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Article

Dynamics of Organic Nitrogen Compound Mineralization in Organic Soils under Grassland, and the Mineral N Concentration in Groundwater (A Case Study of the Mazurian Lake District, Poland)

by
Jan Pawluczuk
1 and
Arkadiusz Stępień
2,*
1
Department of Soil Science and Microbiology, University of Warmia and Mazury in Olsztyn, Plac Łódzki 3, 10-719 Olsztyn, Poland
2
Department of Agroecosystems and Horticulture, University of Warmia and Mazury in Olsztyn, Plac Łódzki 3, 10-719 Olsztyn, Poland
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(3), 2639; https://doi.org/10.3390/su15032639
Submission received: 28 December 2022 / Revised: 24 January 2023 / Accepted: 28 January 2023 / Published: 1 February 2023
(This article belongs to the Special Issue Biodiversity and Ecosystem Services for Environmental Sustainability)
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Abstract

:
Peatlands serve numerous functions, with one of the main ones being the retention of nutrients, including mineral nitrogen. A field study in organic soils was carried out in the Mazurian Lake District situated in north-eastern Poland (53°37′50′ N, 20°28′51′ E). In the test peat soils, the rate of organic nitrogen compound mineralization varied significantly depending on the season. The dynamics of the organic matter mineralization process were most strongly influenced by the soil use and the season. The mineral N release was higher in organic soil under intensively used grassland. In this soil, much more mineral nitrogen was released during the summer and spring periods. The dominant form in the organic nitrogen compound mineralization processes was N-NO3. The highest dynamics of the organic nitrogen compound mineralization were noted at a soil moisture content ranging from 57% to 59%. The interdependence of the soil moisture content and the rate of organic nitrogen compound mineralization fluctuated over the study period, with an increase in spring and a decrease in summer. A correlation was demonstrated between the nitrate form content in the peat soils under intensive cultivation and the nitrate-nitrogen concentration in groundwater, while no correlation was demonstrated between the N-NO3 content in the soils under a forest and the N-NO3 concentration in the groundwater of these soils. The results provide the basis for the conclusion that the land use type (forest, grassland), which determines the morphological structure of organic soils, affects the intensity of the infiltration of different mineral nitrogen forms.

1. Introduction

The early post-glacial relief is largely made up of cave-in areas formed from melting glacier water, which are currently filled with organic deposits to form wetlands and marshes [1,2]. These are hydrogenic habitats [3] which, in the world literature, are referred to as wetlands [4]. Wetlands are one of the forms of early post-glacial areas found throughout a large part of northern Europe, from north-eastern Germany to northern Poland, the Baltic countries, Belarus, Russia, and even Asia and North America [1,5]. The role of wetlands and their functions in the environment are increasingly appreciated [6,7,8]. Peatlands are one of the more important wetland forms in Europe. In Poland, one of the larger peatland concentrations is the macroregion of the Mazurian Lake District situated in north-eastern Poland. Peatlands, due to the functions they serve in the habitat, including water circulation and accumulation [9,10], effects on element circulation [11,12], water purification [13], protection of biodiversity [14], and effects on the climate [15], are covered by full protection [5,16].
Research into the biophysiochemical processes occurring in peatlands, in terms of their economic use and environmental impact, is becoming crucial [4,17,18,19]. Not only are peatlands natural water retention reservoirs in the environment [9,10], but, due to the hydrobioaccumulation processes that occur in them, they also accumulate organic matter [20], which is rich in plant nutrients, particularly nitrogen [11,12]. The organic matter contained in peats exhibits filtration and sorption properties [21]. Thanks to these properties, peats play the role of natural filters that reduce the flow of nitrogen compounds and other nutrients in the environment and thus contribute to its purification [13].
The filtering capacity of the organic matter accumulated in peats (and the intensity with which it retains and absorbs various nutrients) is determined by the peatland type, the type of organic substance, the degree of their transformation, and the use to which peat soils are put when used in agriculture [22,23]. Peat organic matter is among the labile soil formations that are very susceptible to changes associated with the amount of water found in them [20,24,25]. Therefore, the moisture content and aeration of the habitat determine the specific balance of the organic matter in which accumulated element forms are found and, in particular, determine the mineral N content [26]. Peat soils in which the accumulation of organic matter takes place under the dominant influence of a periodic or permanent excess of water exhibit a positive organic matter balance. At this organic soil development stage, both organic matter mineralization and nitrogen release decrease [27]. In peat soils in which the moisture content has clearly been reduced due to the natural or anthropogenic lowering of the groundwater level, an intensive process of organic matter mineralization takes place [28,29].
The nitrogen contained in organic compounds is released during the process of organic soil matter mineralization [30]. At the final stage of the organic nitrogen compound mineralization process, N-NH4, which can be taken up by plants, is released. Under specific conditions, i.e., low soil water content and oxygen availability, the ammonium ions (N-NH4) are transformed into nitrates (N-NO3) via the nitrification and oxidation process [31]. Nitrates are virtually not absorbed by the soil sorption complex, and, under favourable physico-chemical circumstances, they are leached out of the root zone, which may be a source of groundwater pollution [32]. Mineral nitrogen compounds, which are formed in the organic matter decomposition processes, are a very important link in the nutrient cycle in natural ecosystems [33]. In organic soils under grassland, it is important that the meadow vegetation makes maximum use of the mineral N released [34]. The process of organic nitrogen compound mineralization under specific conditions, if uncontrolled, can proceed with excessive efficiency, thus contributing to the loss of nitrogen, which is a valuable, yield-forming fertilizing element [35].
The aim of the study was to investigate the dynamics of organic nitrogen compound mineralization in organic soils located under different habitat conditions with differing management modes and the effects of such differences on the concentration of this element in the groundwater.

