WATER ABSORPTION OF ORGANIC MATTER TAKEN FROM HORIZONS OF ECTOHUMUS OF FOREST SOILS UNDER NORWAY SPRUCE STANDS

Jarosław Kucza, Justyna Urbaś
Department of Forest Engineering, Agricultural University of Cracow, Poland

ABSTRACT

Organic matter from ectohumus horizons of soils under Norway spruce stands may be divided into two parts: (1) able to absorb such an amount of water that its weight by volume together with absorbed water is over 1 Mg·m^-3, and (2) unable to attain such absorption. In the litter horizon (Ol) and fermentation horizon (Of) the mean time of water absorption by organic matter, able to attain its assumed level, depended on the development phase of a Norway spruce stand. Organic matter taken from soils under stands of age classes I and II turned out to have the best water absorption properties. The assumed water absorption was attained by organic matter in the shortest time in the following cases: in the litter horizon (Ol) from under the 35-year-old stand, in the fermentation horizon (Of) from under the 14-year-old stand, and in the humification horizon (Oh) from under the 21-year-old stand. Ranges of the mean time of water absorption were: for the litter horizon (Ol) from 1.66 to 4.14 days, for the fermentation horizon (Of) from 0.79 to 3.12 days, and for the humification horizon (Oh) from 0.63 to 1.19 days. The relationship between the percentage of organic particles, unable to attain the assumed water absorption, and the stand age in the litter horizon (Ol) was similar to that of the fermentation horizon (Of). The matter taken from the youngest stands, i.e. 14-28 years of age, was characterized by the smallest amount of organic particles unable to attain the assumed water absorption. The amount of these particles expressed in per cent of sample’s total weight was: in the litter horizon (Ol) from 1.55 to 16.16%, in the fermentation horizon (Of) from 5.72 to 26.50%, and in the humification horizon (Oh) from 0.27 to 3.36%. Water properties of organic matter in the litter (Ol) and fermentation (Of) horizons were significantly correlated with the index of the degree of decomposition expressed by the ratio between nitrogen and carbon – (N/C) 1000. No such relationship was found in the humification horizon. This study is of a practical importance for forest hydrology.

Key words: forest hydrology, Norway spruce stand, soil organic matter, water absorption, absorption time, index of the degree of organic matter decomposition - (N/C) 1000.

PREFACE

Introduction

Retention of precipitation water by organic horizons of forest soils is one of the elements of water balance in forest stands [21]. This problem is important not only from the hydrologic point of view [12, 3, 4, 21, 6, 8, 9], but also from the point of view of soil science [5, 15, 16, 19, 20] and ecology, understood as living conditions of organisms.

Organic matter of forest soils forms one of the water reservoirs of forest environment [21], characterized by a great dynamics of reception and returning of precipitation water. These processes are the two important components in water exchange between atmosphere, forest stand, and soil.

Total capacity of the water reservoir of organic horizons (capillary water capacity) is relatively well known [21, 6]. The method of its filling after individual rainfalls has been initially recognized. It depends on the actual degree of filling with water [15, 21, 22], rainfall intensity [21], as well as time between successive rainfalls [9]. The time element is connected with speed of water absorption by particles of organic matter of forest soils being in different stages of transformation. Phenomena of adsorption and absorption, having a physical and mechanical nature [16], seem to be indirectly dependent on chemical changes associated with decomposition. The process of decomposition may change physical properties of the outer coat of an organic particle, as well as properties of matter in its interior. Research on absorption [16], wetting angle, and capillary ascension of water in organic matter [19, 20], carried out on samples taken in pine stands, proved that these properties worsen with the degree of organic matter decomposition. Results of experiments on retention of precipitation water, conducted under conditions of precipitation simulation, carried out on organic soil monoliths of untouched structure taken in Norway spruce stands of different age [8, 9], suggested somewhat different conclusions concerning the relationship between retention capacity of organic horizons of ectohumus and the degree of their decomposition. They permitted to make a hypothesis about the effect of stand species composition, stand development phase, as well as intensity of organic matter decomposition on this phenomenon.

