1. Introduction
Organic soils are very susceptible to transformation and degradation [1,2,3,4,5,6,7,8,9,10,11,12], which is particularly important in agricultural areas. This is due to the specificity of these soils, which are predominantly formed from plant remnants at various stages of decay [13,14,15,16]. Soil organic matter (SOM) undergoes microbiological changes, the rate of which depends mainly on the substrate quality, temperature, and the amount of water. In organic soils, two soil processes can be identified: SOM accumulation (positive SOM balance) and SOM mineralization (negative SOM balance; SOM depletion). When the water level is high (within 10 cm of the soil surface), anaerobic conditions prevail, and the plant remnants are accumulated as peat. However, when the water level decreases, air enters the soil top layer, aerobic conditions begin to predominate, and the organic mass is decomposed, thus reducing the SOM (decession phase) [2]. Recently, considering the idea of sustainable development and peatland rewetting, attention has been paid to a zero SOM balance, which is referred to as the equilibrium phase [17,18]. The evolution of organic soils depends on the amount of water and fertility of the site. The inflow and accumulation of water in organic soils depends on the climate and geomorphology, as well as agricultural management [2,15,19].
Peatlands in various regions of the world have been drained by man, leading to radical changes in organic soil-forming process [9,20,21,22,23,24,25,26]. The lack of land and the hunger after the Second World War significantly intensified these transformations [8]. Drainage of organic soils may also occur in natural conditions, due to both climate change (warming and drought periods) and changes in hydrology, e.g., deepening of the river bed as a result of deep-seated erosion. Such episodes of the natural desiccation of peatlands occurred many times in the Holocene and are recorded in the stratigraphy of peat deposits [16,25]. Degradation of organic soils caused by their drainage carries a high risk to the environment and agricultural management. There are trade-offs between drainage (also for agricultural purposes) of organic soils and ecosystem services. Organic soils are productive, and fertile, but when drained, their water retention and filtration are disturbed, the carbon can no longer be stored (and is released), and plant and animal habitats are lost. Drained organic soils have reduced water storage capacity, and organic matter mineralization releases greenhouse gases into the atmosphere, as well as nutrients to groundwaters [2,9,12,21,25,26,27,28,29]. Degraded organic soils have limited agricultural usefulness, and are exposed to wind erosion and fires (which are difficult to extinguish). They are therefore a serious economic and environmental problem [8,23,30,31,32,33].
In Poland, since the 1950s, intensive, multi-directional (comprehensive) studies on the impact of drainage on properties of peat soils have been conducted [1,2,3,4,5,7,8,11,12,18,19,24,27,29,34,35,36,37]. The need for such research resulted from the intensification of the agricultural use of peatlands, especially fen (low-moor) peatlands [2,34,35,36,37]. During these studies, physical and chemical changes occurring in agricultural, drained organic soils were described. However, these studies have not been available to a wider audience (most of them were in Polish), despite the great applicable character and usefulness of the obtained results for farmers and further international research. The aim of this review is to provide a research overview on agricultural organic soils in Poland, and we accomplish this by (1) exploring the specific character of organic soils, (2) providing a history and definition of the mursh-forming process, (3) showing the position of organic (peat) soils in the Polish Soil Classification, (4) describing the transformations of organic matter after drainage, (5) revealing the influence of mineral matter in organic soils after drainage, and (6) promoting a research agenda to fill the current gaps in the research of drained organic soils. Given the role of organic soils in the global carbon cycle, a better understanding of the processes taking place in drained organic soils is of increasing importance. In order for farmers to rationally use and protect these soils, knowledge about their geographical distribution, current area, ongoing processes, and susceptibility to transformation is needed [15,25].
In the paper we present the knowledge on mursh soils, as this may not be available to the wider audience, in an attempt to fill the scientific gap. The Polish achievements, i.e., the description of the effects of drainage on peatlands, are significant but hardly known. We reviewed scientific literature on drained organic soils used for agriculture, depicting the origin of mursh formation and the specificity of murshes and mursh soils.
2. Specific Character of Organic Soils
Organic soils are classified as being significantly different from mineral soils [38,39]. This distinction results from the dominant role of organic matter in the soil mass. Organic matter consists of various proportions of plant remnants and amorphous humus, and is unstable in aerobic conditions. These proportions change during the evolution of these soils. The second difference is the high water content in organic soils, which in a natural state (before drainage) is close to the total water capacity, i.e., total porosity. Total porosity in peat soils is high and often exceeds 90%, whereas in mineral soils it is frequently in the range of 30–55% [40]. Therefore, organic soils can be treated as a specific water reservoir, which slows down the outflow of water from the catchment area [2,41].
Organic soils are rich in organic matter, commonly formed from peats, organic muds, and organic bottom lake deposits (detrital gyttja, sapropel). However, peat soils are the most common. In soil taxonomy and the WRB (World Reference Base for Soil Resources) classification, organic soils are referred to as Histosols. When classifying organic soils, the content of SOM or organic carbon, and the thickness of the organic layer are usually taken into account [16]. However, the criteria for these two parameters differ in international and national classification systems as well as within scientific disciplines. According to different scientific approaches, the minimum thickness of organic layer (peat) required to classify the soil as organic can range from 10 cm to 100 cm [15]. In general, organic soil is defined as a minimum of 30–40 cm in depth, and has an organic carbon content > 12–20%. The soil classifications [38,39] also take into account the clay content (which modifies the required organic matter content) and the consecutive days of water saturation.
