Humus forms, carbon stock and properties of soil organic matter in forests formed on dry mineral soils in Latvia

Over the last decades, more attention has been paid to carbon accumulation in soil, more recently, to soil humus forms, as they indicate environmental conditions and state of soil organic matter. There is insufficient information on the impact of soil and forest type on the chemical properties of soil, soil organic matter and humus form. Knowledge about the chemical properties of humus is crucial for modelling C and N accumulation and storage in forest soils. On this account, the aim of this study was to char­ acterize soil humus forms, humus chemical properties and C stock and to determine the spatial distribution correlations between soil humus forms in forests formed on dry mineral soils. We studied humus forms in 44 sampling sites located in different types of forests. Soil samples were collected from genetic O and A (EA) horizons and analysed for organic carbon (CORG) and total nitrogen (NTOT) content and NaOH extractable organic matter. Generalized linear model analysis showed that the distribution of the psammomor and mor humus forms is related to oligotrophic forest types, while glaciogenic and glaciolimnic sediments constitute the main precondition for the occurrence of the mull humus form. The psammomor and mor humus forms have the lowest CORG stock in the topsoil, and more than 75% of the total CORG is accumulated in the O horizon. The mull humus form soils have the highest CORG stock in the min­ eral topsoil, accumulating 80% of the total topsoil CORG stock. The Ah horizons of the mull humus soils also have a significantly lower CHS­to­CORG ratio.


INTRODUCTION
Soil plays a key role in all terrestrial ecosystems, espe cially its most dynamic part, soil humus, which provides living organisms with structural elements and energy (Zanella et al. 2018d). The importance of soil humus has recently been stressed in a series of 'Humusica' publi cations on topics related to classification (Zanella et al. 2018c, ecology, organic matter decomposition (Zanella et al. 2018b), agriculture, human impact and environmental challenges (Zanella et al. 2018a. Studies on soil humus have resulted in common knowledge that its formation is a complex process in fluenced by several abiotic factors, such as climate, parent material, soil properties (Vesterdal 1999;Ponge et al., 2011), and biotic factors, the most significant of which are vegetation, plant communities, the quality and amount of litter (Peltier et al. 2001;Albers et al. 2004;Niemi et al. 2007) and the activity of soil micro and macroorganisms (Smolander & Kitunen 2002;Kanerva & Smolander 2007). The sum of all factors and the state of the ecosystem result in a soil humus form as the morpho logical feature of the organic and underlying organomineral horizons of the topsoil (Zanella et al. 2011).
Soil humus forms also indicate changes in soil pollution with heavy metals (Korkina & Vorobeichik 2016), and N variability and pathways in soil and eco system (Trap et al. 2011). Humus forms may be used as indicators to detect climate change and the impact of environmental pollution on forest ecosystems. However, wider and more common use of humus forms as an eco logical indicator is limited due to incomplete knowledge about their dependence on ecological condi tions and the impact of soil and forest types on the chemical properties of soil, soil organic matter and the humus form concerned. The description, classification and mapping of humus, along with information about its chemical properties, also constitute crucial knowledge for modelling C and N accumulation and storage in forest soils. This knowledge can also help evaluate the impact of land management and pollution on a forest ecosystem. That being the case, the aim of this study was (i) to characterize soil humus forms, humus chemical properties and soil organic carbon stock in forests formed on dry mineral soils and (ii) to determine the spatial distribution correlations between soil humus forms in the said forests.

Study sites
Research of humus forms by soil sampling was conducted in 44 sampling sites in different types of forests formed on dry mineral soils from 2009 to 2015. Twentyeight sampling sites were randomly established in the former European secondlevel forest monitoring programme sites. Additional 16 sampling sites were established in territories where the age of forests exceeds 60 (Fig. 1). Altogether seven sampling sites were established in each of the Cladinosa-callunosa, Vacciniosa, Myrtillosa, Hylocomiosa and Aegopodiosa forest types (Buss 1997), and nine sampling sites were established in the Oxalidosa forest type.

Sampling and analysis
The digging of soil profiles was performed in all sampling sites. Soil profiles were described according to Guidelines for Soil Description (FAO 2006). Soil classification was made according to the international FAO (IUSS Working Group WRB 2015).
