THE ENZYME ACTIVITY OF THE FOREST SOILS OF SOUTHERN POLAND AS A MEASURE OF SOIL QUALITY

Kazimierz Januszek
Department of Forest Soils, University of Agriculture in Cracow, Poland

ABSTRACT

The author proposed the evaluation of forest soil quality based on the soil enzyme index (SEI_DPU). The activity of the dehydrogenases, proteases and urease at the natural moisture soil samples from each horizons was determined, and then the activities were computed on the oven-dried soil mass in a column of 1 dm² × 1m (10 dm³) of soil profile. The aim of this study was to examine the proposed index in 10 different subtypes of forest soil in southern Poland. Broadly speaking, dystrophic soils  (Haplic Podzol, Hyperdistric Luvisol, Hyperdistric Cambisol) showed  low values in the soil enzyme index, and eutrophic soils (Endoeutric Cambisol, Rendzic Leptosol, Hypereutric Luvisol) the highest values. The value of the SEI_DPU to the highest degree was determined by urease activity, to a lower degree by protease activity and the lowest degree by dehydrogenase activity. The received values of SEI_DPU sorted investigated soils according to their quality, and an interpretation of order is possible, according to our present knowledge. Effect of differentiated  plant communities on the differentiated values of investigated SEI_DPU in the same or similar soil subtypes are discussed in the paper.

Key words: forest soil types, soil enzyme index, soil quality.

INTRODUCTION

Soil quality is conditioned by the properties of the soil and the kind of climate and microclimate. All soil properties, such as macro- and micromorphological, physical, chemical, physico-chemical, biological, microbiological and biochemical, play an important part in the formation of soil quality. These properties are related to each other and are affected by the kind of parent rock and the specific climate conditions. In trying to determine soil quality, taking into account the properties which come into play here as integral parts is impossible. Biological, microbiological and biochemical properties depend on physical, chemical and physicochemical soil properties and the type of climate and microclimate of the environment. Besides,  Trasar-Cepeda et al. [28] stated  that soil biological and biochemical properties are highly sensitive to environmental stress and thus can be used to assess soil quality.

Hitherto there have been few attempts to develop soil quality indices based on biological properties [1,7,12,17,20,26,29]. In the majority of cases these assessments were made based on the activity of tillage soil horizons [17,20,26]. Arable soils differ from forest soils in the morphology of a soil profile and, as a consequence, soil properties. Arable soils leave a relatively homogeneous, mineral-humus tillage horizon, reaching usually 30 cm deep. Forest soils, according to their biological activity and  subsequent rate of turnover of organic matter entering to soil, leave or do not leave surface organic layers. The comparison of soil properties in differentiated soil profiles on the basis of surface layers is groundless and leads to incorrect conclusions.

Usually the highest microbiological activity in soils takes place in the surface horizon [23,27].  Studies carried out by one author [14] showed that the highest enzyme activities in some subtypes of forest soils in deeper horizons are noted and endopedons lying to 1 m  deep leave appreciable enzyme activity and can have a serious impact on the rate of transformation in soils and,  consequently, on soil quality. For that reason, in these studies the activity of enzymes in natural moisture soil samples from particular horizons of profiles were determined. Then enzyme activities were counted on the basis of oven-dry weight soil mass  volume, in a column at a area of 1 dm²,  and to 1 m deep.

The authors of papers which evaluate soil quality based on of enzyme activity agree that determination of a single soil enzyme could not be used to determine plant productivity [21,22,29]. From own investigations on enzyme activities in mor, moder and mull humus horizons of forest soils of the Western Beskid Mts. [13] it appears that activity of dehydrogenases, urease and proteases in account in the soil volume, reflected correctly the biological activity of the investigated types of humus. It was found that dehydrogenase, urease and protease activities expressed in terms of soil volume in a column of 1dm² – 1 m (10 dm³), fully reflected the quality of forest soils of southern Poland [14]. To estimate quality of the  soils, the following enzyme soil index was proposed:

SEI_DPU = 0.01 × DhA + PA + UA,

where: DhA – dehydrogenase activity in mg TPF 10 dm^-3 24 hs, PA – protease activity in g N-NH₂ 10 dm^-3 20 hs^-1, and UA – urease activity in g N-NH₄ 10 dm^-3 24 hs^-1.