Peat soils show considerable variability of properties, which depends primarily on their origin and the botanical composition of the organic material, and then on the admixture of mineral material [2,13,14,16]. Traditionally, depending on the geomorphological location, hydrological conditions (water supply), and chemical status (fertility), they are divided into raised (high-moor), transitional, and fen (low-moor) peats. This general division separates ombrotrophic bogs from minerotrophic fens. Intermediate properties are exhibited in transitional peats. In the temperate zone, e.g., in Central Europe, soils formed from low-moor peats predominate, and are mainly used for agriculture. Fens are located in river valleys, in terrestrialized lakes (post-lake and lake-side peatlands), and in local depressions of various origins (Figure 1). Fen types have different botanical compositions of peats, degrees of decomposition, contents of mineral admixtures (degree of silting), SOM contents and nutrient status [2,42,43,44,45].
3. Mursh-Forming Process
The drainage of peatlands initiates a series of processes that significantly change the natural soil structure and properties. In Poland, these changes were defined as mursh-forming process and the resulting soil material was termed mursh (also known as moorsh). The term mursh (in Polish mursz) originally meant the material formed during the process of rotten wood decaying in aerobic conditions. This word was used in Poland in the 19th century with various meanings, including an agricultural one: “the kind of dry sandy peat” [46]. In soil science, the term “mursh” was introduced by Miklaszewski [47], and then developed by Tomaszewski [48], who described the mursh-forming process as changes to peat after drainage. Tomaszewski [49] first accurately noticed that after peat drainage “(…) organic-mineral connections, with the predominance of highly fragmented organic matter over clayey mineral matter, are formed. The coagulated organic-mineral mass has, to some extent, colloidal properties, while in the period of dry weather, part of the coagulated organic-mineral mass becomes an irreversible colloid, loses its ability to peptize, hardens and, as a consequence, deteriorates the physicochemical and biological properties of the murshing peat mass”. He further stated that the humus of mursh had worse physicochemical and biological properties compared to the humus of chernozems.
For a long time now researchers and users of drained organic soils have paid attention to the changes taking place in the soil mass after drainage. They were referred to as: metamorphosis [50,51], ripening [52], and weathering [53]. The need to distinguish organic soils transformed through drainage has been also raised by other researchers [54,55,56]. In the 1990s, the term “moorsh” was introduced as an English equivalent of the Polish word “mursz” [2,57]. For the first time the English notation “mursh” appeared in the title of the paper by Wondrausch [58]. Also in the literature, such terms as “secondary transformation” and “secondary humification”, which include changes in peat (mineralization and humification) after drainage, began to be used [59,60,61]. These terms refer to the changes in organic matter and are an analogue to the term “primary transformation”, which includes changes taking place during the accumulation of peat, and is expressed in “the degree of peat decomposition”.
The changes taking place in drained organic soils can be termed as soil development [62]. The soil material formed during mursh-forming processes in field conditions has an earthy appearance, similar to humus material of mineral soils. Hence the terms “earthing” and “earthification” were coined to describe this phenomenon [43,63]. In German research, the following soil development stages were identified: earthyfied fen (Erdfen), weak moorshyfied fen (Fenmulm), and moorsh (Mulm) [62]. In the soil classification of England and Wales [64], within peat soils, raw peat soils and earthy peat soils were separated, in which “drainage, with or without other ameliorative measures, has led to the formation of a fully ripened and humified earthy topsoil”. Earthy topsoil is defined as “a ripened (non-fluid) peaty surface layer that is at least 20 cm thick and contains less than 15 per cent of visible plant remains (fibers) other than resistant woody fragments. It overlies an organic subsurface horizon and normally has a distinct granular or subangular blocky structure”.
In the USA the term “muck” has been broadly used [65], and the process of its formation was termed “muck-forming process” or “mucking” [13]. However, the term “muck” mainly has a practical meaning—it covers a wide spectrum of agriculturally used, drained peat soils formed from highly decomposed peat (sapric). As provided by the definitions in dictionaries (Supplementary Material), a “muck” is a soil material containing highly decomposed plant residues, is darker in color than peat and/or contains more mineral particles. These definitions, however, do not take into account the changes taking place in SOM after drainage, i.e., the formation of a permanent granular structure with grains of various diameters, which fundamentally modify the properties, especially physical ones. The term “muck”, despite its widespread use, has not found much use in soil taxonomy [38].
In Polish terms, mursh-forming processes occur in drained peat soils (and other soils rich in organic matter, i.e., organic mud soils, detrital gyttjas) following changes in their water conditions. A major change that occurs after drainage is the disappearance of peat’s biomorphic structure and the development of the fine single-grain structure. During mursh formation, humus substances are transformed due to repeated cycles of drying and wetting or thawing. These structural changes can be described as the pedogenesis of organic soils after drainage. Then, a coagulation of humus substances and the formation of hard refractory granular aggregates occurs. Mursh granular aggregates have a lower volume of mesopores, which significantly affects the water conductivity and retention, and differs them from the crumb aggregates in the mollic horizon of mineral soils [39]. Granular mursh aggregates are hydrophobic, especially when they are dry during summer droughts, and consequently do not absorb water. Hydrophobicity negatively affects the water conditions in the entire soil profile [66,67,68], which is particularly important in agricultural areas, because this process is irreversible (newly formed aggregates have high hydrophobicity and durability even after re-rewetting, therefore, distinguishing mursh soil material seems to be justified).
In recent years, the concept of the mursh-forming process as a genetic soil-forming process [36] has been reflected in the WRB soil classification system as the principal qualifier “Murshic”. In the new edition of the WRB [39], among the 22 principal qualifiers concerning the Reference Soil Group “Histosols”, “Murshic” and “Drainic” occur interchangeably, the first of which describes structural changes in soil, and the second describes general conditions of drainage. Additionally, among the 28 supplementary qualifiers, newly added is the term “Mulmic” describing mineral soil material ≥ 10 cm thick, that has developed from water-saturated organic material after drainage. It has an organic carbon (OC) content in the range of 8–20%, and has a single grain structure or a subangular or angular blocky structure with an aggregate size of ≤2 cm. In the WRB [39] there are no qualifiers that would concern further degradation changes and decreasing amounts of OC below 8%. The mulmic supplementary qualifier also applies to the Reference Soil Group of gleysols and phaeozems. This supplementary qualifier enables identification of drained organic soils in which the amount of OC was reduced. In the Polish Soil Classification [69], mulmic material corresponds to murshic material with a lower OC and to some semimurshic materials with a higher OC.