During the field studies soil parent material and the dominant tree species of the forest stand in the sampling site were described. The forest site types were described according to the Latvian forest ecosystem classification (Buss 1997).
Topsoil was described according to the European Humus Form Reference Base (EHFRB; Zanella et al. 2011) humus form classification. The field works were carried out to determine properties of soil O and A horizons (thickness, structure, material, pH).
Soil samples were collected from genetic horizons of topsoil: from the organomineral (Ah, AEh, EAh) and organic (O, H) horizons. Mineral soil samples for deter mination of soil bulk density were collected with the core sampler (D = 3.5 cm). Mass of the litter (organic O) horizon was calculated from samples collected with the metal frame (20 cm × 20 cm).
Airdried soil samples were sieved through a 2mm sieve and prepared for physical and chemical analyses. The soil particle size was determined by pipette analysis; before analysis samples were treated with 0.1 M NaOH (van Reeuwijk 1995). The soil pH H2O was measured in water suspension with a glass electrode pHmeter WTW inoLab (10 g soil sample to 50 mL water) (Burt 2004). The total organic carbon (C ORG , %) was determined using a total carbon analyser Shimadzu TOCVcsn solid sample module. The total nitrogen (N TOT , %) was determined using the modified Kjeldahl method (ISO:11261, 2002).
Humic substances (HS) from soil were extracted by using procedures recommended by the International Humic Substances Society (Tan 2005). Alkaline extracts were diluted with deionized water (1:100 volumetovolume ratio). Obtained solutions were analysed for carbon content in the humic substances (C HS , %). Fluorescence emission spectroscopy was used to determine the organic matter humification index (HIX). Emission spectra for all aqueous solutions were recorded (scan speed 500 nm/min, excitation λ = 350 nm, slit width 10 nm, wavelength range from 380 to 650 nm) with Elmer Fluorescence Spectrometer LS 55. The HIX was calculated as fluorescence intensity ratio at 510 to 460 nm wavelength (Kalbitz et al. 1999;Kalbitz & Geyer 2001). For humic acid (HA) purification, 99 mL of HS alkaline extract was acidified with HCl (37.2% HCl to H 2 O = 1:2) solution to pH<2. The precipitated HA from fulvic acids (FA) were removed by filtration. The FA solution was analysed with the Shimadzu TOCVcsn analyser for carbon content in FA (C FA ). The carbon content of HA was calculated by the formula C HA = C HS -C FA . The C HS /C ORG ratio was calculated from C content in HS (C HS , %) and C content in soil (C ORG , %); the C HA /C FA ratio was calculated from C content in HA (C HA , %) and C content in FA (C FA , %) (Martin et al. 1998); the C/N ratio was obtained from organic carbon (C ORG , %) and total nitrogen (N TOT , %) content in soil (Carter & Gregorich 2007). A generalized linear model (GLM) analysis was carried out to determine the correlations between soil humus forms and environmental factors (Quinn & Keough 2002). Binary values were added for all factor types (0 -not observed in the sampling site; 1 -observed in the sampling site). The GLM in R 2.11.1 software was used to investigate the correlations (the level of significance p < 0.05) between humus form spatial distribution and forest type, dominant tree species and soil parent material.

Statistical analysis
Physical and chemical properties of topsoil were included in the data statistical analysis. Statistical analyses (arithmetic mean values, standard deviation, ratios) were calculated using Microsoft Excel 2016 software.
Analysis of variance (Oneway ANOVA) was performed to compare properties (C ORG , N TOT , C/N, C HS , C HS /C ORG , HIX, C t ha -1 , pH H2O ) of soil O and A (EA) horizons between different humus forms. The significance of the differences was determined with Tukey's HSD post hoc test (α = 0.05). Calculations were performed using SPSS PASW Statistics 18 software.

Properties and forms of humus in forests formed on dry mineral soils in Latvia
The topsoils in the studied forests have developed different morphological properties. A humus layer with a thin O horizon and underlying AE (E) horizons has formed in the Cladinosa-callunosa forests. The mean thickness of the humus layer there is 9.8 cm ( Table 1). The mean humus layer thickness of the soil in the Oxalidosa and Aegopodiosa forest types exceeds 20 cm. According to the EHFRB humus form classification (Zanella et al. 2011), a mull humus form has formed in 17 of the 44 studied sites, moder humus -in 8 forest soils, mor -in 10, while a psammomor humus form was characteristic of nine studied soils in oligotrophic forests.
