Forest soil characterisation

Józef Wójcik

The value for soil pH was within a range 2.55 in the ectohumus of rusty podzolic soil on the POP located in the Rokita Forest District in the Baltic natural-forest region and 7.71 at a depth of 40-80 cm in the leached brown soil in the Tomaszów Forest District in the Małopolska natural-forest region and was changing with the depth.

The raw humus was generally most acid while the value for pH of the mineral layers was increasing with the increase of the depth. The greatest vertical variability in the pH value was noted for soils poorly developed from loose rocks in the Elblšg Forest District in the Baltic natural-forest region where pH for the raw humus was 2.67 and for the 40-80 cm layer it was 5.36. The pH value of soils measured on POPs (in fraction < 2.0 mm) in 2003 only slightly differed from pH measured in 1999 (in fraction < 1.0 mm).

The pH value of the raw humus and mineral layers depended on the dominant species in a stand (Fig. 26). The most acidic were the ectohumus horizons of soils under pine stands (the mean pH 3.03). The mean ectohumus pH under spruce, beech and oak stands was respecti-vely: 3.12, 3.38 and 3.42. However, the amounts of the raw humus in soils under the broadleaved stands were negligible or absent. The influence of the dominant species on pH of soil mineral layers somewhat varied. The order of tree species from the most to the least acidifying ones for the 0-5 cm layer was as follows: spruce (pH 3.20) > pine (pH 3.27) > beech (pH 3.52) > oak (pH 3.58). The lowest pH at the depths of 10-20, 20-40 and 40-80 cm were found for soils under spruce stands, however pH for soils under pine stands was surprisingly high. Not less than 100 soil samples under pine stands were taken for the analysis, so the result could not be accidental. The data given in the literature and monitoring results of 1995 and 1999 indicate that little acidifying effect of pine on forest soils was caused by the change in measurement methods. Pine stands in Poland generally occupy coarse-grained soils. In a 10 g sample, the amount of the clayey fractions deciding about soil pH is scarce, and the number of chemically indifferent fractions is high ranging between 2.0 and 1.0 mm.

The mean value for pH of the ectohumus horizons of fresh lowland forest habitats varied with their fertility (Fig. 27). The most acid raw humus (pH 2.74) occurred in the soils of fresh coniferous forest habitats (Bśw), less acid were the ectohumus horizons of soils of mixed coniferous forest habitats (BMśw) and fresh mixed deciduous forest habitats (LMśw) (pH 3.09 and 3.13, respectively), while the least acid (pH 3.43) - of soils of fresh deciduous habitats (Lśw). The pH values of the top mineral layer (0-5 cm) in the soils of Bśw, BMśw and LMśw were similar and of Lśw it was slightly less acid (pH 3,42). For deeper soil layers (5-80 cm) pH was increasing with the increasing fertility of the habitat, for example at a depth of 20-40 cm, the mean value for pH was 4.38 for Bśw, 4.22 for BMśw, 4.12 for LMśw and 4.00 for Lśw. The soils of coniferous forest habitats contain, as a rule, high amounts of coarse fractions which cause "erosion" of a sample.

The content of total nitrogen was always the highest in the raw humus and it decreased with the decrease of the depth. The content of this element in the ectohumus horizons ranged from 3.51 g.kg-¹ in the pseudopodsolic soil from the Strzałowo Forest District in the Mazury-Podlasie natural-forest region to 20.94 g.kg-¹ in the typical pseudogley soil from the Krotoszyn Forest District in the Wielkopolska-Pomerania natural-forest region. Irrespective of the layer, the highest content of nitrogen was found in the soils under spruce stands, followed by the soils under beech and oak stands and the lower content - in the soils under pine stands (Fig.28). The mean amount of nitrogen, the main biomass-producing nutrient, was found in the soils of lowland fresh forest habitats and was closely connected with their fertility. Nitrogen content of the soil ectohumus horizons and all mineral layers increased in the following way Bśw < BMśw < LMśw < Lśw.

The content of organic carbon in the examined soil layers decreased with the depth and in the ectohumus horizons it ranged from 73 g.kg-¹ for the pseudo-podsolic soil from the Olsztyn Forest District in the Mazury-Podlasie natural-forest region to 461 g.kg-¹ in the podzolic soil from the POP located in the Bielsko Forest District in the Carpathian natural-forest region, and at a depth of 20-40 cm from 0.585 g.kg-¹for brown rusty soils (cambic arenosols) in the Olkusz Forest District in the Małopolska natural-forest region to 51.0 g.kg-¹ in the typical acid brown soil in the Międzylesie Forest District in the Sudeten natural-forest region.

The amount of nitrogen in soils depended on the dominant species in a stand (Fig. 29). The ectohumus of soils under spruce stands revealed the highest carbon content (350 g.kg-¹). The mean content of this element in the ectohumus horizons of soils under pine, beech and oak stands was distinctly lower (280, 274 and 267 g.kg-¹, respectively). It has to be emphasised that this form of humus was infrequent in broadleaved stands, and its thickness when compared to coniferous forests was low and did not exceed 0.5 cm in beech stands.

The mean content of organic carbon in the mineral layers of soils under a given dominant species was the highest in soils under spruce stands. In the layers from the depths 0-5, 5-10, 10-20, 20-40 and 40-80 cm these values were respec-tively: 88, 44, 27, 18 and 13 g.kg-¹ and in every case they were twice as high as the values for soils under stands with the prevalence of other species. The lowest organic C was found in the mineral layer soil under pine stands (27, 15, 10, 5 and 3 g.kg-¹ respectively for the layers: 0-5, 5-10, 10-20, 20-40 and 40-80 cm).