2. Materials and Methods

2.1. Experimental Site—Łańsk (Mazurian Lake District, Poland)

The study was carried out in Łańsk (53°37′50″ N, 20°28′51″ E), in the Mazurian Lake District situated in north-eastern Poland, in three representative soil pits. The data was collected in 2008 and 2018. Due to nonsignificant differences between the years of research, the main material presents the averages of the results obtained in 2008 and 2018. The results for each individual year of study are included in the supplement [Supplementary Tables S1–S7]. The measurements were taken during four seasons (spring—May, summer—August, autumn—November, winter—January).

2.2. Habitat Conditions and Soil Characteristics of the Site

At the site under study, the downslopes do not exceed 6% in over 88% of the area, which results in a slightly undulating relief. In the geological formations, outwash sediments which exhibit great homogeneity in their grain size are dominant, and 90% of the mineral formations are dominated by fluvioglacial sands and gravels. In the study area, organic soils account for 20% of the soil cover. The organic soils found there are formed from low moor peats with an average area of 3.5 ha. The formation of peat soils on the site under study involved groundwater which percolated through sandy formations to accumulate in basin-shaped depressions and form an extensive underground reservoir. In the scientific literature, this type of hydrological supply of wetlands is referred to as soligenous [3]. The organic soils on the site under study differed in their mode of management and morphological structure. On a reclaimed peatland under degraded grassland (Soil 1), deep, fibrous, moderately moorshified peat and muck soil formed from moderately decomposed low moor reed peat are found (R2—30–60%; R—the degree of organic matter decomposition). A peat and muck sapric soil (Soil 2) is formed from highly decomposed sedge low moor peat (R3 > 60%). In both the peat and the muck soils (Soil 1 and Soil 2) located in a drained lake basin, clear differences occur in the soil profile’s uppermost layers. Under the degraded meadow (Soil 1), humus muck with a fine granular structure is found, while, in the soil under the managed meadow (Soil 2), proper muck with a coarse granular structure is found. The peat soil under the birch-alder swamp forest (Soil 3) was formed from highly decomposed sedge peat (R3 > 60%), underlain at a depth of 16 cm by highly decomposed alder peat (R3 > 60%).
Based on geomorphological research, the type of hydrological supply was determined in the field [36]. The prognostic moisture-soil complexes and potential moisture hydrogenic habitats, as identified by Okruszko, Piaścik [37], were determined for the organic soils. At locations representative of the site under study, soil pits were dug, in which the soil type was identified based on the samples collected from the soil profile layers from a depth of up to 150 cm. The locations of the soil pits were selected using the sample plot method developed by Richling [38] and the standard plot method [39]. In the soils under the degraded meadow (Soil 1), the groundwater level ranged from 27 to 100 cm below ground level. Under the managed meadow (Soil 2), during all four seasons in the years under study, the groundwater level remained at a depth ranging from 25 to 75 cm below ground level. In the soil under a birch-alder swamp forest (Soil 3), there was water on the surface, and the water level lowered to an average depth of 5–20 cm only in summer.

2.3. Determination of Physical and Chemical Characteristics

In the soil pits, soil was collected from the layers of 5–10, 15–20, 25–30, and 35–40 cm into 100 cm−3 cylinders for the determination of its physical and chemical characteristics. Samples were collected from each pit in four replications. The soil moisture content was determined using the oven drying method at a temperature of 105 °C [40].
The total nitrogen in the soil was determined using the Kjeldahl method. The mineral N (N-NO3, N-NH4) content in the soil was determined following a 14-day incubation at a temperature of 28 °C while maintaining the current moisture content [41]. The N-NO3 content was determined in a 1% K2SO4 extract, and the N-NH4 content in disulfonic acid with Nessler’s reagent. For the measurements of the groundwater level and the collection of water samples for chemical analyses, piezometers were installed in the vicinity of the soil pits. The piezometers were installed to a depth of 150 cm. For the water samples collected, the following were determined: N-NO3 using the colorimetric method with disulfonic acid, and N-NH4 using the colorimetric method with Nessler’s reagent [42].