Hylocomiosa forests have the highest variability of humus forms: there is mor humus in one, moder humus in two and mull humus in four of the Hylocomiosa forest sites ( Table 2). The Cladinosa-callunosa and Vacciniosa forest soils have the psammomor or mor humus forms, while the Myrtillosa forest soils have the mor or moder humus forms. There is no mor humus form in the nutrientrich forests. In the studied Oxalidosa forest sites, three soils have the moder and four soils have the mull humus forms. Soils in all studied Aegopodiosa forest sites have the mull humus form.
Using generalized linear models (GLMs), a significant correlation of spatial distribution was found between the Cladinosa-callunosa forest type and the psammomor humus form and between the Vacciniosa and Myrtillosa forest types and the mor humus form. At the same time, the spatial distribution of the moder humus form was not linked to a specific forest type (Table 2).
Geological conditions make another significant factor that defines the spatial distribution of soil humus forms. The GLMs show a significant correlation between marine sediments and the psammomor humus form and between glaciogenic deposits and the mull humus form (Table 3). Mor humus develops in soils on glaciofluvial and marine deposits, aeolian dunes and limnic material, and moder humus develops in soils on limnic material, glaci olacustrine and glaciogenic deposits, although none of these correlations are statistically significant.
More significant correlations between humus forms and soils are explained by the WRB reference soil group (Table 4). A GLM revealed a significant correlation between Arenosols and the formation of psammomor and mor humuses in Latvian forests. Luvisols account for the spatial distribution of the mull humus form. All sites of Stagnosols and Gleysols have the moder humus form, while sites of Planosols and Retisols have the mull humus form, although binary GLMs do not let us consider these correlations as significant.
The dominant tree species in a forest stand is a statistically less significant factor for the development of a particular humus form compared to the forest type, Quaternary deposits or WRB reference soil group (Table 5). Nevertheless, the mor or moder humus forms develop in spruce (Picea abies) and birch (Betula pendula) stand soils and mull humus develops in oak (Quercus robur) stand soils. The GLM analyses allow us to maintain that the development of the mull humus form is not associated with pine (Pinus sylvestris) forests.
The organic matter content and properties of mineral topsoil are largely dependent on soil formation processes. The mineral topsoil (E or EAh horizons) of the psammomor and mor humus soils have a lower C ORG content: the mean C ORG content does not exceed 0.91%. Organic matter in the soils with the moder and mull humus forms is accumulated in the Ah horizon. The mean C ORG content in the Ah horizons of soils with moder humus in the Myrtillosa type forests reaches 1.36%, while in the soils in the Myrtillosa type forests it is 4.4%. The mean C ORG content in the mull humus soils is 2.6% (Table 6).
Like with the C ORG content, the N TOT content in mineral soil depends mainly on soil formation processes. The N TOT content is less than 0.1% in the E horizons of soil with the psammomor humus form and varies from 0.03% to 0.18% in soil with mor humus. The moder and mull humus soils have more than 0.2% N TOT in the Ah horizon ( Table 6).
The mean C/N ratio in the mineral topsoil decreases in the following sequence: psammomor > mor > moder > mull. The mean values are 14.8 > 13.1 > 9.2 > 7.4, respectively.
According to the characteristics of the O horizon, statistically significant differences exist in the mull humus form soils, which have a significantly lower C ORG content in the O horizon than the mor humus form soils ( Table 6). The C/N ratio in the O horizon of the mull humus form soils is significantly lower than in all other humus form soils (Table 6).
In addition, no statistical differences among the studied humus form soils were found when comparing the mean HIX, C HS /C ORG and C HA /C FA values.

Organic carbon and total nitrogen stock in humus forms in forests formed on dry mineral soils in Latvia
The O horizon and mineral topsoil play an important role in the accumulation of organic matter in forest soils. The C ORG and N TOT stock in soil is dependent on the content of organic matter, litter mass and bulk density of mineral soil. Nutrientrich coniferous forests (Vacciniosa, Myrtillosa and Hylocomiosa) have a higher litter mass (Table 1). Soil bulk density in mineral topsoil varies from 0.86 to 1.16 g cm -1 .