The first comprehensive description of the mursh-forming process (the general theory of the mursh-forming process) was presented in the thesis of Henryk Okruszko [42] Muck soils of valley peatlands, and their chemical and physical properties. Okruszko [2,42,57], as well as Okruszko and Ilnicki [36], described the morphology of the peat-mursh soil profile, distinguishing the surface mursh horizon (M) and peat horizon (T). The mursh horizon could be divided into three layers: M1, M2 and M3. The M1 layer is a sod or turf layer consisting of organic particles (grains). Its structure changes from amorphous through granular to fine-grained (silt-like), and two types of soil aggregates can be identified: (i) 5–10 mm in diameter, mostly hydrophobic, porous, and often made of worm casts, (ii) particles not exceeding 4 mm in diameter, highly susceptible to wind erosion, and breaking down easily into smaller fragments. In the case of strongly transformed, degraded soils, aggregates of diameter of 1–2 mm predominate in this horizon. As a result of humus mineralization, the aggregates break down into increasingly fine grains, and the mineral fraction of sand and silt is released [70] (Figure 2).
The M2 layer is composed of loosely stacked (arranged) particles with a diameter of 2–10 mm and sharp edges. Soil aggregates are almost exclusively made of compacted humus. The size of soil aggregates gradually increases down the soil profile. The M3 layer is the transition between mursh and the peat beneath. This layer has unaltered cohesion, which disintegrates under the pressure into larger aggregates with sharp edges (diameter more than 20 mm). Peat occurs inside the aggregates and is surrounded by amorphous humus. The decrease in the size of aggregates with the ongoing process of mursh-formation is typical: from coarse blocky subangular or angular blocky to fine granular and coke-like. It is termed as the pulverization of murshes and is an advanced mursh-forming process. The diameter of aggregates in mursh soils can, therefore, be an easily measurable indicator of the progress of the mursh-forming process and soil quality. Researchers from Germany take a similar position on this matter [71,72]. Instead of recognizing the size of aggregates as an indicator, researchers are favoring the fact that in the assessment of the quality of mineral soils, great importance is attached to their structure and their aggregate structure [73].
Based on the morphological diversity and chemical composition, the following types of murshes can be identified: (1) based on the progress of mursh-forming process: (a) peaty, (b) humic, and (c) grainy (proper); (2) based on mineral admixtures: (a) ferruginous, (b) calcareous, and (c) silted; (3) based on origin: (a) of peat origin, (b) of mud origin, (c) and of organic gyttja origin. However, it is also essential to recognize the murshes on the basis of the progress of pedogenic changes (degree of transformation) (Figure 2, Table 1). Peaty mursh can be treated as an initial material developed after drainage. Further drainage leads to the development of humic mursh, whereas grainy or proper mursh is typical of long and intensively drained peat soils. Sometimes within this type, so-called degraded mursh with very fine, hydrophobic soil aggregates can be distinguished. From the beginning of scientific interest in the process of mursh-formation in Poland, based on micromorphological methods [51,74], numerous forms of murshes: initial porous, initial compact, coprogenous, amorphous, detritic, proper, comminated, and root mursh were distinguished [75].
4. Organic (Peat) Soils in the Polish Soil Classification
The Polish Soil Classification (PSC) [69] defines organic soil materials as those which are saturated with water for at least 30 consecutive days in most years (or drained) and contain ≥12% OC. The adopted criterion of OC content results from the fact that in soils with a sandy mineral part and with ≥12% of OC, SOM constitutes more than half of the soil volume and affects its properties [40,76]. In the PSC [69] the SOM in organic soils starts within 30 cm from the surface and has a combined thickness of at least 30 cm within the 60 cm from the soil surface. This amount of organic matter makes the soil capable of delivering ecosystem services typical for organic soils. The murshic (Polish: murszik) horizon is an organic, surface soil horizon developed as a result of permanent drainage and the pedogenic transformation of primary organic materials (peat, organic mud, gyttja) [69]. It meets the following parameters: (i) consists of organic material (≥12% OC) meeting the criteria of sapric peat, (ii) in a drained state, has horizontal and/or vertical cracks and permanent pedogenic aggregate structure in ≥50% of the soil horizon volume, and (iii) has a thickness of ≥10 cm.
To classify a soil as mursh, the murshic horizon must have a thickness of ≥30 cm, and based on the origin, nature, and thickness of the organic horizon, the following subtypes of mursh soils are distinguished: mineral-mursh, fibric, hemic, sapric, gyttja, mud, and shallow mursh soils. In peat soils, when the murshic horizon is less than 30 cm thick, the subtype “mursh” is distinguished. The PSC [69] enables the identification of further evolutionary stages of drained organic soils, in which the arenimurshic horizon (Polish: arenimurszik) occurs. This is a surface mineral horizon containing <12% OC developed in sandy material that has the texture of sand or loamy sand. In this horizon, the humified organic matter does not form permanent connections with sand grains, and at least 10% of sand grains are devoid of humus sheaths. Aggregates of organic matter in a dry state can be easily separated from sand, e.g., by blowing.
