Keywords
pot experiment, total N, nitrate, nitrite, ammonium, nutrient supply capacity
This article is included in the Agriculture, Food and Nutrition gateway.
pot experiment, total N, nitrate, nitrite, ammonium, nutrient supply capacity
Nitrogen (N) is an essential element for plants, takes various forms in soil1, and is one of the most limiting nutrients for various crops2–4. Nitrogen is present in soils as inorganic nitrogen (nitrate, ammonium, and dinitrogen) and organic nitrogen (urea and amino acids)5.
Conversions between organic and mineral N forms are primarily affected by soil microorganisms, which enable conversion to forms that plants can uptake6. In addition, some pollutant N forms are present in soil, so nitrogen conversions may indirectly affect human health and load the environment1. For this reason, quantifying studies aim to better understand nitrogen form ratios in soil and contribute to reaching sustainable agriculture practice. Recently, Jakab published a similar study7 quantifying another vital element for plants, phosphorus, in soil from the same region.
Humus is a main fertility component of the soil, 65–75% of its structure being made up of organic matter8,9. We decided to include humus content in this N-related study as it represents a known indicator of soil quality10.
Calcium chloride (CaCl2)-soluble nitrogen forms are the most available N ions for assimilation by plants11,12. Therefore, we quantified all N forms in CaCl2 solution. Additionally, we determined nitrate-N via a potassium chloride (KCl)-based method.
The main objectives of our study were: (a) to map out all easily absorbed nitrogen forms for better understanding of available nutrient composition and (b) to determine the ratio of pollutant (nitrite) and absorbable (ammonium ion, nitrate) N forms in humus sand soil in pot experiments.
Randomized soil sampling (approx. 60kg; 15 cores from control parcel) was done at the Pallag Experimental Station of Horticulture at the University of Debrecen, Hungary on May 20, 2020, according to Hungarian national standard MSZ 08020213 from the -20 cm of topsoil using a vane. Next, samples were transferred for measurements to a greenhouse located at Department of Agriculture, at the University of Debrecen. To measure chemical parameters, samples were sieved through a 2mm mesh.
We determined soil moisture content gravimetrically, according to Klimes-Szmik14, drying the soil samples at 105°C for 24 hours and weighing the mass loss. To evaluate texture, Arany-type plasticity index was measured, using the methodology recommended by Hungarian national standard MSZ-08 0206/1-7815; briefly from a burette distilled water is added to 100g sample until soil reaches the upper limit of its water holding capacity. Soil pH was determined in 1 mol L-1 KCl solution (soil/water = 1.0/2.5 wt./wt.), according to Buzás16 using a glass electrode (Model Seven2Go Advanced Single-Channel Portable pH Meter, Mettler, Toledo. We quantified humus content with potassium dichromate, according to Székely17: to calculate humus% we first determined organic-C%. The latter was measured following Hungarian standard MSZ-08 0210-7718; briefly, 10 mL K2SO4 was added to 1.0 g of soil sample in a 300 mL Erlenmeyer flask, then 0.10 g Ag2SO4 aqueous solution was added, boiled for 5 minutes, cooled to room temperature, and titrated with 0.2 N Mohr's salt solution. Ferroin was used was the indicator, given that its color change is reversible, pronounced, and fast. We calculated C% according to Equation (E1):
where a is the difference between blank and soil sample titration (mL), multiplied by the Mohr-salt factor and b represents the weight of the soil sample (g).
Next, humus% was calculated according to Equation (E2):
All measurements of soluble N forms in soil were conducted according to Hungarian standard MSZ 20135:199919; briefly, to determine KCl-NO3 (mg/kg) content, the sample was prepared with 1 mol L-1 KCl of soil solution (74.5g KCl were added to 1 L distilled water20. Spectra were recorded with a Model PU 8610 UV/VIS kinetics spectrophotometer by Pye Unicam (Cambridge, GB). For assessing total soluble nitrogen (UV degraded) content, soil samples were extracted with 0.01 mol L-1 CaCl2 solution. Soil samples were mixed with sodium tetraborate buffer, then oxidized with excess K2S2O8 and passed into an ultraviolet digester. Nitrate was reduced on a cadmium-copper column and then converted to a colored azo compound via the Griess-Ilosvay reaction; dissolving 0.5 g of sulfanilic acid and 0.05 g of 1-naphthylamine in 150 mL of dilute acetic acid. A colored solution was obtained, which was analyzed by visible photometry at 540 nm (SKALAR photometry (San Plus Analyser, S.F.A.S).