2.4. Statistical Analyses

The study results were collected using Excel software and subjected to a statistical analysis that was conducted in the Statistical 13 PL program. In order to compare the significance of the average values, an ANOVA variance analysis was conducted. To compare the average intragroup values, a Tukey post-hoc test was employed. The significance level was assumed to be p < 0.05. The significance level was determined for the interaction between such factors as the soil type (the use), the season, and the soil layer (the depth). In order to examine and describe the interdependence between two variables in the form of the significance of the strength and direction of the relationship, linear regression analysis was applied. First, the Pearson correlation coefficient (r) was calculated, and the linear regression equation was determined. The correlation between the two characteristics is shown in the scatter plots. The significance level in the linear regression analysis was assumed to be p < 0.05. In order to assess the interdependence to be able to find out to what extent one characteristic (x) determines the variation of a resultant characteristic (y), determination coefficients were calculated in accordance with the following formula: the determination coefficient = r2 ∙ 100%, where r is the Pearson correlation coefficient. A regression analysis in this form was conducted to present the relationships between the moisture content, the Ntotal content, and the mineral N content (N-NO3 and N-NH4) in the soil, and to show the relationship between the N-NO3 and N-NH4 contents in the soil and the N-NO3 and N-NH4 contents in the water. The F-values of ANOVA are presented in Table 1.

3. Results

3.1. Morphological Structure of the Soils

The organic habitats of Soil 3 under the birch-alder swamp forest were in their natural state, and, in terms of the prevailing moisture conditions, they were classified as a wet prognostic moisture-soil complex (Table 2). Soil 1, under intensively used grassland, and Soil 2, under extensively used grassland, were classified as a drying prognostic moisture-soil complex. Soil 1 and Soil 2 were in the decession (enhanced aerobic peat mineralization) phase, while Soil 3 was in the accumulation phase.

3.2. Ntotal Content in Soil

A significant difference was demonstrated in the Ntotal content, depending on the soil type, the depth, and the interaction between the soil type and the depth (Figure 1).
The Ntotal contents of Soil 1 and Soil 2 increased up to the depth of 25–30 cm and amounted to an average of 3.17 g 100 g−1, while, in the layer at 35–40 cm, the content was lower and amounted to 3.02 g 100 g−1. The Ntotal content in Soil 3 decreased from 3.39 to 2.61 g 100 g−1 as the depth of soil profile increased.

3.3. Moisture Content in Soil

A significant difference was demonstrated in the moisture content depending on the soil type, the depth, and the season, and on the interactions between the soil type and the depth and between the soil type and the season (Figure 2). The most favourable air and water conditions for the organic nitrogen compound mineralization process were noted in Soil 1, while the least favourable conditions were found in Soil 3. Irrespective of the soil type or the season, the statistical analyses confirmed the moisture content of the soil increased with an the depth increased. The test soils showed the lowest moisture content in the summer. The determined water properties of the soil did not differ statistically between spring, autumn, and winter. In Soil 1, the most favourable conditions for mineralization were found in the proper mucks (5–10 cm), in which the moisture content amounted to 60.3%. Unfavourable conditions for mineralization occurred in the organic layers of Soil 3 (5–40 cm) and of Soil 2 (25–40 cm), in which the moisture content ranged from 82.6% to 89.9%. Soil 1 was characterized by low moisture content in all seasons. In both Soil 2 and Soil 3, similar moisture contents were found, except in summer.

3.4. Mineral Nitrogen Content in Soil (N-NO3, N-NH4)

The contents of mineral nitrogen compounds (N-NO3 and N-NH4) varied significantly among the test soils, the layers, and the seasons, as well as in the interactions between the test factors (Figure 3, Table 3). The highest rate of N-NO3 release was noted in Soil 1 (12.2 mg L), while the lowest was in Soil 3 (1.6 mg L−1). The significantly highest N-NO3 contents in the soil were noted in summer (14.3 mg L−1), while the lowest contents were found in winter (1.0 mg L−1). Irrespective of the soil type and the season, the N-NO3 content decreased as the soil depth increased. Irrespective of the depth, the factor determining the release of N-NO3 in the soils was the season. Irrespective of the season, in the soils in the decession (enhanced aerobic peat mineralization) phase (Soil 1 and Soil 2), the N-NO3 release was significantly the highest in the layer of 5–10 cm (the uppermost muck layers Mt). No significant differences were demonstrated in the N-NO3 content between the layers in Soil 3. According to an analysis of the interaction of depth × season, the highest N-NO3 content was demonstrated in summer in the layer of 5–10 cm (22.03 mg L−1), and the lowest in winter, in the layers of 25–30 and 35–40 cm (0.84 mg L−1). Analysis of the interaction of soil type × layer depth × season showed that significantly the most N-NO3 was released in summer in Soil 1 in the layer of 5–10 cm (32.83 mg L−1). The lowest N-NO3 content was noted in winter in Soil 3 in the layer of 25–30 cm (0.58 mg L−1), but the differences were not statistically proven.
The N-NH4 content showed similar relationships with the soil type, the depth, and the season to those demonstrated in the analysis of the N-NO3 content. The highest N-NH4 contents were noted in Soil 1 (3.88 mg L−1), while the lowest ones were found in Soil 3 (2.31 mg L−1) (Figure 4, Table 4). The factor determining the N-NH4 release in the soils was the season. The significantly highest N-NH4 contents in the soil were noted in summer (5.72 mg L−1), while the lowest ones were found in winter (1.37 mg L−1). The analysis of the effect of layer depth revealed that the highest N-NH4 content was noted in the layer of 5–10 cm (4.17 mg L−1) and that the content decreased in the deeper test layers. Irrespective of the season, in Soil 1 and Soil 2, the significantly highest N-NH4 release was in the layers of 5–10 cm, and the N-NH4 content decreased in the deeper layers. In the process of organic nitrogen compound mineralization in Soil 3, no significant differences were demonstrated in the N-NH4 contents between the layers of 5–20 cm and 25–40 cm. As regards the interaction of factors: soil*season, the study demonstrated that the highest N-NH4 content was in summer in the soils in the decession (enhanced aerobic peat mineralization) phase (Soil 1 and Soil 2) and was lower in spring and autumn and the lowest in winter. An analysis of the interaction of factors: depth*seasons demonstrated that the most N-NH4 was released in summer in the top layers, while the least was released in winter in the layers of 35–40 cm. The analysis of the interaction: soil type*layer depth*season revealed that most N-NH4 was released in summer in Soil 2 in the layer of 5–10 cm (9.83 mg L−1), but the content of this nitrogen form did not differ significantly from the contents noted in the layers of 5–10 cm and 15–20 cm in Soil 1. The least N-NH4 was released in winter in Soil 3 in the layer of 5–10 cm (0.69 mg L−1).