Subject, aim, and scope of research

Water absorption ability of organic matter, especially in the horizons where the effective capillary system has not been developed, is for forest communities an important element from the point of view of formation of microclimatic conditions, as well as from the point of view of water balance [7].

The time of water absorption of organic matter taken from the horizons of ectohumus of mountain forest soils under Norway spruce stands was the subject of this study.

The aim of the study was to demonstrate variation of the time of water absorption of organic matter in individual horizons of ectohumus, depending on the development phase of the Norway spruce stand and intensity of decomposition processes, expressed by the ratio between nitrogen content and carbon content [11].

Investigations were carried out on samples of the mor type ectohumus taken under the lower mountain zone Norway spruce stands situated within the drainage area of Dupniański rivulet in the Beskid Slšski Mountains. These forests are the part of the Wisła Forest District of the Katowice Region of State Forests [8]. The basic investigations on water absorption of organic matter and determination of carbon and nitrogen contents in the horizons of ectohumus were carried out on 12 permanent sample plots under Norway spruce stands of different age classes [12], and in addition, on a plot under the 50-year-old stand (plot X). In the case of spruce needles from the litter horizon (Ol) the studies also included material collected under the 8-9-year-old spruce regeneration (plot M7). Also additional comparative studies were carried out on 3 samples of dried (in temperature of 50-60°C) Norway spruce needles collected from a tree about 50 years of age (plot X), and on a sample of fresh fallen needles collected under the 119-year-old stand (plot 4).

A general forest characteristics of the study material collecting site is given in Table 1 [24].

METHODS

The essence of experiments carried out during this study, according to the study aim stated above, was the measurement of time of water absorption by particles of organic matter from the horizons of ectohumus. The introduced criterion of water absorption time was understood as a time interval from the moment of immersion of dry matter in distilled water (of temperature 18-22°C) to the moment of its falling to the bottom of a beaker, i.e. to the moment of surpassing by soaked particles of weight by volume of 1.0 Mg·m^-3. The surpassing of this level did not necessarily mean the end of water absorption by organic matter, but it was a clear signal that a certain (high) degree of water saturation was attained.

Material for investigations was taken from soil monoliths of untouched structure cut in the soil cover to cylinders of stainless steel, 12.5 cm in diameter and 11 cm in height. When taking soil monoliths the care was taken to make sure that their structure and volume were similar to structure found in open pits (for studies on skeletal content of soil profiles), 1 m² in area, made in the neighborhood, and recognized to be representative for the study plots. In the laboratory, the soil monoliths were divided into genetic horizons of ectohumus from which, after drying in temperature of 50-60°C, samples of organic matter were taken for studies on water absorption. In the case of the litter horizon (Ol) all spruce needles, without twigs, present in a monolith, were taken for the experiment. The reason for this procedure was the size of twigs which would not fit to beakers without breaking them to pieces, and also the fact that needles could make from 68 to 86% of organic fall [12], and therefore it was possible to treat them as the basic building material of the organic soil horizons under Norway spruce stands. In samples from the litter horizon, needle content varied from 64% in the monolith from the plot 2 to 94% in the monolith from the plot M7 [25]. Masses of samples from this horizon varied from 3.25g (plot M7) to 36.48g (plot 4) [25]. Masses of samples from the fermentation and humification horizons, taken for investigations after their earlier mixing (in order to make them representative for the monolith), varied from 19 to 27g in the case of the fermentation horizon, and from 39 to 40g in the case of the humification horizon [25].

The ectohumus samples, prepared in such a way, and the dried additional samples were placed in 1-liter beakers with distilled water of temperature of about 20°C (18-22°C). Keeping of organic matter at a constant depth of submersion was secured by a perforated aluminum disk. Date and hour of sample submergence was the starting point of studies consisting of successive separation of fallen (waterlogged) organic matter particles after a determined time of water absorption. The waterlogged particles were taken from the beaker’s bottom after 12, 24, and 48 hours, and then dried in temperature of 50-60°C till a constant mass was obtained. The time of subsequent collecting of fallen matter depended on its amount. In the case of periods between two consecutive collections of waterlogged organic particles longer than 48 hours, distilled water in the beakers was replaced with a fresh one. Experiments were conducted till no fallen particles of organic matter were found at the beaker’s bottom for 7 consecutive days. The floating part of the sample remaining after this period of time was considered to be unable to attain the assumed water absorption.