In the PSC [69], the evolutionary sequence of soil horizons with decreasing organic matter content is the following: mursh (Polish: mursz), semimurshic (Polish: murszowaty), and postmurshic (Polish: murszasty). The semimurshic horizon contains 6–12% OC and the postmurshic horizon contains 1–6% OC. The postmurshic horizon is similar to other humus horizons of arable soils, however humus does not form permanent bonds with mineral sand grains and can be mechanically separated [70,74,77]. These horizons may be developed as a result of drainage and the pedogenic transformation of the surface horizons of gley soils, or as a result of advanced transformations of intensively drained organic soils, especially those with a small thickness of organic layer.
In the graphic soil profiles in Figure 3, the inflow of mineral matter to soils is shown. It is assumed that in organic soil, organic matter is formed mainly in situ by photosynthesis. The first soil profile represents a hemic peat soil [69], Rheic Hemic Histosol [39], which is wet organic soil with organic matter being accumulated, where the groundwater level is close to the surface. Profile 2 is a hemic murshic soil [69], Murshic Hemic Histosol [39], which is a drained soil where the groundwater level is dozens of centimeters below the surface. It is assumed that the ideal groundwater level for a production of dry matter in grassland used as fens lies between 40 and 50 cm below the surface [17]. However, in practice, the groundwater level is often in the range of 70–80 cm below the surface during the growing season, due to intensive evaporation and a decreased summer groundwater level, which simultaneously presents irrigation difficulties. Further evolution of drained soils (profile 3) with intensive, long-term drainage leads to the loss of organic matter, reduction in the thickness of the organic layer, and subsidence of the peatland surface [12]. The PSC classifies such a soil a thin murshic soil [69], and the WRB lists Histic Gleysol (arenic, drainic) as an example [39]. According to the WRB classification, this soil no longer belongs to the Reference Soil Group of Histosols, but to gleysols. When the groundwater level is permanently deep (profiles 4 and 5), further decreases of organic matter occur and the organic layer becomes shallower, and is mixed with mineral subsoil during tillage, which is referred to as peatland disappearance [78].
5. Transformations of Organic Matter after Drainage
The effects of the transformation of organic matter during the mursh-forming process have been studied since the second half of the 20th century [2,5,42,79,80,81,82,83,84,85,86,87,88,89,90,91]. After drainage of the organic soil, oxidation of the SOM occurs, the quantity of soil carbon decreases, and the ash content increases. Organic matter undergoes microbial decomposition leading to CO2 and N2O emission to the atmosphere, as well as the production and increased availability of DOC and DON (dissolved organic carbon and nitrogen) in waters. Consequently, the loss of organic matter occurs, and the peatland subsides.
Peat soils in fens contain large amounts of nitrogen. Characteristic is the increase of total nitrogen in murshes (usually by 0.5–1.0%), and the decrease of organic carbon, which reduces the C:N ratio. This ratio is 16.3–17.8 in peats, and lower in murshes—12.1–12.5 (Table 2). Therefore, the C:N ratio can be treated as a simple indicator of the progress of the mursh-forming process [10,92]. The accumulation of nitrogen in murshes may be higher by 30–35% than in the peats from which they develop [2]. The mursh-forming process decomposes peat and delivers higher nitrogen availability. The transformation of nitrogen compounds depends on peat type and habitat conditions (mainly the water levels), and the type of vegetation [93]. In peats, nitrogen occurs in organic compounds in plant remnants, whereas in murshes it is mostly from fulvic acids and hydrolyzing compounds (hemicellulose and cellulose) [79]. As a result of mineralization, organic nitrogen compounds are transformed into mineral forms (N-NH4, N-NO3) and may be uptaken by plants. Due to nitrification processes, nitrous oxide is emitted to the atmosphere. Nitrogen compounds are accumulated in surface layers and bound by humus substances [2,35,44].
Okruszko et al. [94] found that the peat-mursh soils used for agriculture as grasslands (with groundwater levels during the growing season of 70–90 cm) released 574.1 kg of nitrogen per ha annually; 204.9 kg from this amount was uptaken by grassland vegetation and collected hay, and the remaining 369.2 kg could theoretically remain in soil organic matter. However, the authors stated that 63.4 kg of nitrogen were lost annually (washed into groundwater or emitted as N2O). In Poland, the amount of nitrogen released annually as a result of mineralization of organic matter of drained organic soils varies widely (60–400 kg ha−1). However, frequently, especially in deeply drained alder peats (rich in nitrogen), the amount of released nitrogen reaches 800–1000 kg ha−1, which is significantly higher than the amount that can be taken up by intensively managed grassland (300–400 kg ha−1) [2]. Since not all of the remaining nitrogen is fixed in humic compounds, it causes a threat to the environment (groundwater eutrophication, greenhouse effect).
Table 2General characteristics of soil materials.