The C ORG stock in the O and A (EA) horizons in the studied forest soils varies from 10.2 t ha -1 up to even 117.9 t ha -1 . The lowest C ORG stock in a soil humus form was found in the Cladinosa-callunosa forest type, while the stock of C ORG that a humus form can store in the Hylocomiosa forest type reached 73.9 t C ha -1 on average ( Fig. 2A). The Cladinosa-callunosa forests also have the lowest N TOT stock in a soil humus profile: it was just 0.35 t N ha -1 in one of the study sites located in pine (Pinus sylvestris) tree stands. The highest N TOT stock in a humus profile was found in the Oxalidosa and Aegopodiosa type forest soils: for example, the O and Ah horizons in the Oxalidosa birch (Betula pendula) forest soil contain 18.1 t N ha -1 , and the Aegopodiosa forest soil humus profile in a mixed oak (Quercus robur), birch (Betula pendula) and ash tree (Fraxinus sp.) stand contains 11.9 t N ha -1 .
A comparison of the mean C ORG stock in the humus profiles in different forest types shows the lowest humus C ORG stock in the Cladinosa-callunosa forests ( Fig. 2A). The Vacciniosa and Myrtillosa forest types have relatively similar mean C ORG stocks in the humus profile: 35.6 and 38.9 t C ha -1 , respectively.
The Hylocomiosa forests have the highest mean humus C ORG stock. However, they also have a high C ORG stock variability within the studied forests: soils with mull humus in a mixed oak (Quercus robur) and pine (Pinus sylvestris) forest store 28.5 t C ha -1 , whereas in the mor humus topsoil in a pine (Pinus sylvestris) tree stand it is 117.9 t C ha -1 . The O and Ah soil horizons in the Oxalidosa forest contain 64.2 t C ha -1 and in the Cladinosa-callunosa forests is significantly lower than the C ORG stock in the soils of the other studied forest types. The analysis also shows significant differences in the C ORG stock in the humus profile between the Vacciniosa and Oxalidosa forest types. A comparison of the mean humus C ORG stock in different humus forms shows that soils with psammomor humus have a significantly lower C ORG stock than the soils with other humus forms (Fig. 2B). The mean C ORG stock in the O and EAh (E) horizons of soils with psammomor humus is 19.3 t C ha -1 , in the mor humus soils 40.8 t C ha -1 , in the moder humus soils 61.6 t C ha -1 and in the mull humus soils 49.1 t C ha -1 .
A soil humus form impacts the C sequestration mechanisms in the soil. In the soils with the psammomor and mor humus forms, a significant amount of the total C ORG stock is accumulated in the O horizon (78.9% and 84.6% of the total C ORG stock in the psammomor and mor humus profiles, respectively). The O horizons of the moder humus soils accumulate on average 60.9% of the total C ORG stock (Fig. 2B). In the mull humus soils, a substantial share of the organic carbon accumulates in the mineral topsoil, where the mean C ORG stock in the Ah horizon is 38.8 t C ha -1 (80.4% of the total humus organic carbon stock, significantly higher compared to other humus form soils).
Compared to the organic carbon stock, the total nitrogen stock in the topsoil of the studied soils varies among forest types in a different way. The highest humus N TOT stock was detected in the Oxalidosa forest soils, where it reaches 9.53 t N ha -1 (Fig. 3A). The mean N TOT stock of 1.03 t N ha -1 in the Cladinosa-callunosa forest soils is significantly lower than in the Myrtillosa, Hylocomiosa, Oxalidosa and Aegopodiosa forest soils. Statistically significant differences were also identified between the Vacciniosa and Hylocomiosa, between the Vacciniosa and Oxalidosa and between the Myrtillosa and Oxalidosa forest soils.
A comparison of the mean nitrogen stock in different humus form soils shows significant differences among all studied humus form soils (Fig. 3B) In contrast to the organic carbon accumulation, the soil O horizon plays a less important role in the accumulation of total nitrogen in a humus profile. The O horizon accumulates 54.9% of the humus N TOT stock in the psammomor humus soils, 65.1% in the mor humus profile, 34.3% in the moder humus profile and just 11.6% in the O and Ah horizons of the mull humus soils (Fig. 3B).