Samples taken from the litter horizon (Ol), as well as additional samples, were considered to be a pure organic matter. The material from the fermentation and humification horizons had mineral impurities, and their percentage had to be taken into account during the analysis of results to assure that they refer to the 100% organic matter.

To achieve this aim reduction calculations had to be made. They were based on roasting losses (Polish standard: PN-88/B-04481) in spruce needles and twigs (being the main components of the organic fall) collected from the litter horizon (Ol), and dried at temperature of 105-110°C. Burning of organic matter took place at temperature of 700°C. The mean roasting losses of these components amounted to 96.01%. The proportion between the 100% organic matter and its percentage loss after roasting (96.01%) was used in recalculation of the results obtained after burning of mixtures of organic and mineral particles fallen in beakers, and originating from the fermentation (Of) and humification (Oh) horizons. The recalculation was made [25] for two time intervals of particle fall onto the beaker’s bottom (12 and 24 hours), since it was assumed that the period of 24 hours is long enough for all mineral particles to fall in water environment. The method of determination of organic substance content using losses after roasting was chosen for several reasons, in spite of some disadvantages [14]. The parent rock of investigated profiles consisted of Istebna sandstones and conglomerates of a relatively small content of a binding agent. The suspended matter fallen after 12 and 24 hours contained a considerable percentage of organic matter [25], and made a small part of the total sample. Mineral parts had fallen in most samples after 12 hours, and this indicated a small content of clay parts which could contain water in their composition, chemically bound, and freed during roasting. It was assumed that roasting losses of water chemically bound with clay particles were so small that they did not significantly alter the calculations of the 100% organic matter content in samples.

In the case of fermentation (Of) and humification (Oh) horizons the investigations described above were repeated in May 2003 for 13 plots. In each plot two monoliths were taken, 2 meters away from earlier sampling place. Conducting of two additional series of investigations for the fermentation and humification horizons was caused by necessity of checking the reliability of results of the first series, it was especially necessary to make certain that during their execution no errors were made during separation between the fermentation and humification horizons. In two additional research series for both horizons the samples of organic matter dried at temperature of 50-60°C weighed about 35g. Data on water absorption of organic matter from the fermentation and humification horizons in form of the mean from three research series were used in statistical analyses.

Determination of carbon and nitrogen content in respective horizons of investigated profiles was completed in June 2002 using separate samples, originating from 5 soil monoliths (cut with cylinder 3.8 cm in diameter), after their earlier division into horizons. These monoliths were cut 1 meter away from the place of sample collecting for experiment on water absorption of organic matter. Determination of carbon and nitrogen content was carried out in the Department of Forest Ecology, Agricultural University of Cracow using a Leco macro-analyzer CNS-2000. Results of this determination are presented in this paper in two ways.

The first method was the ratio between carbon content and nitrogen content (C/N) considered in soil science to be a measure of the degree of soil organic matter decomposition [1]. It was used in a descriptive analysis of results of this study.

The second method was a reversed ratio, i.e. the ratio between nitrogen content and carbon content [11] expressed in per mille, and named as the index of the decomposition degree – (N/C) 1000 [10]. This method was used in construction of mathematical formulae describing relationships presented in the aim of this study, as being a better method from the point of view of calculation and presentation procedures.

It was attempted to confirm by the analysis of regression the significance of graphically suggested interpretation of investigated relationships based on results of this study. Mean values from three measurement series were used for the statistical analysis of results concerning water absorption of organic matter in the case of the fermentation (Of) and humification (Oh) horizons, accepting their relatively small variation as an indication of a natural non-repetition, and not as inaccuracy in sample preparation. Values of coefficients occurring in formulae were calculated using the least squares method in the process of their identification. Results of statistical tests of concordance between values calculated from equations and measured ones are presented in form of a table.