| Property, Unit | Sedge-Reed Peat 25–30 cm | Humous Mursh 5–10 cm | Alder-Wood Peat 55–60 cm | Proper Mursh 15–20 cm | Calcareous Mursh |
Ferruginous Mursh |
Mursh Of Mud Origin | Sandy Mursh | Semimurshic Material | Postmurshic Material |
|---|---|---|---|---|---|---|---|---|---|---|
| Source | [95] | [96] | [97] | [98] supplemented | [99] supplemented | |||||
| LOI, % | 89.9 | 85.3 | 79.3 | 72.0 | 43.9 | 32.1 | 51.4 | 39.6 | 13.9 | 5.8 |
| BD, Mg m−3 | 0.167 | 0.249 | 0.145 | 0.367 | 0.580 | n.d. | 0.757 | 0.540 | 0.951 | 1.211 |
| TP, % | 89.3 | 84.5 | 91.3 | 79.1 | 72.9 | n.d. | 68.0 | 74.8 | 60.6 | 51.3 |
| OC, % | 55.98 | 54.12 | 63.72 | 58.07 | 21.83 | 7.42 | 24.03 | 21.37 | 6.67 | 2.73 |
| TN, % | 3.15 | 4.47 | 3.91 | 4.64 | 1.58 | 0.47 | 2.00 | 1.43 | 0.53 | 0.25 |
| C:N | 17.8 | 12.1 | 16.3 | 12.5 | 13.8 | 15.8 | 12.0 | 15.1 | 12.6 | 11.0 |
| pHH2O | 6.1 | 5.4 | 6.0 | 5.4 | 7.6 | 6.0 | 6.0 | 6.4 | 6.2 | 5.8 |
| pHKCl | 5.5 | 4.9 | 5.3 | 5.0 | 7.3 | 5.0 | 5.3 | 5.7 | 5.3 | 4.9 |
| CEC, cmol(+) kg−1 | 174.5 | 190.7 | 146.1 | 153.9 | n.d. | 72.4 | 99.7 | 105.0 | 48.4 | 19.1 |
| BS, % | 68.3 | 42.2 | 77.7 | 63.8 | n.d. | 68.3 | 65.4 | 64.7 | 54.6 | 55.7 |
| CaCO3, % | 0.0 | 0.0 | 0.0 | 0.0 | 30.5 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 |
| Catot, g kg−1 | 33.3 | 32.0 | 22.4 | 30.3 | 238.9 | 17.7 | 42.6 | 10.9 | 3.2 | 1.6 |
| CaHCl, g kg−1 | n.d. | n.d. | n.d. | n.d. | n.d. | 16.80 | 15.36 | 9.50 | 2.80 | 1.40 |
| Fetot, g kg−1 | 20.1 | 38.7 | 11.5 | 44.5 | 17.8 | 346.2 | 40.5 | 32.7 | 12.1 | 9.8 |
| FeHCl, g kg−1 | 16.5 | 32.0 | 4.5 | 14.3 | n.d. | 56.1 | 23.3 | 17.1 | 4.7 | 3.8 |
| Ptot, g kg−1 | 0.5 | 1.0 | 0.5 | 2.4 | 1.3 | 0.88 | 1.41 | 1.20 | 0.8 | 0.4 |
| PHCl, g kg−1 | n.d. | n.d. | n.d. | n.d. | n.d. | 0.26 | 0.36 | 0.30 | 0.15 | 0.09 |
| Mgtot, g kg−1 | 0.3 | 0.2 | 1.4 | 1.3 | 3.4 | 0.4 | 0.60 | 0.30 | 0.09 | 0.05 |
| MgHCl, g kg−1 | n.d. | n.d. | n.d. | n.d. | n.d. | 0.32 | 0.39 | 0.26 | 0.06 | 0.04 |
| Ktot, g kg−1 | 0.2 | 0.3 | 0.8 | 0.4 | 0.5 | 0.05 | 1.20 | 0.20 | 0.10 | 0.08 |
| KHCl, g kg−1 | n.d. | n.d. | n.d. | n.d. | n.d. | 0.02 | 0.30 | 0.13 | 0.06 | 0.05 |
Explanation: LOI—loss-on-ignition, BD—bulk density of dry soil, TP—total porosity, CEC—cation exchange capacity, BS—base saturation, CaCO3—calcium carbonate, determined according to the Scheibler’s method, tot—total content of elements, HCl—content of forms soluble in 0.5 M HCl extract.
The transformation of organic matter during the mursh-forming process is dynamic and depends on site conditions [34]. During the first phase after drainage, lasting several (or even dozens) years, intensive humification of SOM occurs, and some organic substances (cellulose, lignin, bitumen) are transformed to humus substances (first to fulvic and then to humic acids), and these new substances are enriched in oxygen functional groups, e.g., carboxylic, alcoholic, phenolic [100]. As a result, surface mursh horizons contain more humic compounds, which are traditionally divided into fulvic or humic acids. These compounds have different molecular weights, carbon and nitrogen contents, and stabilities, i.e., resistance to decomposition (humic acids have a resistance to decomposition than fulvic acids). In the first stages of plant decay, fulvic acids are formed and prevail [79], and in subsequent stages they are transformed (during condensation and polymerization) into other, more complex compounds (humic acids). However, in degraded murshes, the opposite process may occur, i.e., disintegration of humic acids to simple organic compounds. Therefore, it was assumed that the ratio of humic to fulvic acids may be a fairly specific index of the direction of changes taking place in drained organic soils [2]. In eutrophic sites, more fulvic acids are formed, whereas in the sites with lower trophism, polymerization and condensation of humic acids occurs [85]. Wójciak [85] also reported that in murshes formed from moss and sedge peats, humic acids prevailed and the ratio of humic acids to fulvic acids in some soils exceeded 3:1. In rush and alder peats, which are typical for eutrophic sites, the ratio of humic acids to fulvic acids was approximately 1:1. Kalisz et al. [87,88] stated that ratio of humic acids to fulvic acids is higher in the first stage of organic matter transformation in mursh soils and lower when the process is advanced.
The mursh-forming process also contributed to the decrease of humins and the increase of hot-water extractable carbon (HWC), which is a labile fraction of carbon compounds corresponding with the biomass of microorganisms [88,101]. Therefore, hot-water-extractable organic matter (HWOM) may be a good indicator of changes occurring in drained organic soils [102]. Becher [103] reported that after drainage, organic soils contain more humic acids and labile carbon fractions. Moreover, in murshes, humic acids contain more hydrogen, nitrogen, and phosphorus, and less carbon; the particles of humic acids are less oxidized; and humic acids contain more hydrophobic factions, as compared to peats. Water soluble fulvic acids represent the most mobile humus fraction and, according to Leinweber et al. [104], their molecular chemical composition and thermal properties may be objective ecological indicators of land use and peat degradation.