3.5. Mineral Nitrogen Content (N-NO3, N-NH4) in Groundwater

The concentration of mineral nitrogen forms (N-NO3, N-NH4) in the groundwater of the test organic soils was determined by the soil type, the season, and the interaction between these factors (Figure 5 and Figure 6). The study results show that the dominant form in the water was the form N-NO3. From Soil 1 and Soil 2, more N-NO3 than N-NH4 penetrated into groundwater. The highest N-NO3 and N-NH4 concentrations were noted in the groundwater collected in Soil 1. Significantly lower N-NO3 contents were demonstrated in Soil 2 and Soil 3, while the lowest N-NH4 content was noted in Soil 2. Irrespective of the soil type, the lowest N-NO3 and N-NH4 contents in groundwater were noted in winter (0.096 and 0.135 mg L, respectively). The highest N-NO3 concentration in groundwater was noted in spring and summer in Soil 1 (an average of 0.378 N-NO3 mg L−1), while the lowest was in winter in Soil 3 (0.054 mg L−1). The highest N-NH4 concentration in groundwater was noted in autumn in Soil 1 (0.358 mg L−1).

3.6. Dependencies between the Examined Factors

The analysis of the correlations between the variables showed that there were significant negative relationships between soil moisture and the content of N-NO3 (r2 = −0.8509), N-NH4 (r2 = −0.8928) in the soil and the content of N-NO3 in water (r2 = −0.8929) (Table 5). In the case of N-NO3 content in the soil, a mutual negative correlation was also observed with the Ntotal content in the soil (r2 = −0.9033). Mutual negative relationships between the content of N-NO3 and N-NH4 in the water were also found (r2 = 0.9272).
The course of the relationship between the soil moisture content and the N-NO3 and N-NH4 contents is represented by a rectilinear regression which indicates that a higher soil moisture content is correlated with lower N-NO3 and N-NH4 contents in these soils (Figure 7a,b). Significant relationships, however, were only noted in Soil 1 and Soil 2. The calculated determination coefficient (r2) shows that the N-NO3 content in Soil 1 and Soil 2 was determined from 36% to 40%, and the N-NH4 content from 53% to 61%, by the soil moisture content. The determination coefficient shows that, in Soil 3, the Ntotal content determines 34% of the N-NO3 and 56% of the N-NH4 content and, in the other soils, the N-NO3 and N-NH4 contents were not determined by the Ntotal content (Figure 7c,d). The linear regression analysis of the effect of the N-NO3 and N-NH4 contents in the soils on the N-NO3 and N-NH4 contents in groundwater indicated the occurrence of significant positive relationships in Soil 2 with both the N-NO3 and N-NH4 contents, in Soil 1 with only the N-NO3 content, and in Soil 3 with the N-NH4 content (Figure 7e,f).

4. Discussion

4.1. Morphological Structure of the Soils

The areas under study are covered by the range of the Pleistocene glaciation known as the Weichselian glaciation [43,44]. These areas are characterized by a diversity of landscapes and the variation of habitat conditions, as documented in studies by Ben and Evans [45], Dembek [46], and Wierzbicki et al. [47]. In the peat and muck soils, namely, Soil 1 and Soil 2, located in a drained lake basin and differing in their intensity of use, clear differences occur in the soil profile’s uppermost layers. The differences in the morphological characteristics of the uppermost layers of the test peat and muck soils were affected by the habitat conditions, particularly the sustained groundwater level as well as the soil moisture content and the associated dynamics of organic matter mineralization, which was also demonstrated in a study by Bieniek et al. [48], Rydelek [49] and Oleszczuk et. al. [50]. The lowering of the groundwater table in organic soils, either naturally or anthropogenically, makes them usable for agriculture, resulting in changes in their morphological characteristics and the transformation of these soils into organic-mineral soils [51].