The index tested some subtypes of podzol and  brown soil, and gave reliable evaluation of its quality [14].

The aim of the study was a verification  of the usefulness of the SEI_DPU to assessment of soil quality on the base of other forest soil subtypes of southern Poland, extremely differentiated according to their quality. The study included the following soil types: podzol, lessivés, brown, rendzin and extremely fertile humus-deluvial soil, found in a complex of arable chernozem, which is unusual in forest areas. The aim of the study was also to investigate the effects of forest stand composition on soil enzyme activity and soil quality. Therefore, enzyme activities of podzols, lessivés and acid brown soils under conifers and deciduous forest stand were investigated.

AREA DESCRIPTION, METHODS AND MATERIAL STUDIED

The investigations were carried out in the summer periods of the years 1997–2001.  The studies on the same subtypes of soil were conducted on the same day of the same year. The study included:

• two proper podzol soils (Haplic Podzols), the first one under the monoculture of pine forest stand (plot No. 1) and the second under the pine forest stand with an admixture of birch and beech understory (plot No. 2) in forest compartment No.: 58c and 60a respectively, of Rudy Raciborskie Forest District (Table 1);

• two lessivés soils,  the first  podzolized lessivés soil (Hyperdistric Luvisol) under a larch forest stand (plot no 3) and the second browned lessivés soil (Epidystric Luvisol) under the forest stand of oak and hornbeam as the climax vegetation (plot No. 6), location in the forest compartment 57l and 51i, respectively of Miechów Forest District (Table 1);

• two acid brown soils (Hyperdistric Cambisols), the first under a monoculture spruce forest stand (plot No. 4) and the second under the beech forest stand (plot No. 5), located in the forest compartments  70f and 70g, respectively of Ustroń Forest District (Table 1);

• two leached brown soils (Cambisols) , the first with the fir-beech forest stand as the climax vegetation, located in the area of control unit No. 121 of Forest Didactic Station Krynica – Epidystrict Cambisol (plot No. 7) and the second under the lime forest stand as a climax vegetation located in the area of "Obrożyska" Reservation of Piwniczna Forest District – Endoeutric Cambisol (plot No. 8);

• chalk rendzina (Rendzic Leptosol) under a larch-hornbeam forest stand (plot no. 9) located in forest compartment 18a of Krasnystaw Forest District (Table 1),

• humous deluvial soil (Hypereutric Luvisol) under ash forest stand (plot No. 10) located in  forest compartment 58c of Miechów Forest District.

For every plot, a soil profile has been done, which have received the same numbers as their plots (Table 1). In all profiles the soil horizons were marked according to methods used in soil science. The contents of the soil skeleton (gravel and stone) in field conditions, in volume percent and with reference to total soil mass, were estimated visually. Soil samples from every horizon were collected in plastic bags, and samples of natural structure, to cores of 250 cm³ volume, in order to determine the soil bulk density were collected as well. In soil horizons with plenty (> 60%) of soil skeleton, soil without skeleton was collected in its field condition to measuring cylinders,  imitating the natural density by compression. Soil samples from surface horizons were collected from nine places around the soil profiles and then homogenised. The samples from other horizons were collected inside soil profiles. The samples were transported to the laboratory on the same day and stored in a refrigerator at 4°C. In a laboratory, one part of these samples was sieved through a 2-mm sieve. In samples with a natural moisture content, before no more then a few days, the activity of the following enzymes was colorimetrically determined:

• dehydrogenases by the Lenhard method [24] using the Casida et al. procedure [4];

• proteases by the Hoffmann and Teicher method  [11];

• urease by the Romeiko and Malinska method [11] with Čunderova modification,  using phosphate buffer at pH 6.7, 5% urea solution as substrate and 24-hour incubation.

All incubations were carried out at 37°C. The moisture content was determined by weight method. Enzyme activities were expressed in terms of oven dry soil volume in a column of 1dm² and natural soil depth to a maximum of 1 m of deep. In these calculations the volume of soil skeleton was deducted and oven dry soil mass was calculated according to soil bulk density.