Peatlands in their natural state store carbon and are the sinks for carbon dioxide (CO2), but are also sources of methane (CH4) [32]. Drainage of peat soil reduces CH4 emissions, but changes the peatland into a source of CO2 due to aerobic peat mineralization [31]. It is estimated that about 1 Pg year−1 of CO2 is emitted from drained peatlands globally, which is equivalent to 10% of the CO2 emissions from the entire agriculture, forestry and land use sectors [105]. The most important factors regulating CO2 emissions from drained organic soils are organic matter and air in the topsoil. The rate of peat mineralization and CO2 emissions depends on the peat type, groundwater level, and temperature [12,30]. The emission rate is the highest for peatlands developed from alder peats and the lowest developed from Sphagnum peat. The release of CO2 changes seasonally in line with temperature changes. Therefore, the highest emissions are recorded between June and September. Oleszczuk et al. [30] also stated that peat mineralization and CO2 emission are most intense when the water level is 90 cm below the soil surface. Further lowering of the water results in drying of the upper peat layers, impeding peat mineralization, and reducing CO2 emission [17]. Rewetting (reswamping) of formerly drained peatlands may reduce CO2 emissions and restore the carbon sink function in peatlands [33].
6. Mineral Matter in Organic Soils after Drainage
In the evolution of drained organic soils, in terms of the influx and transformation of mineral substances (Figure 3), the influx of nutrients with atmospheric water is more or less constant over time and relatively insignificant compared to other sources. Also, the inflow of aeolian deposits is of less influence [106]. On the other hand, in undrained, wet organic soils, the influx of mineral matter together with the water supply (surface water inflow, spring groundwater inflow) is important. The content of mineral components in the waters flowing into the fens or transitional mires are very diverse, which is reflected in the vegetation and the nature of the accumulated peat. In general, the chemical composition of the mineral substrate as a result of biogeochemical cycles affects the nutrient content in the surface layers [107]. This relationship is stronger the shallower the peat deposit is. Drainage eliminates surface inflow, and a new source of mineral matter appears in the form of capillary infiltration of groundwater. Mineral compounds are released during the mineralization of organic matter. Changing the air and water conditions initiates the transformation of mineral compounds towards oxygenated forms. In case of agricultural use, there may also be an influx of minerals along with fertilizers and pesticides. Intensive mixing of soil components takes place and the share of mineral components increases, and organic matter is reduced as a result of mineralization. In case of shallow organic soils, mechanical tillage and mixing of soil surface layers with mineral subsoil is important. Plowing significantly increases the aeration of soil, and accelerates decomposition of organic matter [51]. Further intensive drainage eliminates the inflow of nutrients by capillary rise, because the groundwater level is too deep. This is particularly important in organic soils developed on a sandy subsoil, which is frequent in Poland. Mineral compounds can be leached out of the root zone, and the uptake by cultivated plants leads to soil impoverishment of nutrients and soil acidification. Further depletion of SOM leads to a change in the position of soils in the soil classification systems (from organic soil types to mineral ones).
The assessment of plant-available nutrients in organic and mineral-organic soils (3–20% SOM) used as meadows, is possible in an extract of 0.5 M HCl [108,109], which turned out to be particularly useful for determining phosphorus fertilization needs at meadows located on drained peatlands [110]. The 0.5 M HCl enables extraction of mainly freshly formed (precipitated) mineral compounds, and to a lesser extent elements associated with humus. According to various authors [95,106,111,112], 74–89% of calcium, 25–71% of magnesium, 30–96% of iron, 29–75% of aluminum, 35–67% of potassium, and 13–38% phosphorus are extracted using 0.5 M HCl. The studies of Okruszko [2,113,114] revealed that the solubility, and thus the availability of chemical compounds for plants, decreases with the progress of the mursh-forming process, which is related to their aging, i.e., the transition from amorphous (colloidal) to crystalline forms.
The most important change concerns the loss of SOM, as it is 5–7% lower in murshes than in the peats lying below them. This is associated with an increase in bulk density, which in murshes is greater than 0.2 Mg m−3, and a decrease in total porosity (Table 2). The decomposition (mineralization) of SOM is associated with, among others, secretion of large amounts of H+, which changes soil reaction (decreasing the pH), as well as base saturation. This leads to changes in the sorption complex. In the first stage after drainage, the value of cation exchange capacity (CEC) increases, because humus acids formed in large quantities show greater cation sorbing capacity than the plant tissues present in peat. However, during the mursh-forming process, the CEC value decreases, which is associated to an increased share of hydrogen and transformations of the organic sorption complex. For example, the CEC in peat amounts to 157.8 cmol (+) kg−1, whereas in murshes developed from them it is merely 137.1 cmol (+) kg−1. The average CEC of mursh in the soil profile was only 86.9% of the capacity of the underlying peats [115]. High CEC in organic soils may lead to strong binding of cations and, for example, deficiency of copper to plants [116,117].
The chemical composition of peat, including additions of mineral alluvial and colluvial sediments, as well as content of Fe, Ca, Mg, and P, determines the formation of humus-mineral and humus-metallic bondings [2,80,85,87,95]. In murshes, iron hydroxide forms complex bondings with organic compounds [82,84,95]. Mineral-organic bondings may be formed during various stages of organic matter transformation [42]. In Poland, as early as the 1950s, studies of mineral-humus connections in drained organic soils were undertaken using physical fractionation in heavy fluids [70,82,118]. The developed method [118] included the separation of four fractions with a specific density (g cm−3): >2.88, 2.88–1.94, 1.94–1.59, and <1.59. Higher content of the heavy fraction indicates that the evolution of soils after drainage will move towards black earths (with the mollic horizon), in which organic matter is stable and humus is permanently connected with the mineral fraction. The research carried out by Kalisz et al. [87,119] and Długosz et al. [120] indicate that the addition of a fine-grained mineral fraction (silt, clay) has a positive effect on the stability of organic matter in drained peatlands.