4.2. Mineral Nitrogen Content in Soil

The study results relate to the dynamics of organic nitrogen compound mineralization in organic soils formed from peat in areas intensively or extensively used as hay meadows, or retained in their natural state. The authors’ own study showed that the mineralization of organic nitrogen compounds in peat soils located under different habitat conditions, under intensively and extensively used grassland in its natural state, varied based on the particular season and the mineral-organic and organic layers under study. Similarly, as demonstrated by Rydelek [52] and Stolarczyk et al. [53], the course and intensity of matter mineralization in organic soils are significantly affected by the moisture content of the soil and the soil type, the degree of organic substance transformation, and the soil use. The results of the authors’ own study indicated that organic soils classified as drying prognostic moisture-soil complex (Soil 1 and Soil 2) are characterized by a high susceptibility to organic nitrogen compound mineralization. It may be that the sites with higher overall microbial activity and N mineralization potential release more ammonium when subjected to reduced conditions because of an ammonium accumulation in pore water resulting from the decreased activity of the nitrifying bacteria. Nitrification and nitrate production are low in anoxic environments, and, if any nitrate is still formed, it is utilized as terminal electron acceptors. This can result in the nitrate release from waterlogged peat soils ceasing altogether [54].
At the sites under study, the highest rate of organic nitrogen compound mineralization was noted in summer in mucks (Soil 1), where the moisture content amounted to 55.2%. The dynamics of the mineralization of organic nitrogen compounds could be caused, among other things, by the changing population of soil microorganisms involved in the release of nitrogen from organic matter [55]. The abundance and activity of soil organisms are linked to the prevailing environmental conditions at a given time, which influence the physicochemical processes in the soil [56]. Oleszczuk et al. [50] point out that peat consolidation, shrinkage, mineralization, swelling, and subsidence are processes inherent to drained peatlands and linked to groundwater level. In peat soils, in which the moisture content was clearly reduced due to the natural or anthropogenic lowering of the groundwater level, the process of enhanced aerobic peat mineralization, referred to as decession, takes place [28,29]. During this organic matter chemical transformation phase, the biomass mineralization process, which contributes to its loss occurs intensively. This results in the decomposition of the previously accumulated organic substances and, consequently, in the reduced thickness of this organic layer in the soil [28].

4.3. Mineral Nitrogen Content in Groundwater

The mineral N concentration in the groundwater of organic soils is mainly determined by the dynamics of its release in the process of organic matter moorshification, and the sustained water level. In addition, the mineral N amount in the groundwater is determined by the filtering capacity of the organic matter, which is, in turn, dependent on the peatland type, organic substance type, the degree of their transformation, and the agricultural management method [57,58]. This process can proceed with varying degrees of intensity in particular seasons and years [57,59].
The study results demonstrate that the soligenous type of hydrological supply, which is a post-Pleistocene evolution of regional hydrology and is predominant at the site under study, as shown in a study by Wierzbicki et al. [47], can contribute to a low moisture content of the organic soils located in particular in the slightly undulating relief. The nitrate concentration in the groundwater under natural grassland in regions with temperate climates is usually below 2 mg dm−3 [60]. The lower mineral N concentration in the groundwater of Soil 2 results from the greater phytosorption of mineral N by young meadow vegetation regrowing after each mowing and the reduced dynamics of the organic matter mineralization process. This was also demonstrated in a study by Ilnicki [61], who also stated that nutrients could be absorbed by soil particles as well as transformed by bacteria (denitrification) and later used by vegetation. The detailed consideration of water sources in a study by Walton et al. [57] indicated that the average nitrate removal efficiencies were highest for groundwater (76 ± 25%) and lowest for river water (35 ± 24%).
On the other hand, Vagstad et al. [62] link this directly with the geomorphology of the site. The results of studies by both Ilnicki [61] and Vagstad et al. [62] can confirm the processes taking place in the organic soils under study. This is also confirmed by the results of a study by Fotyma [63], who conducted research into the temporal and spatial variations in the nitrogen emissions in the soil and its use in agricultural crop production, as well as its dispersion. The results of the authors’ own study provide the basis for the conclusion that the land use type (forest, grassland), which determines the morphological structure of organic soils, affects the intensity of the infiltration of different mineral nitrogen forms. This was confirmed in a study by Daniels et al. [64], who demonstrated that ammonium leaching was greater in the acidic upland peatlands of Great Britain. Part of the N-mineral is released in the form of ammonium, which is not readily available to plants and remains in the soil, and part is released in the nitrate form, which is the more labile form and is leached into the groundwater after phytosorption [65].