RESULTS AND DISCUSSION

Results of experiments on water absorption of organic matter from three horizons of the mor type ectohumus from under Norway spruce stands, and also of additional samples are presented in Tables 2, 2a, and 2b. Results of determination of ratio between carbon and nitrogen content (C/N), and values of the index of the degree of decomposition – (N/C 1000 in the investigated horizons are shown in Table 1. The analysis of results was carried out separately for each of the three investigated horizons of ectohumus.

• Litter horizon (Ol)

It is evident from data in Table 2 that some needles from the litter horizon (Ol) were not able to attain the assumed water absorption, even after 31 days of soaking.

The maximum time during which waterlogged needles were found to be falling to the bottom of beakers was 24 days. Most of sinkable needles fell after 6 days. The mean time of water absorption (calculated as the mean weighted by mass) of the sinkable part of organic matter was different for different profiles, and it was found to be from 1.66 days in the profile 2 (27-year-old stand) to 4.14 days in the profile M7 (8-9-year-old regeneration). The percentage of unsinkable matter was different in individual profiles. It varied from 1.55% in the profile 2 (27-year-old stand) to 16.16% in the profile 4 (119-year-old stand). The results presented in this paper showed that organic matter in ectohumus profiles may be divided into two parts: one able, and another unable to attain the assumed water absorption. Results of additional experiments carried out on dried living Norway spruce needles of three consecutive years of their formation, as well as on a fresh fall of organic matter intercepted at the height of 1 meter above the ground, did not show such variability (Table 2). In these experiments all samples were wholly able to attain the assumed water absorption during a relatively narrow time interval (2.92-3.67 days). It proved that there was a change in absorptive properties of needles under influence of processes taking place in the initial stage of their decomposition.

In the litter horizon (Ol) the ratio C/N (Table 1) varied widely from 24.81 in the profile D (17-year-old stand) to 44.50 in the profile O (69-year-old stand). This range manifested different initial intensities of decomposition processes in the horizon under discussion.

The analysis of results of investigations on water absorption of needles from the litter horizon (Ol), as well as on the degree of their decomposition, suggested that between the investigated properties of organic matter there may exist functional associations which may be described by mathematical formulae. A different needle susceptibility to water absorption, as shown during this study, permitting to divide needles into able and unable to attain the assumed water absorption, caused that the investigated relationships were analyzed separately for these two components.

Relationship between the mean time of water absorption by Norway spruce needles from the litter horizon, able to attain the assumed absorption level, and the stand age

The position of points in the coordinate system (Fig. 1a) suggested that dependence of the mean time of water absorption of needles from the litter horizon (Ol), able to attain the assumed absorption level, on the age of forest stand, has its extreme (minimum). This dependence was described by the formula:

(1)

where:
Tn_Ol – mean time of water absorption of Norway spruce needles from the litter horizon, able to attain the assumed absorption level [days];
W – stand age [years].

This relationship was highly significant – age of the stand from which tested needles originated explained 80.0% of variation of the value Tn_Ol. The standard error of differences (Tn_Ol calculated – Tn_Ol measured) was 0.285 days. The mean error of estimation was 11.0%. Statistical tests of concordance between values calculated from the formula and measured ones are shown in Table 3. The minimum value of the mean time of water absorption of needles, able to attain its assumed level, occurred in the case of stand about 35 years of age, characterized by a full crown closure, a lack of forest floor vegetation, high crowns successfully absorbing solar radiation, and a specific microclimate [23]. Progress of the assumed form of the function describing this relationship may be explained by relations between the development of biomass in forest site and the age of stands. A cyclic character of this development [2] would decide about the organic fall, and possibly also about actual possibilities of its decomposition, depending on intensity of a biological cycle in the forest site [18].

Relationship between the mean time of water absorption by Norway spruce needles from the litter horizon, able to attain the assumed absorption level, and the index of the degree of decomposition

The position of points in the coordinate system (Fig. 1b) permitted to suppose that the function describing the dependence of the mean time of water absorption of needles, able to attain its assumed level, on the index of the degree of decomposition (C/N 1000) has its extreme (minimum). The following formula was developed for this dependence:

(2)

where:
Tn_Ol – as in formula (1);
Ws_Ol – index of the degree of decomposition of organic matter from the litter horizon (Ol) – (N/C 1000 [value without denomination].