Spatial distribution of soil humus forms
According to the humus form classification (Zanella et al. 2011), the psammomor, moder and mor humus forms develop in forests on dry mineral soils in Latvia. A forest type plays a significant role in the spatial distribution of soil humus forms. Psammomor humus develops in the Cladinosa-callunosa type forests, the mor humus forms are related to the Vacciniosa and Myrtillosa forest types, and mull humus is formed in the Oxalidosa and Aegopodiosa forest types (Table 2). These results coincide with studies in Estonia (Kõlli 2013) and show that a forest type is one of the major factors effective in the boreonemoral zone. At the same time, the formation of moder humus under different forest conditions, mainly in the Myrtillosa and Oxalidosa forests, indicates that other factors also influence the development of soil humus forms. Correlations between humus forms and geological conditions (Table 3), as well as studies on soil diversity in Latvia (Kasparinskis & Nikodemus 2012), show that geological conditions are a significant abiotic factor that determines the availability of nutrient elements in soil and soil reaction. Like in other studies, the parent material rich in base cations determines the formation of the mull humus forms, whereas the acidic parent material determines the formation of mor humus .
From all of the studied factors, the dominant tree species has the smallest impact on the spatial distribution of humus forms. Specific tree species do not determine the formation of a definite humus form. Based on the study results, we are able to say that the mull humus forms do not develop in the pine (Pinus sylvestris) forests and the formation of mor humus is less likely in a broadleaf forest. A similar conclusion, namely, that the formation of a humus form is influenced not by specific tree species but by the proportion of coniferous and broadleaf trees, has been made in other studies conducted in the boreal and boreonemoral zones (Vesterdal et al. 2008;Kõlli 2013;Labaz et al. 2014).
Overall, the results of the study demonstrate the need for further research to include more detailed and more precise information concerning the morphological, chemical and physical properties of soil and humus. In addition, soil humus forms should be described according to the second level (for example, eumull, mesomull, etc.) of the EHFRB classification system (Zanella et al. 2011).

Chemical properties of soil humus forms
The C ORG content in the O horizon of the studied soils varies from 15.3% to 46.7%. Similar differences, where the mull humus forms had a significantly lower C ORG compared to the moder and mor humus forms, were detected in a study in Poland (Labaz et al. 2014). In our study, these differences were caused by the low C ORG content (< 20%) in the O horizon in the oak (Quercus robur) stand soil. Other studies have not found significant differences in the C ORG content between oak and coniferous litter (Vesterdal 1999;Remy et al. 2016). Accordingly, these results indicate that litter sampling must be performed more precisely in broadleaf forests.
The mean C ORG content in the O horizons of the psammomor, mor and moder humus forms (Table 6) does not differ from the mean C ORG content of 37.1% in European forests (De Vos et al. 2015) and 28.6% in forest soils in Latvia (Bārdule et al. 2009).
The N TOT content in the O horizon of the psammomor and mor humus soils (Table 6) is lower than that found in other studies in Latvia (Terauda & Nikodemus 2006;Bārdule et al. 2009) and Europe (Korhonen et al. 2013). In moder and mull humus, however, it is quite close to that reflected in the results of other studies (Bārdule et al. 2009).
These differences in the C ORG and N TOT content also determine the differences in the C/N ratio in the O horizon. A statistical analysis shows that only in mull humus the C/N ratio is significantly lower than in all other humus form soils (Table 6). The C/N ratio in the O horizon of the mull humus forms is below 30 and corresponds to the ecological conditions described in the humus classification system (Zanella et al. 2011). Furthermore, the mean C/N ratio in the O horizon of the psammomor (C/N > 40) and moder humus forms (C/N = 30-40) corresponds to the classification, while the mean C/N ratio in mor humus is below 40. These differences may be related to the fact that some of the study sites are located not far from the city of Riga and may therefore be exposed to N deposition through atmospheric pollution (Hosseini Bai et al. 2015). Differences and variations in the litter C/N ratio have been corroborated in other studies in Europe. For example, the C/N ratio in the O horizons of mor humus in mountains in Poland is around 27 (Labaz et al. 2014), and the results of a study in Estonia show that the C/N ratio in the O horizon of moder humus may be around 20 (Kõlli 2013). These results urge us to suggest that even if litter is suitable for fast mineralization, a colder and wetter climate in the boreonemoral zone can slow down the decomposition process Bayranvand et al. 2017).