5. Conclusions

In the test peat soils, the rate of organic nitrogen compound mineralization varied significantly depending on the season. The uppermost soil layers, in which peat muck characterized by a medium transformation degree was found, exhibited high dynamics of the mineral nitrogen release. The dynamics of the organic matter mineralization process were most strongly influenced by the soil use and the season. The mineral nitrogen release was higher in organic soil under intensively used grassland. In this soil, high levels of mineral nitrogen were released during the summer and spring periods. The dominant form in the organic nitrogen compound mineralization processes was N-NO3. In the test organic soils, the highest dynamics of organic nitrogen compound mineralization were noted at a soil moisture content ranging from 57% to 59%. The interdependence of the soil moisture content and the rate of organic nitrogen compound mineralization fluctuated over the study period, with an increase in spring and a decrease in summer. Irrespective of the use and the layer depth, this interdependence was more than two-fold greater for N-NO3 than for N-NH4. A correlation was demonstrated between the nitrate form content in the peat and muck soils under intensive cultivation and the nitrate-nitrogen concentration in the groundwater, while no correlation was demonstrated between the N-NO3 content in the soils under a forest and the N-NO3 concentration in the groundwater of these soils.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su15032639/s1, Table S1: Ground water levels [cm], 2008 and 2018; Table S2. The Ntotal content in the soil, [g 100g−1], 2008 and 2018; Table S3. Soil moisture content [%], 2008 and 2018; Table S4. The N-NO3 content in the soil [mg L−1], 2008 and 2018; Table S5. The N-NH4 content in the soil [mg L−1], 2008 and 2018; Table S6. The N-NO3 contents in groundwater [mg L−1], 2008 and 2018; Table S7. The N-NH4 contents in groundwater [mg L−1], 2008 and 2018.

Author Contributions

Conceptualization, A.S. and J.P.; methodology, J.P.; validation, A.S. and J.P.; formal analysis, A.S. and J.P.; investigation, J.P; resources, J.P; data curation, A.S.; writing—original draft preparation, A.S. and J.P.; writing—review and editing, A.S. and J.P.; visualization, A.S.; supervision A.S. and J.P.; project administration, J.P.; funding acquisition, A.S. and J.P. All authors have read and agreed to the published version of the manuscript.