This relationship was significant. Variation of the value Ws_Ol in the litter horizon in the tested sample explained 70.0% of the value Tn_Ol. The standard error of differences (Tn_Ol calculated – Tn_Ol measured) was 0.349 days. The mean error of estimation was 13.4%. Statistical tests of concordance between values calculated from the equation and measured ones are shown in Table 3. The relationship presented above showed that the shortest time of water absorption occurred in the case of organic matter (from the horizon under discussion) of which the index of the degree of decomposition was 34.9 (C/N =28.65), and also that the identical time of absorption may be characteristic for matters of different indexes of the degree of decomposition. At this stage of research (without additional chemical analyses) only the hypothesis may be made that the decomposition processes taking place in the horizon under discussion may assume “different directions”, and the percentage of matter unable to attain the assumed water absorption may result from its hydrophobic properties [19, 20]. This conclusion of hypothetic character should be verified during future research.

Relationship between the percentage of unsinkable Norway spruce needles in the litter horizon and the stand age

A great variation in the percentage of needles from the litter horizon (Ol) unable to attain the assumed water absorption (Nz_Ol), and the position of points in the coordinate system (Fig. 1c), permitted to suppose that this relationship may be described by a formula of similar construction as the formula (1):

(3)

where:

Nz_Ol – percentage of unsinkable Norway spruce needles in the litter horizon (Ol) [in % weight];
W – as in formula (1).

This relationship was highly significant – the development phase of the stand, expressed by its age, in the tested sample of needles in the litter horizon (Ol), explained in 92.1% the variation of the value Nz_Ol. The standard error of differences (Nz_Ol calculated – Nz_Ol measured) was 1.33%. The mean error of estimation was 13.7%. Statistical tests of concordance between values calculated from the equation and measured ones are shown in Table 3. The minimum value of the percentage of unsinkable needles (according to the curve in the graph) occurred in the case of the 28-year-old stand, which may be considered to be close to the one calculated during identification of the formula (1).

Relationship between the percentage of unsinkable Norway spruce needles from litter horizon and the index of the degree of decomposition

The distribution of points in the coordinate system (Fig. 1d) suggested that the investigated dependence of the percentage of spruce needle, unable to attain the assumed water absorption, on the index of decomposition of organic matter in the litter horizon (Ol) may be described by a formula of structure similar to that of the formula (2):

(4)

where:
Nz_Ol – as in the formula (3);
Ws_Ol – as in the formula (2).

The relationship described by the formula (4) is significant. Variation of the index of the degree of decomposition (Ws_Ol) in the litter horizon (Ol) in the tested sample explained 71.0% of variation of the value Nz_Ol. Standard error of differences (Nz_Ol calculated – Nz_Ol measured) was 2.56%. The mean error of estimation was 26.3%. Statistical tests of concordance between values calculated from the equation and measured ones are presented in Table 3. The smallest percentage of Norway spruce needles unable to attain the assumed water absorption occurred at the index of the degree of decomposition equal to 34.12 (C/N = 29.31), and therefore very close to the value obtained by identification of the formula (2).

The relationships described by formulae (1), (2), (3), and (4) permitted to make the following general conclusions:

1. The tested samples of Norway spruce needles from the litter horizon under stands 28-35 years old had the best absorptive properties (the shortest water absorption time), and the smallest percentage of needles unable to attain the assumed level of absorption,

2. Samples of the index of the degree of decomposition of organic matter equal to about 34.5 (C/N = 29.0) were characterized by the shortest time of water absorption of needles able to attain its assumed level, as well as by the smallest percentage of needles unable to attain such an absorption level.

• Fermentation horizon (Of)

In the fermentation horizon (Of) the investigated properties of organic matter underwent the changes in comparison with the litter horizon (Ol) (Table 2a).