The second part of collected samples was dried at room temperature and then the samples were sieved through a 2-mm sieve. In these samples physical, physicochemical and chemical properties necessary to the undertaking of typological diagnoses were determined. To determine of these properties the following methods were used [18]:

• texture using the Bouyocous-Casagrande method, as modified by Prószyński method;

• soil pH in H₂O and 1M KCl solution with soil to solution ratio in organic soil as 1:5, in sand 1:1, and in loam soil as 1:2.5 by potentiometric method;

• cation exchange capacity (CEC)  was calculated as a sum of the total acidity by Kappen method and sum of base cations (BC) in 1M CH₃COONH₄ at pH 7.0 as extractor and degree of base saturation (BS% = BC/CEC×100) was calculated;

• organic C by the Tiurin method with Soil Science Department of Agriculture University modification (without use of catalyzer and  longer time of oxidation),

• total nitrogen by the Kjeldahl method.

RESULTS AND ANALYSIS

In Table 1 soil units are given which are based on the standards of international criterions [32] and the potential and actual plant community as well. Soil skeleton and fine earth content and soil bulk density have been shown in Table 2. Soil reaction and sorption properties are shown in Table 3. Content of organic carbon and total nitrogen, C:N ratio in surface horizons and activity of investigated enzymes in horizons to 1 m of deep, represented by weight units,  are presented in Table 4. The investigated soil units in Table 1-4, ordered according  to increasing quality and evaluated based on  physical, physic-chemical and chemical properties and according to already attributed (Table 1-4) quality in agriculture and forestry practice.

In organic soil layers (Ofh and Oh horizons in profiles 1-5, Table 4) the  highest dehydrogeases, proteases and urease activities were noted in raw humus of acid brown soil under the beech forest stand (profile 5, Table 4) with respective values of: 40.96 mg TPF 100 g^-1 24 hs^-1; 1690.24 mg N-NH₂ 100 g^-1 20 hs^-1 and 24.44 mg N-NH₄ 1 g^-1 24 hs^-1.

The lowest out of all investigated enzyme activities was noted in organic soil layers in different soil,  dehydrogenases in raw humus of acid brown soil under the spruce monoculture (0.134 mg TPF 100 g^-1 24 hs^-1, profile 4, Table 4), proteases in raw humus of podzol soil under the deciduous forest stand (61.823 mg N-NH₂ 100 g^-1 20 hs^-1, profile 2, Table 4), urease in raw humus of lessivés soil under the larch forest stand (0.759 mg N-NH₄ 1 g^-1 24 hs^-1, profile 3, Table 4).

Of all the mineral-humus horizons  (Ah horizons in profiles 6-10, Table 4), the highest dehydrogease activity in leached brown soil was noted under the deciduous forest stand (39.13 mg TPF 100 g of soil^-1 24 hs^-1, profile 7, Table 4) and protease and urease activities in rendzin soil (respectively: 503.40 mg N-NH₂ 100 g^-1 20 hs^-1 and  195.58 mg N-NH₄ 1 g^-1 24 hs^-1, profile 9, Table 4). The lowest dehydrogenase activities in horizon Ah of humus-deluvial soil (8.94 mg TPF 100 g^-1 24 hs^-1, profile 10, Table 4) and protease and urease activities in Ah horizons of leached brown soil under the lime forest stand (respectively: 148.82 mg N-NH₂ 100 g^-1 20 hs^-1 and 3.37 mg N-NH₄ 1 g^-1 24 hs^-1, profile 8, Table 4) were noted.

In all profiles of investigated soil subtypes dehydrogenase activities, according to weight units,  decreased together alongside the increasing depth of the soil profiles (Table 4). The pattern of protease and urease activities in investigated soil profiles was not so regular. Sometimes the activities of protease and urease were higher in deeper horizons than in shallower laying. For example, in lessivés soils there were noted higher activities respectively of urease and protease, in argic horizon Bt and lower activity in luvic horizon Eet (profiles 3 and 6, Table 4). Similarly, in acid brown soil profile (profile 4) higher urease activity in deeper horizons (Bbr2, BC) and lower activity in shallower horizon (Bbr1) were noted. Similarly phenomenon can be observed in the activity of protease in podzol soils (profile 1 and 2, Table 4).