Of all the metallic elements, organic soils contain the largest amounts of calcium and iron (Table 2). The amounts of calcium in murshes are related to Ca release during peat decomposition. However, Ca is displaced by hydrogen in the sorption complex during mursh formation, and also leached down the soil profile, therefore it is rarely accumulated in the topsoil (murshes). The Ca content in murshes correlates with the content in peats and depends on local geochemical conditions and water flow. In the first period after drainage, the Ca content may be even higher in murshes than in peats. However, with the progress of the mursh-forming process, Ca decreases, which is referred to as their decalcitation. The annual Ca leaching from drained organic soils may reach 260–840 kg ha−1 [2,95]. During mursh-formation, iron accumulates (primary it comes from the decomposition of peat plant remains), providing 433 kg Fe2O3 ha−1 annually [81]. However, the main source of iron comes from water capillary rise, which provides Fe2+, later oxidized to Fe3+ in the aeration zone. Murshes contain mainly “free” (Fe in easily soluble or colloidal compounds) and “bound” (Fe bound humus) iron forms [95].
In some soils a particularly high accumulation of calcium or iron in murshes occurs, which justifies the need to distinguish carbonate and ferruginous murshes. Liwski et al. [121], on the basis of total content of calcium and iron distinguished, calcareous murshes (over 4.3% Ca) and ferruginous murshes (over 4.2% Fe). The iron content in ferruginous murshes is usually 7–10.5% (up to 14–17.5%) [2]. In carbonate murshes, the accumulation of calcium carbonate had taken place before peat drainage, and in peats it originated from groundwater or surface water. Such murshes were developed from peats lying on limestone in river valleys at loess areas [122], or from carbonate spring formations, or from shallow peats lying on carbonate lake sediments [96]. Carbonate murshes have higher content of magnesium, and their sorption properties result from the predominance of calcium and magnesium cations. Other chemical characteristics of these soil formations are primarily determined by the properties of the peats from which they originated, and the SOM content. Both calcium and iron can also precipitate above the groundwater level in drained organic soils. Under specific conditions, a meadow lime horizon and/or a layer of iron ore deposited directly below the shallow organic horizon may be formed. Such accumulation is related to the discontinuity of water capillary rise between the mineral subsoil (usually sandy) and organic topsoil, and can also impair the physical properties of soils. In the ferruginous concretions that build iron ores, the content of elemental iron can reach up to 28% [118]. An increased contents of calcium carbonate or iron compounds are associated with a lower SOM content, which modifies soil properties. Ferruginous murshes may have a higher content of phosphorus, while other properties remain similar to murshes with similar SOM content. If the mursh contains vivianite, the peat-mursh soil does not require P fertilization. Unlike accumulation of iron compounds, vivianite precipitation occurs only during peat formation. The content of phosphorus extracted with 0.5 M HCl in murshes with vivianite is in the range of 870–1300 mg P kg−1, but may reach 2200 mg P kg−1 [2].
Generally murshes, apart from ferruginous and calcareous murshes (Table 2), contain similar amounts of iron and calcium. However, during long term drainage, iron prevails, and calcium is leached out. The presence of large amounts of chemically active (reactive) calcium compounds (also magnesium to some extent) and iron (also aluminum to some extent) in drained organic soils affect, among others, the solubility and availability of phosphorus for plants [111,113,114,123,124]. Considering the above, mursh soils can be classified as either of the lime type (pedocal) or the sesquioxide type (pedalfer), but most often they belong to the mixed type (calalfer) [113].
Phosphorus accumulates as a result of the mursh-forming process. Therefore, its content is usually higher in murshes than in peats. The mursh-forming process leads to the release of phosphorus from peat and increases its availability to plants [113,114]. The share of phosphorus in mineral forms in its total content increases in murshes (33–35%) in relation to peat (26–28%) [112]. In murshes, phosphorus is present in different proportions and various combinations: with humus (approx. 60–90% of total P), iron, and calcium. The complexation of iron and aluminum with humus compounds reduces the P sorption capacity, which means that phosphorus in murshes can occur in forms available to plants [95]. However, as a result of the long-term drainage at agricultural areas, phosphorus-iron compounds increasingly gain in solidity and their availability for plants diminishes [2].
Typical for peats and the murshes is the scarcity of potassium [13]. The process of mursh formation does not significantly affect the distribution of potassium in the soil profile [2]. The content of magnesium in organic soils is several to dozens times lower than that of calcium and behaves similarly to calcium. Aluminum, on the other hand, behaves similarly to iron and its content increases in murshes, which is caused by its sorption by colloidal organic substances [2]. Piaścik [95] indicated that the total aluminum content in mursh soils is 3–15 times lower than that of iron, and mursh horizons contain 1.5–3 times more Fe than the underlying peat.
The calculated coefficients of enrichment (for unfertilized soils) in mineral elements in the surface (mursh) layer (0–5 cm) in relations to underlying peat (45–50 cm) proves the above statements [125]: 4 for aluminum and zinc, 3–4 for potassium, 2–3 for manganese and cobalt, 1–2 for magnesium, iron, chromium, and copper. However, for calcium it is only 0.6, which suggests leaching of Ca in drained organic soils.
The properties of murshes can also be modified by the addition of a mineral clastic fraction and, consequently, mud-derived murshes and sandy murshes usually found in river valleys can be distinguished [98,120] (Table 2). A significant addition of mineral particles decreases SOM content, and modifies soil physical (bulk density increases, total porosity decreases) and chemical properties. Generally, murshes developed from organic muds contain more potassium, which results from the presence of K in clay minerals. Chemical properties of sandy murshes are influenced by the decreasing SOM and significant addition of sand and silt. These murshes are a transition to semimurshic and postmurshic soil materials.