Although most of waterlogged particles fell after 4 days, the time of falling of the remaining sinkable particles was prolonged to 34 days. The mean time of sinking of the waterlogged part of organic matter in this horizon was also diversified among individual profiles, and ranged from 0.79 days in the profile 1 (14-year-old stand) to 3.12 days in the profile 4 (119-year-old stand). The percentage of unsinkable particles considerably increased, even after 41 days. Its variation ranged from 5.72% for the profile D (19-year-old stand) to 26.50% for the profile 4 (119-year-old stand). Thus, it may be concluded that although the mean time of water absorption of sinkable particles of matter from the fermentation horizon (Of) was shorter by about 1 day than that of the sinkable particles from the litter horizon, the percentage of unsinkable organic matter distinctly increased.

In the fermentation horizon (Of), the interval of variation of C/N distinctly decreased in comparison with the litter horizon (Ol), and it ranged from 20.30 in the profile 1 (14-year-old stand) to 29.54 in the profile 4 (119-year-old stand).

The analysis of results, presented above, suggested that between the investigated properties of organic matter in the fermentation horizon there are (similarly as in the litter horizon) functional relations that can be described by mathematical formulae.

Relationship between the mean time of water absorption by organic matter from the fermentation horizon, able to attain the assumed absorption level, and the stand age

When analyzing this relationship in the fermentation horizon (Of) the formula used to describe it was different than that for the litter horizon. This was caused by the distribution of points in the coordinate system (Fig. 2a), as well as by a lack of data for the sample plot with Norway spruce regeneration (M7). Therefore this relationship was analyzed only in the available range of age variation (14-119 years), and it was described by the equation:

(5)

where:
Tn_Of – mean time of water absorption of organic matter from the fermentation horizon (Of) able to attain the assumed absorption level [days];
W – as in formulae (1) and (3).

This relationship was highly significant – the age of the Norway spruce stand, from which tested samples of organic matter originated, explained 89% of variation of the value of Tn_Of. The standard error of differences (Tn_Of calculated – Tn_Of measured) was 0.245 days. The mean error of estimation was 14.5%. Statistical tests of concordance between values calculated from the equation and measured ones are shown in Table 3. Time of water absorption of organic matter particles from this horizon was shortest in the case of young Norway spruce stands of full crown closure. The periodicity of biomass development in the forest site [2], and the continuity of decomposition processes may suggest that in future the investigated relationship will be described by the function of structure similar to that of the formula (1).

Relationship between the mean time of water absorption by organic matter from the fermentation horizon, able to attain the assumed absorption level, and the index of the degree of decomposition

The position of points in the coordinate system (Fig. 2b) permitted to suppose that the function describing the dependence of the mean time of water absorption of organic matter from the fermentation horizon, able to attain the assumed absorption level, on the index of the degree of decomposition (in the variation interval tested) has a monotonically decreasing progress. For this relationship the following formula was elaborated:

(6)

where:
Tn_Of – as in formula (5);
Ws_Of – index of the degree of decomposition of organic matter from the fermentation horizon (Of) (N/C 1000 [value without denomination].

This relationship was also significant. Variation of the index of the degree of decomposition of organic matter (Ws_Of) in the tested sample from the fermentation horizon (Of) explained 61.2% of variation of the value Tn_Of. Standard error of differences (Tn_Of calculated – Tn_Of measured) was 0.468 days. The mean error of estimation was 27.7%. Statistical tests of convergence between values calculated from the equation and measured ones are shown in Table 3. This relationship showed that the shortest time of water absorption occurred in the case of organic matter decomposed to the highest degree, i.e. at the index of 48.03 (C/N) = 21.72. This confirmed an earlier conclusion that the effect of the degree of decomposition on water properties of organic matter under spruce stands is different from that for organic matter under pine stands [16, 19], already within the same genetic horizon.

Relationship between the percentage of unsinkable organic matter in the fermentation horizon and the stand age

The percentage of organic matter unable to attain the assumed water absorption in the fermentation horizon (Of) was considerably greater than that in the litter horizon (Ol). The position of points in the coordinate system (Fig. 2c) suggested that dependence of this percentage in the investigated horizon on the stand age (W) may be described by a straight line formula:

(7)

where:
Nz_Of – percentage of unsinkable particles of organic matter in the fermentation horizon (Of) [% weight];
W – as in formulae (1), (3), and (5).

