Slope aspect and altitude effect on selected soil organic matter characteristics in Beskid Mountains forest soils

Soil plays a key role in carbon storage (Degórski 2005; Nabuurs et al. 2007; Tolunay 2011). It is estimated that the amount of carbon in the soil is 2–3 times higher than that in the aboveground parts of plants (Post et al. 1990; Fornara et al. 2011). The carbon stock in the topsoil is 11.3–126.3 tonnes C/ha (Dixon et al. 1994; Baritz et al. 2010). In forest ecosystems, woody vegetation together with the shrubbery layer contributes to a better maintenance of soil moisture (Wang et al. 2012); moreover, it provides organic matter in the form of organic remains from the aboveground parts of plants and root systems, which enrich the surface horizonof soil through humification and mineralisation processes (Bardgett and Wardle 2010). Accumulation volumes in the soil depend on the type of plant cover and organic matter supplied and on the rate of decomposition processes (Błońska et al. 2021). Natural or human-induced disturbances are associated with the loss of soil carbon (Degórski 2005; Shiels et al. 2006). Soil carbon and nitrogen accumulation outside the vegetation species composition is influenced by pH and soil texture (Fotyma et al. 1998). Additionally, the level of carbon and nitrogen contentsin the soil is influenced by factors such as slope aspect, climate and altitude above sea level (Crowther et al. 2016).

Mountain areas are characterised by a variety of climatic factors, which results in diverse vegetation. Temperature decreases with altitude, which reduces the efficiency of ecosystems (Zhu et al. 2018) and slows down the rate of organic matter decomposition (Parras-Alcántara et al. 2015; Egli and Poulenard 2016; Bardelli et al. 2017). According to Egli and Poulenard (2016), mountain soils are highly dynamic systems that may react sensitively to environmental changes. It is known that slope aspect influences the local microclimate, especially solar radiation intercepted by the slope orientation, there by affecting the biochemical processes in the soil (Barbosa et al. 2015). Research shows that north-facing slopes capture less radiation from the sun and therefore have lower temperatures and higher humidity compared to south-facing slopes (Sewerniak et al. 2017). Slope aspect can affect species composition of ground flora, and thus indirectly involves ali-mentation of different types of litterfall to topsoils of contrasting exposures (Jasińska et al.2019). Climate in general affects the activity and composition of micro-biota and mesobiota involved in organic matter decomposition (Ascher et al. 2012). According to Makoi and Ndakidemi (2008), climate impacts on the microbial biomass and abundance in the soil as well as on most of the enzymatic activities are dependent on the altitude. Enzymatic activity is used to assess the quality and fertility of soils and also to determine the nutrient cycling (Błońska et al. 2021). It correlates closely with soil condition so that changes occurring in the ecosystem can be captured. Carbon substrates and nitrogen are essential in enzymatic reactions (Piaszczyk et al. 2019; Lasota et al. 2020). Dehydrogenases are an integral part of intact cells and provide information about the biologically active microbial population in the soil. A decrease in bacterial and fungal biomass with increasing altitude was observed in a previous study (Margesin et al. 2009). Humus morphology strongly depends on the slope, altitude, climate, biological factors and species composition of forest stands (Zanella et al. 2011; Bayranvand et al. 2017). Łabaz et al. (2014) showed that the humus types were distinctly correlated to specific sets of environmental factors. In addition, the use of chemical properties such as alkaline cations is helpful in assessing soil quality (Brożek et al. 2011).

We hope that a better understanding of the mechanisms and factors influencing the dynamics of carbon and nitrogen in mountain forest soils will enable to predict these phenomena in the future. The present study aimed to determine how slope aspect and altitude above sea level influence carbon and nitrogen accumulation and dehydrogenases activity of forest soils. Forest soils with the same texture and vegetation along altitudinal climosequence were selected for the research. In the present study, we hypothesise that slope aspect and altitude influence the carbon and nitrogen accumulation in forest soils and dehydrogenases activity.