Because of the fact that soils differ in thickness of the raw humus horizon (from 0 to 7 cm) and the density of mineral layers, the organic carbon contents expressed in grams of C per kilogram of soil does not account for the mass of this element accumulated in the examined soils. Thus, the organic carbon storage in the ectohumus horizons (if present) and mineral layer up to a depth of 80 cm was calculated for each soil. This storage expressed in Mg of C per hectare was calculated from the bulk density determined for each POP, and the bulk density values for the deeper soil layers were determined on the basis of the literature review [16, 19].

The mean organic carbon storage for the species (Fig. 30) was the lowest under pine stands (96 g.kg-¹), markedly higher under oak and beech stands (99 and 115 g.kg-¹ respectively) and the highest under spruce stands (259 g.kg-¹). The ectohumus horizons under spruce and pine stand on average stored 20 Mg of C per hectare each while the ectohumus horizons of oak and beech stands - only 6.0 and 0.6 Mg of C per hectare, respectively. The percentage share of carbon in the raw humus under pine stands was as high as 20% in the total soil mass, while the share of ectohumus carbon in the total carbon pool in the soils under spruce and oak stands was respectively 8% and 6% and in the soils under beech stands - below 1%.

The C:N ratio or the quantitative ratio of organic carbon to nitrogen in the soil humus or organic matter is one of the most important indicators of the fertility of forest habitats. The lower the indicator value the more fertile is the forest soil.

The highest value of the organic carbon content to the total nitrogen content ratio was found in non-decomposed ectohumus and ranged from 18 in the glayey brown soil from the Suwałki Forest District in the Mazury-Podlasie natural-forest region and in the typical pseudogley soil from the Krotoszyn Forest District in the Wielkopolska-Pomerania natural-forest region to 37 in the soil developed from loose rocks from the Elblšg Forest District in the Baltic natural-forest region and this value was gradually decreasing in deeper layers to reach 7 at a depth 40 - 80 cm for typical acidic brown soils from the Stuposany and Wisła Forest Districts in the Carpathian natural-forest region.

The mean C:N ratio in the ectohumus and endohumus for the dominant species in a stand varied (Fig. 31). This ratio in the soils under pine stands was the highest in all analysed layers and in the ectohumus it was 26:1 and in the mineral layers it declined to 21:1. The carbon to nitrogen ratio in all soil layers under beech, oak and spruce stands was similar. The mean value for the C:N ratio in the ectohumus under a given species was 20-21:1 and in the layer 40-80 cm this value was 15:1. The reason for this low variation in the C:N ratio between soils under particular tree species has to be seen in that both the pine and spruce stands and the beech and oak stands occupied sites varying in fertility and moisture level. The illustration of such situation are the second-level POPs located in the pine stands in the habitats: Bśw, Bw, BMśw, BMw, BMG, LMśw, LMwyż, LMG or even Lśw, Lwyż, LG, or Lł. Because the C:N ratio is an indicator of habitat fertility, the averaging of the ratio value for the soils under all pine stands growing both in very poor and very rich soils cause that the differences between the soils under particular species are not distinct.

Much more interesting approach to this issue is the analysis of the relationships between the values of the C:N ratio averaged not for the dominant species in a stand but for the forest habitat types. However, even with this approach there are certain constraints emerging from the fact that stands composed of different species grow on soils occupying a given forest habitat. The highest value of the carbon to nitrogen ratio was found in the soils of the fresh coniferous forests (Fig. 32). The mean value of the ratio being 28:1 for the raw humus and 24-25:1 for the endohumus classified these humus horizons to the moder/mor type. The C:N ratio in the types of humus of the fresh mixed coniferous forest habitats was 25:1 in the raw humus and 21:22.1 in the mineral layers which allows to classify them to the moder type. Also the C:N ratio in the humus horizons of the soils of fresh mixed deciduous forest habitats was characteristic for the moder type of humus (on average 23:1 in the ectohumus and19-21:1 in the mineral layers). The soils from the fresh deciduous forests did not contain the raw humus (the exception were two pine stands) and the endohumus horizons of these soils revealed the C:N ratio similar to the typical mull type (14-16:1). The carbon to nitrogen ratio in the organic matter of soils in the mountain forest habitats was usually lower than in the lowland habitats. The C:N ratio for the types of ectohumus of mixed mountain coniferous forest (BMG) and mixed mountain deciduous forest (LMG) was 21-22:1 and for lowlands respectively 19-21:1 and 17:20:1. The lowest value of the C:N ratio were found in the soils of the mountain forest habitats. These soils - like the soils of Lśw - did not contain the raw humus, and the C:N ratio in the mineral layers ranged from 15:1 in the 0-5 cm layer to 10:1 in the 40-80 cm layer.

The changes in the C:N ratio in the years 1996, 1999 and 2003 wee analysed on the basis of mean values for pine stands in the fresh mixed coniferous forest habitats and for spruce, beech and oak stands growing in the fresh deciduous forest habitats which most frequently represented (Fig. 33). The mineral layers of the soils under spruce, beech and oak stands of Lśw habitats were characterised by a relatively stable C:N ratio. In 1999, the change in the C:N ratio in those soils was from -11 to +13 percent when compared to the values from 1996. The changes in the C:N ratio in 2003 when compared to 1996 were also insignificant and oscillated between -11 and 14 percent. The carbon to nitrogen ratio in the mineral layers in the soils under pine stand in the years 1996 and 1999 was similar - the differences were within a range of 13 percent, however this ratio in the soils measured in 2003, i.e. after the change of the measurement method, was higher than that in 1996 by 25-54 %. This change is believed to be caused by the application of a new measurement method - a precise information about changes in the C:N ratio will be possible to interpret after the comparison of the analyses of carbon and nitrogen in the fractions <2.0 and <1.0 mm. .