This relationship was significant – the development phase of a stand, expressed by its age, explained 76.7% of variation of the value Nz_Of. The standard error of differences (Nz_Of calculated – Nz_Of measured) was 2.72%. The mean error of estimation was 21.5%. Statistical tests of convergence between values calculated from the equation and measured ones are shown in Table 3. Percentage of organic particles unable to attain the assumed water absorption increased with increase in stand age.

Relationship between the percentage of unsinkable organic matter in the fermentation horizon and the index of the degree of decomposition

Position of points in the coordinate system (Fig. 2d) permitted to suppose that, similarly as in the case of relationship between the mean time of water absorption in the fermentation horizon (Of) and the index of the degree of decomposition – formula (6), the function describing the dependence of the percentage of particles of organic matter in the fermentation horizon (Of), unable to attain the assumed water absorption, on the values of this index would have a monotonically decreasing progress. For this relationship the following formula was worked out:

(8)

where:
Nz_Of – as in formula (7);
Ws_Of – as in formula (6).

This relationship was significant. Variation of the value of the index of the degree of decomposition Ws_Of explained 68.8% of variation of the value Nz_Of. The standard error of differences (Nz_Of calculated – Nz_Of measured) was 3.15%. The mean error of estimation was relatively large, and amounted to 2.49%. Statistical tests of convergence between values calculated from the equation and measured ones are shown in Table 3. The value of the index of the degree of decomposition (46.7) approximated the result obtained by identification of the formula (6).

• Humification horizon (Oh)

In the humification horizon (Oh) there was a further distinct change of the properties described above. Total time of particle falling was reduced to 15 days (table 2b).

The mean time of water absorption of sinkable organic matter in the tested sample varied from 0.63 days in the profile D (19-year-old stand) to 1.19 days in the profile F (114-year-old stand). The percentage of unsinkable particles of organic matter after 22 days of soaking rapidly decreased, and ranged from 0.27% in the profile 1 (14-year-old stand) to 3.36% in the profile L (94-year-old stand). Since the changes in intensity of decomposition processes were smooth in the entire organic matter of the ectohumus horizons it may be supposed that also changes in water properties were of the same character. A small percentage of unsinkable organic particles, found in this horizon, could result from the effect of matter of the border area between humification and fermentation horizons, or of a certain inaccuracy in dividing the study material.

The humification horizon (Oh) was characterized by a small variation of the ratio C/N (Table 1), ranging from 20.61 in the profile L (94-year-old stand) to 23.07 in the profile D (19-year-old stand).

Results of investigated water properties of organic matter and identification of measures of its decomposition in this horizon became considerably limited. This caused that their statistical analysis became difficult. According to the Student’s test of the hypothesis (at the confidence level of 0.05) that individual values of investigated properties are not significantly different from their mean values cannot be rejected. Besides, this variation may have resulted from horizon border effects. This caused that most of the analyzed relationships assumed a descriptive form, pointing out the tendencies observed.

Relationship between the mean time of water absorption by organic matter from the humification horizon, able to attain the assumed absorption level, and the stand age

The distribution of points in the coordinate system (Fig. 3a) suggested the application of the same form of the formula as in the case of the litter horizon (Ol):

(9)

where:
Tn_Oh – mean time of water absorption by organic matter from the humification (Oh) horizon, able to attain the assumed absorption level [days];
W – as in formulae (1), (3), (5), and (7).

This relationship was significant – age of the Norway spruce stand from which organic matter was taken explained 71.1% of variation of the value Tn_Oh. The standard error of differences (Tn_Oh calculated – Tn_Oh measured) was 0.088 days. The mean error of estimation was 9.5%. The shortest time of water absorption by organic matter from this horizon, similarly as in higher horizons, took place in young Norway spruce stand, about 21 years of age. Statistical tests of convergence between values calculated from the equation and measured ones are shown in Table 3. This relationship was worked out, in spite of statistical reservation, because there was a great similarity to the one worked out for the litter horizon (formula 1), and suggested (as it has been mentioned above) that in future, a similar formula would replace formula (5), describing this relationship in the fermentation horizon (Of).