Funding

Project financially supported by the Minister of Education and Science under the program entitled “Regional Initiative of Excellence” for the years 2019–2023, Project No. 010/RID/2018/19, amount of funding was 12.000.000 PLN. The results presented in this paper were obtained as part of a comprehensive study financed by the University of Warmia and Mazury in Olsztyn, Faculty of Agriculture and Forestry, Department of Agroecosystems and Horticulture (grant No. 30.610.015-110) and Department of Soil Science and Microbiology (grant No. 30.610.005-110).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. The Ntotal content in the soil: (a) an average for the soil type; (b) an average for the soil depth; (c) the interaction of soil type × soil depth. a,b,c,d—Values followed by the same letters do not differ significantly in Tukey’s (HSD) test (p < 0.05). Soil 1—degraded meadow, Soil 2—productive meadow, Soil 3—alder swamp.
Figure 1. The Ntotal content in the soil: (a) an average for the soil type; (b) an average for the soil depth; (c) the interaction of soil type × soil depth. a,b,c,d—Values followed by the same letters do not differ significantly in Tukey’s (HSD) test (p < 0.05). Soil 1—degraded meadow, Soil 2—productive meadow, Soil 3—alder swamp.
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Figure 2. The soil moisture content: (a) an average for the soil type; (b) an average for the soil depth; (c) an average for the season; (d) the interaction of soil type × soil depth; (e) the interaction of soil type × season. a,b,c,d,e—Values followed by the same letters do not differ significantly in Tukey’s (HSD) test (p < 0.05). Soil 1—degraded meadow, Soil 2—productive meadow, Soil 3—alder swamp.
Figure 2. The soil moisture content: (a) an average for the soil type; (b) an average for the soil depth; (c) an average for the season; (d) the interaction of soil type × soil depth; (e) the interaction of soil type × season. a,b,c,d,e—Values followed by the same letters do not differ significantly in Tukey’s (HSD) test (p < 0.05). Soil 1—degraded meadow, Soil 2—productive meadow, Soil 3—alder swamp.
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Figure 3. The N-NO3 content in the soil: (a) an average for the soil type; (b) an average for the soil depth; (c) an average for the season; (d) the interaction of soil type × soil depth; (e) the interaction of soil type × season; (f) the interaction of season × soil depth. a,b,c,d,e,f,g,h,i—Values followed by the same letters do not differ significantly in Tukey’s (HSD) test (p < 0.05). Soil 1—degraded meadow, Soil 2—productive meadow, Soil 3—alder swamp.
Figure 3. The N-NO3 content in the soil: (a) an average for the soil type; (b) an average for the soil depth; (c) an average for the season; (d) the interaction of soil type × soil depth; (e) the interaction of soil type × season; (f) the interaction of season × soil depth. a,b,c,d,e,f,g,h,i—Values followed by the same letters do not differ significantly in Tukey’s (HSD) test (p < 0.05). Soil 1—degraded meadow, Soil 2—productive meadow, Soil 3—alder swamp.
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Sustainability 15 02639 g004a 550Sustainability 15 02639 g004b 550
Figure 4. The N-NH4 content in the soil: (a) an average for the soil type; (b) an average for the soil depth; (c) an average for the season; (d) the interaction of soil type × soil depth; (e) the interaction of soil type × season; (f) the interaction of season × soil depth. a,b,c,d,e,f,g,h,i,j—Values followed by the same letters do not differ significantly in Tukey’s (HSD) test (p < 0.05). Soil 1—degraded meadow, Soil 2—productive meadow, Soil 3—alder swamp.
Figure 4. The N-NH4 content in the soil: (a) an average for the soil type; (b) an average for the soil depth; (c) an average for the season; (d) the interaction of soil type × soil depth; (e) the interaction of soil type × season; (f) the interaction of season × soil depth. a,b,c,d,e,f,g,h,i,j—Values followed by the same letters do not differ significantly in Tukey’s (HSD) test (p < 0.05). Soil 1—degraded meadow, Soil 2—productive meadow, Soil 3—alder swamp.
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Figure 5. The N-NH4 contents in groundwater: (a) an average for the soil type; (b) average for the season; (c) the interaction of soil type × season. a,b,c,d,e,f,g,h,i—Values followed by the same letters do not differ significantly in Tukey’s (HSD) test (p < 0.05). Soil 1—degraded meadow, Soil 2—productive meadow, Soil 3—alder swamp.
Figure 5. The N-NH4 contents in groundwater: (a) an average for the soil type; (b) average for the season; (c) the interaction of soil type × season. a,b,c,d,e,f,g,h,i—Values followed by the same letters do not differ significantly in Tukey’s (HSD) test (p < 0.05). Soil 1—degraded meadow, Soil 2—productive meadow, Soil 3—alder swamp.
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Figure 6. The N-NO3 contents in groundwater: (a) an average for the soil type, (b) average for the season, (c) the interaction of soil type × season. a,b,c,d,e,f,g,h—Values followed by the same letters do not differ significantly in Tukey’s (HSD) test (p < 0.05). Soil 1—degraded meadow, Soil 2—productive meadow, Soil 3—alder swamp.
Figure 6. The N-NO3 contents in groundwater: (a) an average for the soil type, (b) average for the season, (c) the interaction of soil type × season. a,b,c,d,e,f,g,h—Values followed by the same letters do not differ significantly in Tukey’s (HSD) test (p < 0.05). Soil 1—degraded meadow, Soil 2—productive meadow, Soil 3—alder swamp.