The activity of the researched  enzymes in counted  oven dry soil mass  volume in a column of 1 dm² and 1 m deep are showed in Table 5 in  increasing order. The order of  investigated soils in Table 5 is dependent on the kind of enzyme. The highest value of dehydrogenase activity in Endoeutric Cambisol was under the climax lime forest stand (profile 8) and the lowest in Haplic podzol under the pine monoculture stand (profile 1, Table 5).  A relatively low value of dehydrogenase activity in Rendzic Leptosol (profile 9, Table 5) was observed. The highest urease and protease activities were discovered in Hypereutric Cambisol (profile 10, Table 5). The lowest value of urease activity were determined in Hyperdistric Cambisol under the spruce monoculture (profile 4) and protease activity in Hyperdistric Luvisol under the larch forest stand (profile 3, Table 5).

The values of SEI_DPU for investigated soil units on Figure 1 are presented, in increasing order. The sequence of soil according to urease activities of the investigated soils is the most similar to the order according to values of SEI_DPU. Only Haplic Podzol soil (profile 1) and Hyperdistric Cambisol (profile 4) have other  locations. The value of the SEI_DPU for the most part was determined by urease activity (mean 52.3%), to a lesser degree by protease activity (mean 31.9%) and the lowest degree by dehydrogenase activity (mean 15.5%).

In the same soil type, the values of the examined index,  like the activities of investigated enzymes, were in every case  lower in soil under monocultures of  coniferous forest stands  (profile 1, 3 and 4)  and higher in soil under  deciduous forest stands (profile 2, 6 and 5). The values of SEI_DPU of soil under the coniferous forest stand with reference to SEI_DPU values of soil under the deciduous forest stand, in percent, amounted respectively to: 57.7%, 17.2% and 40.5%, mean 38.5%.

DISCUSSION AND CONCLUSIONS

The comparison of enzyme activities which was investigated in the same summer period but in different years can be questionable.  The results of earlier conducted investigations revealed a minor difference in the activity of  the investigated enzymes between the analysed years [13].

The results of the investigations of Gianfreda et al. [8,9] and the revue of Kiss et al. [15] pointed out the individual sorption abilities of enzymes to different minerals and the different residual activities of enzymes after accumulation. Zantua and Bremner, quotation by Bremner and Mulvaney [3] stated, "urease activity increased after addition of jack bean urease or of organic materials promoting microbial activity, but subsequently decreased and stabilized at the level observed before addition of these substances" and the explanation of these findings is that "the urease activity of unamended soils reflects their capacity for protection of urease and that urease in excess of this capacity is decomposed or inactivated" [3].  According to these results, can be hypothesised that soil subtypes can heave a peculiar capacity towards individual enzyme dependence on some kinds of minerals and may be kinds of humus colloids.

The microbial activity of soil, to a large extent, is dependent on the quantity of available carbon, and is reflected by dehydrogenase activity [24]. Nitrogen mineralization rates have been found to be highly correlated with plant nitrogen uptake and net aboveground productivity – Pastor et al. 1984, quotation of White [30]. The potential rate of nitrogen mineralization in soil is reflected by the potential activities of protease and urease [15,24].  Consequently,  the evaluation of soil quality based on dehydrogenase, protease and urease  activities seem to be well justified.

A broad range of received values of SEI_DPU gives the possibility of elaborating on biochemical soil classification. In the Table 6 quality soil classes are proposed, conditioned  by the range of SEI_DPU values. Because on forest grounds soils of low quality dominate, narrow class sections (6 units) are proposed, allowing the possibility to classify poorer forest soil, and to a lesser degree proposed classes can divide rich soil, which is rarely found on forest grounds, but form rather arable ecosystems. Therefore the proposed division is connected exclusively to the forest soil of southern Poland.