Relationship between the mean time of water absorption by organic matter from the fermentation horizon, able to attain the assumed absorption level, and the index of the degree of decomposition

Position of points in the coordinate system (Fig. 3b) may suggest that the dependence of the mean time of water absorption of organic matter from this horizon (Tn_Oh) on the index of the degree of decomposition (Ws_Oh) became reversed as compared with higher horizons (formulae 2 and 6). Time of water absorption by organic matter from this horizon was the shorter the smaller was the value of this index. This conclusion should be verified in future.

Relationship between the percentage of unsinkable organic matter in the fermentation horizon and the stand age

The results only suggested that the percentage (within the variation interval found) of unsinkable particles of organic matter (Nz_Oh), similarly as in the case of higher horizons, had the tendency to increase with increase of age of stands (Fig. 3c).

Relationship between the percentage of unsinkable organic matter in the fermentation horizon and the index of the degree of decomposition

When analyzing the position of points in the coordinate system (Fig. 3d) no conclusion can be made that in the humification horizon (Oh) the percentage of unsinkable particles of organic matter (Nz_Oh) depended on the index of the degree of decomposition (Ws_Oh).

CONCLUSIONS

Research on water absorption by organic matter taken from horizons of ectohumus of forest soils under Norway spruce stands growing in the drainage area of Dupniański rivulet in the Beskid Slšski Mountains permitted to draw the following conclusions, and make hypotheses to be verified during future studies.

1. Organic matter from horizons of ectohumus of soils under Norway spruce stands may be divided into: able to absorb such an amount of water that its weight by volume is over 1 Mg·m^-3, and unable to attain such an absorption level.

2. The percentage of organic matter unable to attain the assumed water absorption is in the horizons of ectohumus quite diversified. The highest percentages were found in the following horizons: the fermentation horizon (Of) – from 5.72 to 26.50% by weight, and the litter horizon (Ol) – from 1.55 to 16.16%. In the humification horizon (Oh) this percentage was marginal – from 0.27 to 3.36%.

3. Occurrence of organic matter of different capacity of water absorption in the ectohumus horizons (built of a homogeneous organic fall) may manifest different directions of decomposition processes occurring simultaneously. The presence in the ectohumus horizons of particles unable to attain the assumed level of water absorption may manifest the hydrophobic properties of organic matter, and confirm many directions of the decomposition process. At this stage of research it may not be concluded what chemical processes are responsible for this variability. But the knowledge about the presence of organic matter particles of various absorption properties in the ectohumus horizons could be utilized in studies on the chemical composition of humus compounds in both components. Discovering the difference between these chemical compounds should explain the directions of the decomposition process, as well as the cause of changes in absorptive properties of organic matter. For this reason the above conclusion is of a hypothetical character, and should be verified during future studies.

4. The shorter is the mean time of fall of organic matter, able to attain the assumed level of water absorption, the more advanced are decomposition processes. Thus, this time decreases with increase of depth of occurrence of the ectohumus horizons.

5. The greater is the depth of occurrence of respective ectohumus horizons, the narrower is the interval of variation of C/N, from almost 20 in the litter horizon (Ol) to 2.5 in the humification horizon (Oh). This fact showed very different initial intensities of processes of decomposition of organic matter in the litter horizon, which became more uniform in the humification horizon. This variability may be caused by microclimatic agents associated with a development phase of the forest stand.

6. During this study the significant relationships were found between the mean time of water absorption of organic matter from the litter and fermentation horizons, able to attain the assumed absorption level, and the stand age, as well as the decomposition advancement expressed by the index of the degree of decomposition – (N/C) 1000. These relationships may be described by regression equations. In the humification horizon no such relations were found.

7. The percentage of organic matter from the litter and fermentation horizons, unable to attain the assumed level of water absorption, were also significantly dependent on the stand age, as well as on the index of the decomposition degree. This dependence may also be described by regression equations. In the humification horizon no such dependence was found.

Results of this study are of practical importance for forest hydrology because they showed that filling with precipitation water of parts of organic matter under Norway spruce stands is diversified, depending on the stand age and the degree of decomposition of individual organic horizons of forest soils.

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