7. Conclusions
The changes taking place in drained organic soils used for agriculture were described. The alterations caused by the mursh-forming process leads to the changes in soil morphology, soil physical properties (including water retention capability), physicochemical properties, and chemical and biological properties. The mursh-forming process is a soil genetic process occurring in drained (artificially or naturally) organic soil. Soil material (mursh) formed in the surface layer differs significantly from the original soil material (peat), and when agriculturally used can also be termed muck [126,127,128,129] (however, this term has not been included in the soil classification systems). Changes in the structure of organic soils due to drainage are irreversible, and the formed aggregates generally show high hydrophobicity and durability even after reswamping, therefore, distinguishing mursh soil material seems to be justified. The mursh-forming process is inherently related to the mineralization of soil organic matter and leads to a reduction of organic carbon in soil. The compounds released during this process are transformed from organic forms to mineral ones, thus becoming available to plants. However, in this form, they can be easily leached into groundwater, causing their eutrophication. At the same time, large amounts of CO2 are released, increasing the pool of this greenhouse gas in the atmosphere. Mursh has many unfavorable properties in terms of agriculture and environmental management. These properties are mainly related to decreased water storage capacity, which significantly limits the hydrological function of organic soils. There are trade-offs between agricultural use of organic soils and ecosystem services. It is important to undertake the measures to mitigate negative changes in agricultural organic soils, and where possible, rewet the peatlands.
The studies carried out so far have provided considerable information, crucial from the theoretical and applicable point of view, which is of great importance in agricultural management. However, there are still some aspects that require further research. The most important of them are:
Development of rules for monitoring the distribution of organic soils and the state of their transformation (degradation) as a result of progressive drainage, using field studies and remote techniques.
Development of ways to limit (reduce and mitigate) SOM mineralization, greenhouse gas emissions, and disappearance of organic soils from agriculturally used areas.
Determination of the effects of rewetting on soil processes and properties of murshes.
Determination of the influence of the mineral component or subsoil on the properties and directions of evolution of drained organic soils.
Research on the impact of transformation of organic and mineral components of drained organic soils on their physical properties, including structure, hydrophobicity, and water retention capacity.
Research on the effect of mursh age on their physicochemical properties, including the availability of nutrients for plants.
Conceptualization, A.Ł., B.K., P.S., B.S. and J.N.; Validation, A.Ł., B.K., P.S., B.S. and J.N.; Writing—Original Draft Preparation, A.Ł., B.K., P.S., B.S. and J.N.; Writing—Review & Editing, A.Ł., B.K., P.S., B.S. and J.N.; Visualization, A.Ł., B.K. and P.S.; Funding Acquisition, A.Ł., B.K. and P.S. All authors have read and agreed to the published version of the manuscript.
Not applicable.
The data presented in the paper are available on request from the corresponding author.
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.
Footnotes
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Enlarge this image.Figure 1. Differences between natural and drained peatlands used for agriculture: (A)—natural fen peatland covered by peat-forming vegetation (rushes of sedge and reed); (B)—meadow on drained fen peatland; (C)—sedge rushes and alder forest on natural low peatland; (D)—drainage ditches on peatland use as meadow; (E)—profile of undrained peat soil (top layer—acrotelm abundant in roots); (F)—profile of drained peat soil (uppermost mursh layer).
Enlarge this image.Figure 2. Examples of soil materials studied; (A)—reed peat (medium degree of decomposition—hemic), 51.6% OC, visible white epiderms of Phragmites australis; (B)—sedge peat (weakly decomposed—fibric), 56.0% OC, visible rootlets of Carex sp.; (C)—proper (grainy) mursh, 48.3% OC, (field conditions); (D)—silted mursh, 26.8% OC (field conditions); (E)—calcareous mursh, 25.7% OC, 30.7% CaCO3 (field conditions); (F)—ferruginous mursh, 22.9% OC, 25.9% Fetot. (air-dried); (G)—degraded (grainy) mursh, 40.5% OC, visible aggregates of various size and mineral particles covered by iron oxides (air-dried); (H)—semimurshic soil material, 8.9% OC, dominance of whitish mineral particles of sand and silt size (air-dried).
Enlarge this image.Figure 3. Successive stages of organic soil transformation showing the inflow of mineral matter to soil; (I)—PSC (2019): Hemic peat soil (Gleba torfowa hemowa), WRB (2022): Rheic Hemic Histosol; (II)—PSC (2019): Hemic murshic soil (Gleba murszowa hemowa), WRB (2022): Murshic Hemic Histosol; (III)—PSC (2019): Thin murshic soil (Gleba murszowa płytka), WRB (2022): Histic Gleysol (Arenic, Drainic, Mulmic); (IV)—PSC (2019): Typical semimurshic soil (Gleba murszowata typowa), WRB (2022): Mollic Gleysol (Arenic, Drainic, Mulmic); (V)—PSC (2019): Postmurshic soil (Gleba murszasta), WRB (2022): Umbric Gleysol (Arenic, Drainic, Humic, Nechic).
Classification of murshes according to the degree of transformation of the mass taking into account its structure.
| Mursh Type | Degree of Transformation | Description of Mursh Mass |
|---|---|---|
| Peaty | Low | Loose structure, tendency to form aggregates cemented by humus; mursh contains fragmented remains of vegetation, is light, and does not get hands dirty |
| Humic | Moderate | Granular or cryptogranular structure, similar to cultivated soils, occasionally silty; mursh solidifies under pressure, crumbles into granules under light pressure, gets hands dirty with fresh humus |
| Grainy (proper) | High | Grainy or cryptograiny structure with visible shape of grains (grains size from several millimetres to 0.1 mm), sometimes firm, angular (degraded mursh); the mursh is loose, friable, does not get hands dirty |
Supplementary Materials
The following supporting information can be downloaded at:
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