The sequence of investigated soil subtypes in increasing order of SEI_DPU values (Fig. 1), deviating  to a small degree from the sequence of subtypes in Table 1–4, is ordered  according to the commonly acknowledged quality of these soils. The Hyperdistric Luvisol under the larch forest stand, received relatively low values of SEI_DPU (Prof. 3, Fig. 1) on the level of Haplic Podzol under pine monoculture (Prof. 1, Fig. 1). A appreciable decrease of enzyme activities can be observed in soil under monoculture conifer trees in the pH of soil, in 1M KCl below 3.5 (compare values of pH and enzyme activities in surface soil horizons of 1 and 2, 3 and 6 and 4 and 5 profiles in Table 3 and 4). These low values of SEI_DPU for podzolized lessivés soil can be explained by the influence of slow decomposing  larch needles. Larch litter has the highest C:N ratio (C:N = 77) among the species which make up forests in Europe [31]. The unprofitable effect of the monoculture of conifer forest stands on the enzyme activity of soil can be observed as well as in the examples of Cambisols under the spruce monoculture and the beech forest stand (Profiles 4 and 5), Haplic Podzols under the pine monoculture and the mixed forest stand (Profiles 1 and 2). Chodzicki [5,6] mentioned about the profitable effect of beech on soils properties in pine forests. The profitable interaction of birch  leaves in comparison to pine needles on nutrients oven time, especially of nitrogen and phosphorus by Berg and Staaf [2] as well as by Ste-Marie and Paré [25] have been described. Steubing [27] has been written on many substances which contain pine needles, in comparison to beech leaves, which  retarded microbiological attacks.

Epidystric Luvisol (browned lessivés soil) received relatively high SEI_DPU values under the climax deciduous forest stand. The lessivé soils are known for their rapid rate of mineralization of soil organic matter and low content of humus, very often accumulated in this soil in a kind of ochric diagnostic horizon. An intensive rate of mineralization of organic matter in lessivés soils can be caused by a higher moisture content in surface horizons, due to a lesser permeability of argic horizons.

The highest value of SEI_DPU, as expected, was noted in Hypereutric Luvisol, deep humus deluvial soil (Profile 10). In soil with neutral and weak alkaline reactions there were, very advantageous physical properties, without any signs of gley process, while susceptible to the microbiological decomposition of leaves ash (C:N = 24) [31].

Doubts as appeared with useful biochemical properties to evaluation of soil quality, as recently have been presented by some researchers [10,28], can result from evaluation of soil quality based on biochemical properties of humus horizons only,  presented on weight but not volume units and leave out in evaluation other horizons in soil profile.

Soil can be seen as a perfect reactionary enzymatic column, naturally restored in enzyme and substrates. The substrates input to soil  have a greater possibility to be incorporated into the accumulated enzymes in soils which are more and deeper enzymatic active. Therefore soil depth, enzyme active, may have a crucial role in the rate of enzyme transformation in soil. This is the reason for the assumption of soil quality, based on biochemical properties, the greater part of the soil profile of not only the humus horizon, which in forest soils is strongly differentiated according to quality and width.

From the studies curried out, it appears that an extremely critical matter is the adaptation of a suitable species composition for a given forest site,  according to soil quality. Unsuitable species compositions of forest stands (excessive concentration of conifer species) in fertile soil can cause soil degradation during a period of one generation. The first phase sees a degradation of biological and biochemical soil properties, then physico-chemical, chemical and physical properties, and in the next phase micromorphological and the end sees a change of morphological properties, which are easily visible [19]. For these reasons,  to gain a true  sense of reliable assessment of biological and biochemical properties of soil, the aim is fast detection of degradation, so that harmful interaction can be detected in the first stages of development. Based on the investigations carried out, may form following conclusions:

• the proposed soil enzyme index (SEI_DPU), established based on dehydrogenase, protease and urease activities, gave reliable evaluation of quality of ten forest soil subtypes of southern Poland, and an interpretation of the order of these soils is possible, according to our present knowledge;

• a broad range of received values of SEI_DPU allows the possibility of elaboration of biochemical soil classification,

• the monocultures of conifer trees, in comparison to deciduous forest stands, had disadvantageous effects unprofitable to the biochemical activity of soil, leading to its degradation. In investigated soils a great fall in enzyme activity can be observed in soil with pH in 1M KCl below 3.5;

• the adaptation of a proper species composition of forest stand to forest site according to soil quality, has a great effect  on the state of soil quality and prevention of soil degradation,

• the evaluation  of soil quality based on proposed SEI_DPU would be tested on soils less drained than elsewhere.

ACKNOWLEDGEMENTS

This work was partly financed by Scientific Research Committee in frame Project No. DS 3389/ZGL/97-99 and DS 3407/ZGL/00. The author would like to thank Grzegorz Domicz, Arkadiusz Fiślak, Rober Grzesik and Andrzej Koczkodaj for help in field work as well Regina Głowacka, Bożena Dobroś and Agnieszka Wojciechowicz for their help with the analysis of the samples.

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Accepted for print: 29.03.2011

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