amônio em solos salinos

Publication Dates

• Publication in this collection
  30 Oct 2009
• Date of issue
  Aug 2009

History

• Received
  Dec 2007
• Accepted
  Apr 2009