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Figure 7. The relationship between (a,b) the N-NO3 and N-NH4 contents in the soil and the soil moisture content; (c,d) Ntotal content in the soil and the N-NO3 and N-NH4 contents in the soil; and (e,f) the N-NO3 and N-NH4 contents in the soil and the N-NO3 and N-NH4 contents in groundwater (calculated at significance α = 0.05).
Figure 7. The relationship between (a,b) the N-NO3 and N-NH4 contents in the soil and the soil moisture content; (c,d) Ntotal content in the soil and the N-NO3 and N-NH4 contents in the soil; and (e,f) the N-NO3 and N-NH4 contents in the soil and the N-NO3 and N-NH4 contents in groundwater (calculated at significance α = 0.05).
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Table
Table 1. F-values of ANOVA.
Table 1. F-values of ANOVA.
ParameterSoilDepthSeasonSoil × DepthSoil × SeasonDepth × SeasonSoil × Season × Depth
Moisture content [%]96.38 *15.90 *20.89 *3.69 *4.97 *0.13 ns0.55 ns
N-NO3—soil [mg L−1]3983.02 *703.45 *4598.26 *153.25 *946.87 *240.20 *47.29 *
N-NH4—soil [mg L−1]221.04 *146.79 *1030.96 *18.75 *109.11 *20.74 *7.25 *
Ntotal—soil [g 100 g−1]11.74 *12.48 *na13.56 *nanana
N-NO3—water [mg L−1]953.314 *na230.410na71.416 *nana
N-NH4—water [mg L−1]1263.19 *na200.50 *na112.39 *nana
* significant p < 0.05; ns—not significant, na—not applicable.
Table
Table 2. Classification and characteristics of the tested organic soils and type of soil use (soil management).
Table 2. Classification and characteristics of the tested organic soils and type of soil use (soil management).
Tested Soil NumberSoil Use
(Soil Management)		Soil ClassificationPrognostic Moisture-Soil ComplexType of Hydrological SupplyPotential Moisture Hydrogenic Habitats
Soil 1degraded meadowDystric Rheic Murshic Fibric Histosoldrysoligenoussoligenous dry
Soil 2productive meadowDystric Rheic Murshic Sapric Histosoldrysoligenoussoligenous dry
Soil 3alder swampDystric Rheic Sapric Histosol (Limnic)wetsoligenoussoligenous wet
Table
Table 3. The N-NO3 content in the soil depending on the interaction of soil type × season × soil depth.
Table 3. The N-NO3 content in the soil depending on the interaction of soil type × season × soil depth.
Tested Soil Number
(Soil Management)		Depth, cmSeason
SpringSummerAutumnSpring
Soil 1
(degraded meadow)		5–1024.93 c32.83 a7.19 jk1.38 p–t
15–2022.82 cd27.92 b5.27 klm1.14 rst
25–3018.83 fg20.52 ef4.65 lmn0.76 t
35–4015.30 h8.30 j3.09 n–r0.84 st
Soil 2
(productive meadow)		5–1021.32 de29.96 b3.90 l–o1.71 p–t
15–2020.76 def20.75 def2.08 o–t1.25 p–t
25–3016.21 h17.0 gh1.77 o–t1.19 p–t
35–4013.00 i5.60 klk1.43 p–t0.93 st
Soil 3
(alder swamp)		5–101.60 p–t3.30 m–p1.32 p–t0.63 t
15–202.46 o-t2.97 n–s1.23 p–t0.67 t
25–302.50 o-t1.34 p–t1.23 p–t0.58 t
35–402.09 o-t1.22 p–t1.21 p–t0.76 t
a–t—Values followed by the same letters do not differ significantly in Tukey’s (HSD) test (p < 0.05).
Table
Table 4. The N-NH4 content in the soil depending on the interaction of soil type × season × soil depth.
Table 4. The N-NH4 content in the soil depending on the interaction of soil type × season × soil depth.
Tested Soil Number
(Soil Management)		Depth, cmSeason
SpringSummerAutumnSpring
Soil 1
(degraded meadow)		5–106.43 def8.81 ab2.87 l–r2.25 n–u
15–205.16 fgh8.57 abc1.89 p–w1.42 s–w
25–304.34 g–k7.30 cd1.15 t–w1.11 uvw
35–403.16 k–p5.61 efg1.05 uvw0.92 uvw
Soil 2
(productive meadow)		5–105.11 fgh9.83 a2.49 m–t2.01 o–w
15–204.88 ghi6.63 de1.56 r–w2.27 n–u
25–303.91 h–l7.51 bcd2.00 o–w1.20 t–w
35–402.63 l–s3.41 j–n1.73 r–w1.16 t–w
Soil 3
(alder swamp)		5–104.58 g–j3.46 j–n1.50 s–w0.69 w
15–204.35 g–k3.26 j–o1.29 s–w1.91 o–w
25–303.65 i–m2.21 n–u1.29 s–w0.77 vw
35–403.84 h–m2.10 n–v1.33 s–w0.80 vw
a–w—Values followed by the same letters do not differ significantly in Tukey’s (HSD) test (p < 0.05).
Table
Table 5. Pearson’s r correlations for the tested variables.
Table 5. Pearson’s r correlations for the tested variables.
VariableMoisture [%]N-NO3—Soil [mg L−1]N-NH4—Soil [mg L−1]N-NO3—Water [mg L−1]N-NH4—Water [mg L−1]Ntotal—Soil [g 100 g−1]
Moisture [%]1−0.8509 *−0.8928 *−0.8929 *−0.71250.5919
N-NO3—soil [mg L−1]−0.8509 *10.65110.70170.3850−0.9033 *
N-NH4—soil [mg L−1]−0.8928 *0.651110.77880.6579−0.4753
N-NO3—water [mg L−1]−0.8929 *0.70170.778810.9272 *−0.4070
N-NH4—water [mg L−1]−0.71250.38500.65790.9272 *1−0.0465
Ntotal—soil [g 100g−1]0.5919−0.9033 *−0.4753−0.4070−0.04651
* Correlation coefficients are significant at p < 0.05.
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Pawluczuk, J.; Stępień, A. Dynamics of Organic Nitrogen Compound Mineralization in Organic Soils under Grassland, and the Mineral N Concentration in Groundwater (A Case Study of the Mazurian Lake District, Poland). Sustainability 2023, 15, 2639. https://doi.org/10.3390/su15032639

AMA Style

Pawluczuk J, Stępień A. Dynamics of Organic Nitrogen Compound Mineralization in Organic Soils under Grassland, and the Mineral N Concentration in Groundwater (A Case Study of the Mazurian Lake District, Poland). Sustainability. 2023; 15(3):2639. https://doi.org/10.3390/su15032639

Chicago/Turabian Style

Pawluczuk, Jan, and Arkadiusz Stępień. 2023. "Dynamics of Organic Nitrogen Compound Mineralization in Organic Soils under Grassland, and the Mineral N Concentration in Groundwater (A Case Study of the Mazurian Lake District, Poland)" Sustainability 15, no. 3: 2639. https://doi.org/10.3390/su15032639

APA Style

Pawluczuk, J., & Stępień, A. (2023). Dynamics of Organic Nitrogen Compound Mineralization in Organic Soils under Grassland, and the Mineral N Concentration in Groundwater (A Case Study of the Mazurian Lake District, Poland). Sustainability, 15(3), 2639. https://doi.org/10.3390/su15032639

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