The C ORG content in mineral topsoil correlates with the N TOT content in the A (EA) horizons. The content of these elements is dependent on soil formation processes as well as on mechanisms whereby soil organic matter is transported to the mineral soil. The mean C ORG content in the EA horizons of psammomor and mor is less than 1%, while in the A horizons of the moder and mull soils it exceeds 2.5% (Table 6). Such a statistically significantly lower C ORG content in the psammomor and mor soils may be explained by intense podzolization. A fraction of the soil organic matter that includes C ORG is translocated to the deeper soil horizons (Grand & Lavkulich 2011;Freyerová & Šefrna 2014). The C ORG content reaches 4.4% in the moder humus soils in spruce (Picea abies) forests and 5.8% in the Ah horizon of the mull humus form in a mixed oak (Quercus robur) and birch (Betula pendula) forest. These results match the findings of other studies (Vesterdal 1999;Vesterdal et al. 2008).
The soil formation processes and the element cycle are also responsible for the differences in the chemical properties of the topsoil organic matter of humus forms. The psammomor and mor humus form soils have a significantly higher proportion of humic substances and a higher C HS /C ORG ratio in the mineral topsoil than the moder and mull humus form soils (Table 6). Humic substances form more than 70% of total organic carbon in the psammomor and mor humus topsoil, while the C HS /C ORG ratio is lower than 65% in the moder and mull humus soils. These differences are caused by different soil faunal activities. The psammomor and mor humus soils have low earthworm and other soil macrofauna activity. Because of this, the mainly soluble fraction of soil organic matter from the O horizon reaches the mineral topsoil (Qualls et al. 2003;Cerli et al. 2008). However, the earthworm activity is high in the mull humus form soils. As a result, non humified organic matter is mechanically brought into the Ah horizon of the soil (Muscolo et al. 2009).
The HS properties, C HA /C FA ratio and HIX are highly variable within the mineral topsoil of the studied soils. Although a comparison of the mean C HA /C FA ratio values does not show any significant differences among humus forms, there is still a higher proportion of the FA fraction in the mor humus topsoil than in other soils (Table 6). These results may be explained by slow litter turnover, which in some cases may reach seven years (Zanella et al. 2011). Such a slow process, when litter gradually goes through a full decomposition cycle, is favourable for the formation of soluble organic compounds (Fröberg et al. 2005;Kalbitz et al. 2006). A higher FA fraction in the psammomor, mor and moder humus mineral soils may also be related to coniferous litter that releases the FA fraction during the decomposition (Vaičys et al. 1996;Qualls et al. 2003). A high proportion of the HA fraction in the mull humus mineral topsoil is a result of more efficient humification in leaf litter (Zech & Kögel Knabner 1994) and stabilization of the humic fraction in organomineral complexes (Piccolo 1996).
In addition, no statistical differences among the studied humus form mineral topsoils were found when comparing the mean HIX values. These results may indicate that the factors affecting the humification pro cesses (soil moisture, litter quality, oxygen, micro organisms, etc.) are relatively similar in forests growing on dry mineral soil.

Organic carbon and total nitrogen stock
The C ORG and N TOT stock in soil is dependent on the content of organic matter, litter mass and bulk density of mineral soil. The research results show that the mean litter mass in the Vacciniosa and Myrtillosa forest types (Table 1) corresponds to the mean mass of the OFH horizon in European forests (De Vos et al. 2015). The mean litter mass in the mull humus soils of the studied Aegopodiosa forests is 3 t ha -1 higher than the calculated mean OL horizon mass in European forests. The greatest litter mass differences can be observed in the moder humus form soils in the Hylocomiosa and Oxalidosa forest types, where the O horizon mass exceeds 200 t ha -1 , which is twice as much as the mean OFH horizon mass in forests in Europe (De Vos et al. 2015). The high mass of the O horizon in Latvian forests is probably related to the climate conditions, as the characteristic excess of precipitation over evaporation impedes the decomposition of organic matter .
The bulk density of mineral topsoil (Table 2) in the Hylocomiosa, Oxalidosa and Aegopodiosa forests is slightly lower than the calculated mean soil bulk density in forests in Latvia (Bārdule et al. 2009). It does not differ significantly from the mean soil bulk density in European forests either (De Vos et al. 2015).