We determined CaCl2-ammonium-N (mg/kg) using a modified Berthelot reaction-based method in which ammonia was initially converted to monochloramine and then to 5-aminosalicylate, according to Buzás16 using 1% EDTA solution and 1 ml 0.06 N NaOH. After oxidation, a green color complex was obtained, with a maximum light absorption at 660 nm. Nitrate-N was determined as described above for the measurement of N content (after reduction, conversion to azo dye, and photometric measurement at 540 nm). All soluble forms were detected by SKALAR photometry (San Plus Analyser, S.F.A.S) in a segmented continuous flow (SCF) system. Finally, we calculated nitrite-N (mg/kg) using Equation (E3), proposed by Buzás16:
To better understand the correlation between humus% (Y) and N-forms (X), we carried out two different tests (i) the Pearson test to establish the relation between the variance (Y) and covariance (XY); and (ii) the Kendall test to verify whether the two variables may be regarded as statistically dependent. All statistical analysis was performed with R Statistical Software 3.5.1 (Foundation for Statistical Computing, Vienna, Austria).
Soil pH was slightly acidic: pH (KCl) 5.5. As KA = 30, according to Arany the soil is considered humus sand17. For these samples, 1.4 % humus content was found, which represents a reasonable value for a sand soil17. Nitrate content was determined using two methods: KCl-NO3 (12.66 mg/kg) and CaCl2-NO3 (13.53 mg/kg), and the average value of nitrate content (12.48 mg/kg) served for the calculation of percentages in Figure 1.
Average ammonium-N was 6.8 mg/kg, which represents 24.7% of the total-N content. In addition, 3.8 mg/kg of nitrite-N (pollutant nitrogen form) was assessed and found to be one order of magnitude less than the nitrate-N content, as expected for well-ventilated sand soil. Organic-N of 4.4 mg/kg contributed by 16.1% to the total nitrogen content of soil (Table 1).
Humus % | N forms (mg/kg) | ||||
Nitrate-N | Ammonium-N | Nitrite-N | Organic-N | Total-N | |
1.4 | 12.5 | 6.8 | 3.8 | 4.4 | 27.5 |
Considering all forms of N under study, the results of correlation analysis evidenced that only the ammonium-N form was in strong correlation with humus%, with r ≈ 0.97 and T ≈ 0.99 (Figure 2).
These promising initial results call for additional experiments to confirm the strong correlation between these two variables. Total-N also showed a good correlation with humus%; however, it was not as strong as that found between humus% and ammonium-N.
Our results reveal that the ratio of nitrate-N and nitrite-N (which is determined mostly by soil oxidation-reduction conditions) is optimal in sand soil samples originating from the Pallag Experimental Station of Horticulture at the University of Debrecen, Hungary. Based on the percentage distribution of soluble N forms present at Pallag Experimental Station, authors recommend using this soil in further pot experiments, given the soil’s optimal nutrient supply capacity20.
Figshare: Supporting data - N forms and Humus% in Sand soil, Nyírség. https://doi.org/10.6084/m9.figshare.12581303.v321
Data are available under the terms of the Creative Commons Attribution 4.0 International license (CC-BY 4.0).
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Is the work clearly and accurately presented and does it cite the current literature?
Yes
Is the study design appropriate and is the work technically sound?
Yes
Are sufficient details of methods and analysis provided to allow replication by others?
Partly
If applicable, is the statistical analysis and its interpretation appropriate?
Partly
Are all the source data underlying the results available to ensure full reproducibility?
Yes
Are the conclusions drawn adequately supported by the results?
Yes
Competing Interests: No competing interests were disclosed.
Reviewer Expertise: Chemical engineering, clean coal technology, process control, modeling, optimization.
Is the work clearly and accurately presented and does it cite the current literature?
Partly
Is the study design appropriate and is the work technically sound?
Partly
Are sufficient details of methods and analysis provided to allow replication by others?
Yes
If applicable, is the statistical analysis and its interpretation appropriate?
Yes
Are all the source data underlying the results available to ensure full reproducibility?
Yes
Are the conclusions drawn adequately supported by the results?
Partly
Competing Interests: No competing interests were disclosed.
Reviewer Expertise: Carbon and nitrogen cycles in terrestrial ecosystems
Is the work clearly and accurately presented and does it cite the current literature?
Partly
Is the study design appropriate and is the work technically sound?
Partly
Are sufficient details of methods and analysis provided to allow replication by others?
Partly
If applicable, is the statistical analysis and its interpretation appropriate?
Partly
Are all the source data underlying the results available to ensure full reproducibility?
Partly
Are the conclusions drawn adequately supported by the results?
Partly
Competing Interests: No competing interests were disclosed.
Reviewer Expertise: soil quality, nutrient management
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