The analyses confirmed the importance of the location conditions, especially the altitude, and to a lesser extent, the slope aspect on the formation of C and N stocks in the surface horizons of mountain forest soils. The GLM analysis confirmed the importance of location in altitude gradient for C and N accumulation in forest soils. In the lowest positions at an altitude of 600 m a.s.l., the lowest C and N stocks were recorded, which increased at an altitude of 1000 m a.s.l. At the highest positions, i.e. at an altitude of 1200 m a.s.l., the stock of carbon and nitrogen decreased due to worsening thermal conditions and lower productivity of vegetation. Previous studies (Yang et al. 2008; Meier and Leuschner 2010; Wiesmeier et al. 2013; Wang et al. 2014).) have proved that the variation in the content of soil organic carbon is associated with a change in temperature and the occurrence of precipitation, which is due to the location in the height gradient. Due to a longer growing season, increase in production of biomass and soil respiration can be expected with decreasing forest altitude (Swetnam et al. 2017).In addition to climatic conditions, vegetation through supplied organic matter is important in shaping C stock in forest soils (Baldrian and Šnajdr 2011; Błońska et al. 2016). Trees provide carbon substrates and nutrients to the soil through the fall of litter and root systems (Błońska et al. 2016, 2021). Lower altitudes have more favourable thermal conditions, which result in a faster rate of decomposition of soil organic matter supplied in greater quantities by trees due to higher productivity. In the height gradient, differences in the amount of organic matter supplied may result from the intensity and differentiation of the development of lower vegetation layers. Altitudinal gradient is well known to be one of the decisive factors shaping the spatial patterns of species diversity (Lomolino 2001). Availability of light, humidity, soil depth and soil properties change in the altitudinal gradient, leading to local variations in the composition of vegetation species (Cirimwami et al. 2019).

The processes of decomposition of soil organic matter involve microorganisms, whose amount and diversity depend on the properties of soils, vegetation and, above all, thermal and moisture conditions (Bardelli et al. 2017). In our present study, we considered the activity of dehydrogenases as an indicator of the number and activity of microorganisms in soils. Higher dehydrogenases activity was recorded in soils at 600 m a.s.l. The higher activity of dehydrogenases in the lowest positions was due to the faster rate of decomposition of soil organic matter as an effect of higher temperature. Previous studies have indicated that the quantity and quality of soil organic matter are quite important in shaping the biochemical activity of soils (Kucharski and Niewolak 1997; Bielińska 2001; Mocek-Płóciniak 2006). The change in altitude leads to a change in temperature, thereby causing a decrease in ecosystem productivity (Zhu et al. 2018). At the higher altitudes of the mountains, as a result of lowering the average air temperature and increasing humidity, the decomposition of plant residues is difficult, which favours the accumulation of carbon in forest litter (Bojko and Kabala 2017). In our study, we noted a significantly lower reserve of C in the soils at the highest altitude, with a simultaneous reduction of enzymatic activity. The low activity of dehydrogenases confirmed the reduction of the decomposition rate of detritus delivered to the soil, which is directly related to thermal conditions. The soils tested in the present study were accompanied by stands with spruce dominance, whose litter often leads to acidification and organic matter accumulation (Paluch and Gruba 2012; Elbe 2014; Gałka et al. 2014). Spruce litter, which contains more difficult to decompose lignin, and the worsening thermal conditions lead to an increase in carbon stock in forest soils. An additional factor influencing the amount of accumulated C and N in the forest soils was the slope aspect. Changes in the slope and intensity of sunrays associated with different slope aspect of the slope affect temperature and humidity fluctuations, which increases the variability of C and N stockpile (Wiesmeieret al. 2019). The north-facing slopes have more favourable humidity conditions than the south-facing slopes (Rawlik et al. 2019). According to Jasińska et al. (2019), litter decomposes faster on north-facing slopes than on south-facing slopes. The reason for that may be different species preferences of plants that influence the decomposition process. In our study, higher dehydrogenases activity was observed in the north-facing slope soils, and this finding can be explained by more stable thermal conditions. The south-facing slopes, despite more favourable thermal conditions, are exposed to stronger insolation, which leads to periodic drying of the accumulated surface humus (Bardelli et al. 2017). Periods of drought cause a decrease in the activity of soil microorganisms, which consequently leads to a slowdown in the decomposition of soil organic matter and the formation of a thick layer of humus. This relationship is supported by other authors who observed that north-facing slopes show higher enzymatic activity (Huang et al. 2015). In our study, we used the CDI and NDI to characterise the intensity of carbon and nitrogen flow from organic to mineral horizons. In soils located on the south-facing slopes, the CDI and NDI were significantly higher, which confirmed the slower mineralisation process.

Our results confirmed the significant influence of the altitude factor on the carbon and nitrogen stocks in the surface horizon of the soils studied. The C stock increased in soils at 1000 m a.s.l. At the highest positions, i.e. at an altitude of 1200 m a.s.l., the stock of carbon and nitrogen decreased due to worsening thermal conditions and lower productivity of vegetation. The soils at the lowest position of altitude gradient showed the lowest reserves of C and N, which was directly related to the rate of soil decomposition of organic matter. At lower position, under more favourable thermal conditions, microbial decomposition was more intensive. There were no statistically significant differences in carbon and nitrogen stocks between exposure variant. In our study, we used dehydrogenases activity to assess the biological activity of soils, which reflected the influence of location factors. The activity of dehydrogenases clearly decreased with altitude; additionally, in the comparison of altitude variants, the activity of dehydrogenases was higher in the soils of colder exhibitions. Knowledge of the carbon accumulation process will allow to optimise the management of forest resources.