The calculated C ORG stock in the O horizon and in the mineral topsoil differs among the studied forest types and humus forms (Fig. 2). The significant differences in the C ORG stock in the O horizon among different humus forms indicate that a humus form must be considered when determining and forecasting a soil C ORG stock. A similar conclusion, i.e. that the humus form is the most significant factor in determining a litter C ORG stock, has also been drawn in Europeanscale studies (De Vos et al. 2015).
Unlike the litter C ORG stock, the studied C ORG stock in the mineral topsoil only partly supports the results of a regionalscale study in Europe (De Vos et al. 2015). According to De Vos et al. (2015), a soil humus form is a more significant factor than a forest type among numerous examined factors that impact C ORG stock in the topsoil. However, the results of our study revealed statistically significant differences in the topsoil C ORG stock in different humus forms as well as in different forest types. The mean C ORG stock in the psammomor humus form topsoil is significantly lower than in the studied mor, moder and mull humus form soils. Moreover, there are significant differences in the topsoil C ORG stock between the Cladinosa-callunosa and Vacciniosa forest types, between the Vacciniosa and Oxalidosa forest types and among the Myrtillosa, Hylocomiosa and Oxalidosa forest types.
In general, our results show a lower C ORG stock in the forest topsoil than the mean C ORG stock of forest soils found in the BioSoil study in Latvia (Bārdule et al. 2009), where the mean C ORG stock was calculated for all forest types, including forests formed on wet mineral soils and organic soils. Nevertheless, the calculated mean C ORG stock of 40.8 ha -1 in the studied mor humus is ap proximately twice as high as the values obtained in Estonia (Kõlli 2013). At the same time, the C ORG stock in mor humus is comparable to Estonian moder-mor humus. These differences may be related to differences in humus classification.
Mor and moder humus accumulates carbon mainly in the O horizon, where the mean C ORG stock was 34.6 and 37.1 t ha -1 , respectively (Fig. 2). These results are higher than the mean C ORG stock in the O horizon in European forests (De Vos et al. 2015). However, other studies report C ORG stocks in coniferous forests exceeding 30 t ha -1 (Cerli et al. 2008;Remy et al. 2016).
In the mull humus soils, organic carbon accumulates in the mineral topsoil. These results well correspond with other studies demonstrating that the mineral topsoil (the Ah horizon) accumulates 64-96% of the total organic carbon in a humus profile (Vesterdal et al. 2012;Kõlli 2013;Remy et al. 2016).
Significant differences recorded in the topsoil N TOT stock between the psammomor and mor humus forms, between the mor and moder humus forms and between the moder and mull humus forms (Fig. 3) indicate that a humus form is an important factor in determining the N TOT stock in the topsoil. The N TOT stocks in the humus forms in the studied forests formed on dry mineral soils in Latvia are similar to those in the soil humus cover in Estonian forests (Kõlli 2013).

CONCLUSIONS
Four humus forms, mor, moder, mull and psammomor (according to the EHFRB classification), develop in the forests formed on dry mineral soils in Latvia. The psammomor humus forms are distributed mainly in the Cladinosa-callunosa forest type, and the distribution of the mor humus forms is related to the Vacciniosa and Myrtillosa forest types. The glaciogenic and glaciolimnic sediments, as well as the nutrientrich sodcalcerous and sodgleyic soils are the main factors responsible for the occurrence of the mull humus form.
The psammomor humus form soils have the lowest C ORG stock in the topsoil (the O and A [EA] horizons). The mean topsoil C ORG stock in psammomor humus is two times lower than in the mor humus form soils. In the psammomor and mor humus form soils, more than 75% of the total C ORG stock is accumulated in the O horizon. The highest topsoil C ORG stock occurs in the moder humus form soils: its O horizon accumulates 61% of the total topsoil carbon stock on average. The mull humus form soils have the highest C ORG stock in the mineral topsoil, where the Ah horizon accumulates 80% of the total topsoil C ORG stock.
The differences in the litter composition and the nutrient cycle affect the properties of soil organic matter. The O horizon of the mull humus form soils has a significantly lower C/N ratio than that of the mor humus form soils. In addition, the proportion of humic substances in the mineral topsoil of the mull humus form soils is significantly lower than in other humus form soils.
Finally, it is worth noting that studies on soil humus forms can provide significant information about forest ecosystems. Further research should include humus forms in forests formed on moist and wet soils. Special attention should be paid to territories where humus forms are dependent on drainage.